planning for subterranean residential development in the uk

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Planning for Subterranean ResidentialDevelopment in the UKJohn McCarthy & Ross KilgourPublished online: 22 Feb 2011.

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ARTICLE

Planning for Subterranean ResidentialDevelopment in the UKJOHN MCCARTHY & ROSS KILGOUR

Abstract

There has been a significant growth in the number of subterranean residential developments overthe past 10 years in the UK, particularly within London. While they may present little visible

evidence above ground, such developments can have significant impacts on sustainability, amenityand environmental quality. Nevertheless, few local planning authorities have developed specificpolicies in response to such developments. This article outlines the results and implications of

research that investigated the approach of four London local planning authorities to suchdevelopment, with a particular focus on the Royal Borough of Kensington and Chelsea, which hasdeveloped more detailed policy for subterranean development than any other authority. Thearticle suggests that more effective outcomes for sustainability, amenity and environmental

quality could be achieved by more general application of policy for subterranean development,with implications also for contexts outside the UK.

Introduction

Subterranean or basement residential development comprises that which is belowground, and for which there may be little or no visible evidence above groundwhen completed. There has been a significant increase in the past 10 years in thenumber of such developments completed in the UK, usually as extensions toresidential properties, although these have been largely confined to a small numberof London planning authorities (Department of Communities and LocalGovernment [DCLG], 2008a). Hence the London Basement Company (a UK-based company specializing in basement conversion and excavation) revealed in2008 that they were experiencing a 30% year-on-year increase in basementconstructions, and that the average size of such constructions was also increasing,from 400 ft2 (37.2 m2) in 2004 to 700 ft2 (65 m2) in 2008 (Strongman, 2008).Moreover, the number of such developments could grow significantly in thefuture, with increasing wealth and competing pressures on land. Typical uses forsuch developments are for playroom, studies, bedrooms, utility and games rooms,swimming pools and gymnasia.While it may be assumed that the impact of subterranean residential

development on amenity, sustainability and environmental quality is minimal, in

John McCarthy, School of the Built Environment, Heriot-Watt University, Riccarton, EdinburghEH14 4AS, UK. Email [email protected]

Planning Practice & Research, Vol. 26, No. 1,pp. 71–94, February 2011

ISSN 0269-7459 print/1360-0583 online/11/010071–24 � 2011 Taylor & Francis 71DOI: 10.1080/02697459.2011.542061

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view of its lack of visibility, its effects can be significant. For instance, suchdevelopment may have a more substantial effect than other types of developmenton the water table, and it may therefore affect the potential for local flooding. Inaddition, it requires a greater amount of embodied energy for construction thansimilarly-sized above-ground development, largely due to the necessary materialsand the need to excavate and remove large amounts of earth. However, onceconstructed, such development is likely to be more effectively insulated thantypical above-ground development. Consequently, such development may notonly provide a less expensive alternative for owners than moving, but may also bemore preferable for planning authorities than above-ground extensions that maycompromise local amenity, character and appearance, and prove less sustainable inthe longer term (Royal Borough of Kensington and Chelsea [RBKC], 2009).Indeed, use of underground urban space more generally may be argued to providea valuable contribution to wider aims for sustainability, since it can enable cities tobe more compact, facilitating better energy efficiency and reducing the needto travel and therefore traffic congestion and atmospheric emissions, albeit withthe associated risk of subsidence and structural damage to existing buildings(Bobylev, 2009).This article investigates the application of planning policy for subterranean

residential extensions in selected local planning authorities in London, so as toreach conclusions on the potential desirability of more general application of suchpolicy. More specific research questions are as follows: to what extent is there aneed for wider application of policy for subterranean residential development?,and to what extent does the application of such policy in RBKC have more generalapplicability?The article is structured as follows. First, the research methods are set out;

second, the increasing prevalence of subterranean development is considered interms of planning applications in four planning authorities; third, the nationalplanning context is set out; fourth, broad environmental and planning implicationsof subterranean development are set out; fifth, the process and effects of planningpolicy are considered in relation to such development in the case of RBKC withpoints of comparison for three other local planning authorities, via interviews withplanners and developers/agents; sixth, this experience is analysed in terms ofimplications of the research for planning policy more generally; and finally,specific conclusions are suggested for policy and practice.

Research Methods

There are 32 borough councils (local authorities) in Greater London, as well as theCity of London Corporation (the governing body for the area of London oftenreferred to as the ‘Square Mile’, which houses many institutions of the UK’sfinancial services industry), all of which have responsibility for land-use planning(see Figure 1). The research that forms the basis of this article investigated theexperience of four London boroughs (primarily RBKC but also the LondonBoroughs of Merton, Wandsworth and Camden) with respect to subterraneanresidential development. These four were selected because they experiencesignificant numbers of planning applications for such development (although some

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such applications have also been received in other London boroughs such asHammersmith and Fulham, Islington, Hackney, Tower Hamlets, Lewisham,Barnet and Ealing), and they provide similar online systems allowing searching ofplanning decisions.The specific research methods were as follows. First, a review of statistical data

for the four selected authorities was undertaken, in terms of data on planningapplication decisions for basement-related applications, and for all planningapplication decisions, from 2001 to 2009. A comparison of these datasets allowedconclusions to be drawn on the popularity of basement-related development (as aproxy for subterranean residential development, or basement extensions) inrelation to all development.Second, a case study of RBKC was undertaken, which made use of

documentary review as well as interviews with two RBKC planning officers,comprising a policy officer who had worked on RBKC’s 2009 SubterraneanDevelopment Supplementary Planning Document and a development controlofficer familiar with applications for subterranean residential development. RBKCwas chosen for this case study because: it has experienced a recent rise in thenumber of planning applications for such development, leading to relatively highnumbers in comparison with overall applications; it has a historic urbanenvironment, which throws into sharp relief the potential problems of such

FIGURE 1. RBKC context map.Source: RBKC (2007). Crown Copyright Ordnance 2011.

Planning for Subterranean Residential Development

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development; and it has gone further than any other authority in adopting planningpolicy and guidance specifically in relation to such development. In fact, only oneother local authority (the London Borough of Wandsworth) had in 2000 adoptedspecific planning policy in connection with subterranean development (DCLG,2008a) (although more recently such policy has been applied in Hammersmith andFulham, and Camden). In addition, one of the authors was employed as a planningofficer for RBKC in 2008/09, during a sandwich year as part of a full-timeplanning course, which enabled in-depth familiarity with the context and issuesdiscussed.Third, interviews were undertaken with a sample of planning officers at two

other London boroughs (Merton and Wandsworth) included in the research, to aidunderstanding more widely of the effects of subterranean residential development,the extent to which existing policies were deemed adequate in addressing suchapplications, and the potential for action to improve practice in managing suchdevelopment. Fourth, interviews were undertaken with a sample of developers/agents involved in applying for planning permission for subterranean residentialdevelopment within the four boroughs considered, to provide triangulatedevidence in relation to the application of planning policy.

Demand for Subterranean Residential Development

An increase in demand for subterranean residential development within GreaterLondon authorities is indicated by the rise in the number of planning applicationdecisions related to such development (decisions by local planning authorities onapplications for planning permission related to subterranean development) in thefour planning authorities considered in this article. These numbers are set outbelow, together with numbers of all planning decisions (Table 1 for RBKC,Table 2 for the London Borough of Merton, Table 3 for the London Borough ofWandsworth, and Table 4 for the London Borough of Camden.) The figures were

TABLE 1. Number of planning decisions per year involving basements:a RBKC, 2001–2009

YearBasementdecisions

Change onpreviousyear (%)

Totaldecisions


Basement decisionsas percentage oftotal decisions

2001 455 2, 841 16.02002 440 73.3 2, 913 2.5 15.12003 423 73.9 2, 707 77.1 15.62004 469 10.9 2, 935 8.4 16.02005 419 710.7 2, 782 75.2 15.12006 525 25.3 3, 172 14.0 16.62007 681 29.7 3, 510 10.7 19.42008 600 711.9 3, 366 74.1 17.82009 483 719.5 2, 781 717.4 17.4

Note: aWith criteria for selection based on the word ‘basement’ appearing in the developmentdescription in the planning decision database.Source: RBKC (2010b).


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obtained from the authorities’ online planning decision databases, and the searchfunction ‘basement’ was used to provide a proxy for subterranean residentialdevelopment in order to illustrate broad trends. The limitations of this methodwere that: the application decisions returned included, for instance, changes of use,new-builds involving basements and details such as internal alterations and airconditioning, as well as new subterranean extensions; and searches were returnedfor all uses, since it was not possible to filter for residential uses only.These tables show that there was a general increase in planning decisions (and

therefore applications) related to basements (a floor or floors of a building that arepartially or completely below the ground floor) between 2001 and 2009 in all fourboroughs (Tables 1 – 4), with RBKC experiencing the highest proportion ofbasement-related applications (Table 1). It may therefore be inferred that there hasbeen a similar increase in decisions (and applications) for subterranean residential

TABLE 2. Number of planning decisions per year involving basements: London Borough of Merton,2001–2009



Totaldecisions



2001 26 2, 556 1.02002 29 11.5 2, 568 0.5 1.12003 29 0 2, 663 3.7 1.12004 36 24.1 2, 704 1.5 1.32005 46 27.8 2, 849 5.4 1.62006 55 19.6 2, 925 2.7 1.92007 77 40 3, 336 14.1 2.32008 112 45.5 3, 119 76.5 3.62009 76 732.1 2, 613 716.2 2.9

Source: London Borough of Merton (2010).

TABLE 3. Number of planning decisions per year involving basements: London Borough ofWandsworth, 2001–2009



Totaldecisions



2001 177 3, 989 4.42002 211 19.2 4, 261 6.8 5.02003 227 7.6 4, 272 0.3 5.32004 261 15.0 4, 556 6.6 5.72005 222 714.9 4, 477 71.7 5.02006 288 29.7 4, 765 6.4 6.02007 377 30.9 5, 311 11.5 7.12008 354 76.1 4, 811 79.4 7.42009 237 733.1 3, 518 726.9 6.7

Source: London Borough of Wandsworth (2010).


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development in these authorities over this timescale, although some suchapplications may have been intended primarily to increase the value of theproperties concerned, and so may not have been implemented (interview,development control planner, RBKC, March 2010). The reasons for such anincrease would seem to be related to: relatively high land and property values,linked to high levels of personal wealth (RBKC has the highest average grosshousehold annual income of any local authority in the UK); and lack ofopportunity to extend above ground, as a result of lack of space or restrictionslinked to planning and related policy. These factors were cited by interviewees aspart of this research, as outlined below. They also help explain why subterraneanresidential development more generally is largely a feature of London and othercities such as Dublin with similarly high historic land and property values, andwhy most applications within London have been within inner London boroughssuch as Camden, Wandsworth and RBKC, rather than outer London boroughssuch as Merton (since more centrally-located boroughs in Greater London tend toexperience higher land values).More specifically, application decisions relating to basements generally peaked

as a proportion of total application decisions in 2007/08, albeit with a slightdecline in 2008/09 possibly as a result of the less ready availability of creditneeded for such costly property alterations (Tables 1–4) (Kilgour, 2010).However, it may be argued that the structural drivers of the increase in suchdecisions (and therefore applications), comprising increasing land values andrestrictions to above-ground development, are likely to intensify in the longer termso that the level of planning applications for subterranean development may beexpected to increase. In addition, new-build residential developments are likely toincreasingly incorporate basements (interview, development control planner,London Borough of Merton, February 2010). These factors would seem to implythe need for development of a clearer policy framework so as to maximize the netbenefits and mitigate possible problems (both of which are discussed below)arising from such development.

TABLE 4. Number of planning decisions per year involving basements: London Borough of Camden,2001–2009



Totaldecisions



2001 336 4, 366 7.72002 344 2.4 4, 298 71.6 8.02003 393 14.2 4, 508 4.9 8.72004 447 13.7 5, 072 12.5 8.82005 469 4.9 4, 843 74.5 9.72006 556 18.6 4, 978 2.8 11.22007 706 27.0 5, 272 5.9 13.42008 633 710.3 4, 763 79.7 13.32009 588 77.1 4, 528 74.9 13.0

Source: London Borough of Camden (2010).


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National Planning Context

While subterranean residential development might be invisible from outside theproperty, in the UK context it constitutes development in principle, defined as ‘thecarrying out of building, engineering, mining or other operations in, on, over orunder land, or the making of any material change in the use of any buildings orother land’ (Cullingworth & Nadin, 2006, p. 149). However, Schedule 2, Part 1,Class A of the Town and Country Planning (General Permitted Development)(Amendment) (No.2) (England) Order 2008 (Great Britain, 2008) sets out‘permitted development’ rights including for certain householder extensions (thismeans that planning permission is not required for such developments).Essentially, a simple form of subterranean residential extension is considered aspermitted development if it satisfies certain conditions. Within conservation areas(areas of special architectural or historic interest where it is desirable to preserve orenhance this), such conditions include that it does not extend beyond the sideelevation wall of the original dwelling house and beyond the front wall of theprincipal elevation of the original dwelling house if it fronts a highway. Outsideconservation areas, such conditions include that the extension does not extendbeyond the front or side elevations of the original dwelling house where theseelevations front a highway, and beyond the rear wall of the original dwellinghouse by more than 4 m in the case of a detached house or 3 m in the case of anyother dwelling house (for single-storey subterranean extensions only). Never-theless, even in cases of permitted development, there is a duty to preserveprotected trees, Listed Building Consent (additional permission needed in relationto alternation, extension or demolition of buildings that are ‘listed’ in view of theirspecial architectural or historical interest) may be needed, and compliance withother relevant legislation such as Building Regulations (which set standards for thedesign and construction of buildings) and the Party Wall Act (which makesprovisions regarding party walls [dividing partitions between buildings] andconstruction and excavation close to buildings) is required (RBKC, 2009).Moreover, in the 2007 Planning White Paper (DCLG, 2007a), the government

proposed the replacement of Parts 1 and 2 of the General Permitted DevelopmentOrder (which sets out permitted development) with a new Householder PermittedDevelopment Order (HPDO). A subsequent DCLG supplementary report onbasement extensions (DCLG, 2008a) recommended the creation of a newbasement extension class within a new HPDO that would extend permitteddevelopment rights for basements meeting criteria in relation to location, height,size, location of light well and impact on the street scene, but would removepermitted development rights for basement proposals within high flood risk zones(RBKC, 2009), and while only a small number of local authorities receivesignificant numbers of planning applications for basements, the overall effect ofsuch a HPDO could be to substantially reduce the number of such applicationsreceived by these authorities. A ‘mini-consultation’ on the proposals includingsuch authorities indicated a broad consensus in favour of such a new basementsclass (DCLG, 2008a).Subterranean development must also be considered in the context of a range of

national planning concerns and consequent national planning policy statements


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that address issues such as the historic environment, noise, transport, housing,waste management, pollution control and archaeology (RBKC, 2009). (National inthis context refers to England since land-use planning is a devolved function ofgovernment in the UK.) Other relevant national policy statements address theincreasingly important issues of climate change and sustainable development, ordevelopment that meets the needs of the present without compromising the abilityof future generations to meet their needs (United Nations World Commission onEnvironment and Development, 1987). Such national policy statements withimplications for subterranean development comprise, for instance, Planning for aSustainable Future (DCLG, 2007a), the Supplement to Planning Policy Statement1 (DCLG, 2007c) and Planning Policy Statement 25: Development and Flood Risk(DCLG, 2010).

Planning Implications of Subterranean Residential Development

It is now appropriate to consider in more detail the broad range of implications ofsubterranean residential development, in relation to planning and related issues,and this is set out below. First, however, it is useful to position subterraneanresidential development as part of wider urban underground space in terms of thedevelopment of such space and the impacts of technology such as tunnelling andexcavation, and the use of underground space as a scarce resource with clearimplications for sustainability, all of which is relevant in many other contextsglobally.In terms of development of underground space, underground construction

benefits from an ancient legacy. Moreover, the application of technology forunderground construction has now been applied globally, with significantunderground buildings developed for instance in Japan, China, Russia, andthroughout Europe, and some 5, 000 private underground dwellings developed inNorth America (Hall, 2004). This has been made possible with the development ofrelated construction technology for excavation and tunnelling. This has beenincentivized by the increasing awareness of the benefits of such technology interms of sustainable development, and many observers such as Celik (1996), Ray(1998), Roberts (1996) and Sellberg (1996) have highlighted the implications ofunderground space development technology for sustainability. In terms of the useof underground space, it is increasingly recognized that this is a scarce andvaluable non-renewable resource since underground structures cannot bedemolished in the same way as above-ground ones. It may therefore be arguedthat such space should be used carefully and considered appropriately in spatialplanning policy at various scales (Bobylev, 2009).In relation to subterranean residential development more specifically, the

following planning-related implications may be identified.

Quality of Life/Structural Stability/Impact on the Historic Environment

Because of the intensive nature of the construction process for subterraneanresidential development, there may be significant impacts on the quality of life oflocal people; for instance, via increased noise, vibration, traffic and dust. In


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addition, the site could be contaminated from its previous use, which could lead tolocal health risks (RBKC, 2009). Moreover, excavation and construction ofsubterranean development may impact on the structural integrity of existing andneighbouring structures. Indeed, in the London Borough of Camden, two semi-detached houses had to be demolished as a result of unauthorized subterraneandevelopment that resulted in subsidence (interview, development control planner,RBKC, March 2010). This is a particularly important issue in connection with thehistoric environment, since Georgian and Victorian terraces (common in parts ofLondon), for instance, usually have relatively shallow foundations that tend tomove naturally in the soil, particularly clay soil (common in many parts ofLondon, including the northern part of RBKC). Consequently, subterraneandevelopment may have effects on these foundations that can impact on thestability of the parent building or adjacent buildings, unless such subterraneandevelopment is designed and constructed appropriately (RBKC, 2009).In addition, such development may have other impacts on the special

architectural or historic interest of listed buildings. This is not the same thing asappearance, and can include ‘the location and hierarchy of rooms and floor levels,foundations, the original purpose of the building, the size and location of anyoriginal basement, cellar or vault, and the integrity of the historic structure’(RBKC, 2009, p. 10). Moreover, the external manifestations of subterraneandevelopment, including ‘light wells and roof lights, structures for safety, accessand ventilation, exposed masonry and trees and landscaping’ (RBKC, 2009,p. 10), may have particular impacts in conservation areas, for instance.

Energy Efficiency and Sustainability

Improving the energy efficiency of homes is an important objective ofgovernment, linked to the broader aim of addressing climate change, and severalpolicy initiatives have been introduced in connection with this objective inEngland. In particular, the Code for Sustainable Homes (CfSH) (DCLG, 2008b)replaced the EcoHomes standard in 2007. The Code is an assessment standardbased on nine specified environmental issues, with a score awarded from Level 1to Level 6, the latter being the highest level of sustainability, equivalent to ‘zerocarbon’ (DCLG, 2008b). At present, all publicly funded new-build housing mustachieve a minimum of CfSH Level 3, and the government plans by 2016 to requireall new-build homes to achieve CfSH Level 6 (DCLG, 2007a, 2007b), althoughthis target may be seen as unrealistic (Taylor & Woolley, 2008).In addition, there is a simultaneous need for existing dwellings to be ‘retro-

fitted’ (via improvements to existing buildings) to improve their energy efficiency,since the presently existing housing stock will continue to comprise the bulk of theUK’s housing stock for a considerable time (Catto, 2008). However, such retro-fitting can be difficult, particularly for older and listed buildings. For instance, theuse of double-glazing to improve insulation may be precluded in such cases, sinceplanning permission or listed building consent may be resisted for the installationof any type of window other than a traditional timber sliding sash in listedbuildings where this was the original window type. Moreover, English Heritage(a non-departmental public body of government with a broad remit for the


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management of England’s historic built environment) suggests that ‘traditional’(pre-1919) buildings need to allow the natural diffusion of moisture betweeninternal and external spaces, and modern building materials obstruct this processby blocking the transmission of air and moisture, although it suggests alternativeretro-fitting methods such as insulation of solid walls, draught-proofing andinstallation of secondary glazing (English Heritage, 2008).The construction of subterranean residential development also requires the use

of more energy and materials than an equivalently-sized above-ground develop-ment, since existing material must be excavated, transported and disposed of atlandfill (since it is typically ‘made ground’ as opposed to natural soil) (Arup,2008). Moreover, such development commonly involves the use of concrete,which is relatively high in embodied energy (approximately 1 MJ/kg) andembodied carbon (approximately 0.16 kg CO2/kg) (Hammond & Jones, 2008).Furthermore, many modern residential basements are ‘tanked’; namely, encased ina protective metal outer casing to prevent dampness seeping into the concrete,which further increases their embodied energy, and thus their carbon footprint.Hence such development usually has a relatively high initial investment duringtheir construction period, as well as an aggregated negative environmental impact(Bobylev, 2009).Nevertheless, such development, once constructed, requires less energy to heat

than similarly-sized above-ground development, since it is thermally insulated bythe ground surrounding it (Anderson & Ward, 1993). It therefore has similarenergy efficiency advantages to earth sheltered housing with passive ventilation,for instance, and so in principle can assist with achieving aims for sustainabledevelopment. Indeed, such environmental benefits from use are common to muchunderground development (Bobylev, 2009). Moreover, subterranean residentialdevelopments are likely to have a longer lifetime than above-ground develop-ments, since they cannot easily be demolished or rebuilt, for instance, as a result ofnew ownership (although they may be further extended), unlike in the case ofabove-ground development. They also often have lower maintenance costs incomparison with above-ground development, resulting for instance from therelatively stable temperature of underground space and the natural protection itenjoys from external influences (Bobylev, 2009). In addition, the removal ofexcavated soil arising from subterranean development to landfill may be regardedas providing a useful source of the necessary daily capping material for landfill, sosuch excavated soil may be regarded as being ‘recycled’ rather than ‘dumped’.Consequently, in assessing the likely emissions impact of a subterranean

residential development, it is desirable to determine its likely lifespan so as toenable the whole life cost of the development to be calculated. The relatively highembodied energy arising from construction could then be set against the energysaving from use resulting from higher insulation standards for heating and coolingduring the whole building lifespan. This might show, for instance, that a basementextension to a Victorian building (which could have a lifespan of centuries) mighthave less of an overall environmental impact over its lifetime than a comparablebasement extension to a new-build dwelling, or an above-ground extension, bothof which could have life spans of only around 60 years (interview, developmentcontrol planner, RBKC, March 2010).


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Flooding

Clearly, flooding (including tidal flooding, surface water flooding and sewageflooding) may have impacts on subterranean development, and such developmentmay in turn impact on ground water flows and the water table, and therefore on thepotential for flooding. Safety is critical in this context, and flood risk zones can beused to determine whether a property is in an area suitable for basementconstruction. However, there is the prospect, linked to climate change, ofsignificant sea level rises in coming decades. It may therefore be argued that theplanning system should resist all subterranean residential development below acertain height above sea level, particularly as climate change models and resultingpredictions for sea level rises become more reliable. Again, a useful distinctionmay be drawn here between subterranean extensions to new properties (or asintegral parts of new-build schemes), and such extensions to older properties, withthe latter having a potentially longer life and therefore facing greater risk in termsof sea level rise, and therefore potential flooding. Ground water flows can also beaffected by subterranean development, since such flows will find an alternativeroute if blocked, and changes in the level of the water table might result, althoughthese are unlikely to be more significant than natural variations (RBKC, 2009).

Social Sustainability

Issues of social sustainability may also arise in relation to subterranean residentialdevelopment, although these are contended. For instance, it may be argued that suchdevelopment can assist in the avoidance of urban sprawl and associated spatialsegregation, as well as in the protection of private garden space that can enhancesocial diversity by encouraging the provision of family housing. However, thedevelopment of subterranean residential development may also be seen as part of atrend for the internalization within private homes of traditionally public activities,with many subterranean residential extensions used, for instance, as gymnasia,swimming pools and home cinemas. Clearly, demand for such development is likelyto be linked to levels of income and wealth, which are relatively high in severalLondon boroughs, including RBKC. Hence this could be seen to contribute to aprocess of residualization (and possible withdrawal) of public facilities, reducedcommunity interaction and possibly increased social segregation. Such effects,while not of the same order of magnitude, may be likened to those arising from gatedcommunities. While Bobylev (2009) indicates that cities with more developedunderground space have better social equity, this assumes such space to be generallyfor public use, while this article considers the private use of such space.Nevertheless, it may also be argued that in areas which are relatively wealthy,

where demand for subterranean residential development tends to arise, people tendalready to use private rather than public facilities, so internalization of uses is notlikely to be significantly increased by such development (interview, policy officer,RBKC, February 2010). However, such effects may be evident in the longer term,particularly where subterranean development acts in combination with otherfactors leading to internalization of uses, such as increasing social segregationbased on income, and increased privatization of public services and facilities.


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The Case of the Royal Borough of Kensington and Chelsea

Context

The case of RBKC will now be considered in terms of its experience ofsubterranean development. The Borough (see Figure 1) is situated in GreaterLondon between the City of Westminster to the east and the London Borough ofHammersmith and Fulham to the west. While it is one of London’s smallest localauthorities, both in size (5 m2 [13 km2]) and population (196, 000), it is also insome respects the wealthiest, and one of the most densely populated (RBKC,2007), not only in London but also within the UK as a whole. Factors such as thehigh level of personal wealth have led to a frequent desire to extend residentialproperties, and such above-ground extensions have had harmful effects such asreduction in garden space (by rear extensions) and alteration of roof lines (bymansard or dormer extensions) (RBKC, 2007). Many recent residential extensionshave been via basements as ‘a direct result of the lack of land; pressure fordevelopment; high land values; strict planning controls; and limited opportunityfor extensions’ (RBKC, 2009a, p. 5). In addition to the indications above of anincrease in basement-related planning applications in RBKC, there has also beenan increase in the number of planning applications specifically for subterraneandevelopment, as indicated in Table 5. However, over 70% of the Borough fallswithin designated conservation areas, and it contains around 4, 000 listedbuildings; consequently, the Borough is highly sensitive to further subterraneandevelopment. Moreover, many original basement developments from theVictorian and Georgian eras have themselves contributed to the Borough’scharacter, which increases the potential for further subterranean development toalter such character (RBKC, 2009).

London Plan and Unitary Development Plan

Until the adoption of supplementary planning guidance, the policy context forsubterranean development in RBKC was provided by the Spatial DevelopmentStrategy for Greater London, or London Plan (Mayor of London, 2008), andRBKC’s Unitary Development Plan (RBKC, 2007). Policies in the London Plan

TABLE 5. Number of planning applications for subterranean developmenta received by RBKC

Year Number of planning applications for subterranean development received by RBKC

2003 642004 892005 852006 1102007 1922008 212

Note: aIncluding new basements and basement extensions, with criteria for selection based on word‘terranean’, ‘swimming pool’, ‘excavation’, ‘underground’, ‘cellar’ or ‘sunken’ appearing in thedescription of the planning application.Source: RBKC (2009, p. 5).


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(which sets out a spatial development strategy for Greater London as a whole) withimplications for subterranean development include those that aim to mitigateclimate change, protect archaeological remains and geodiversity, and minimizeflood risk. For instance, Policy 4A.1 indicates that development should make thefullest contribution to the mitigation of and adaptation to climate change, andminimize emissions of CO2, and Policy 4A.3 states that development shouldminimize energy use by adopting sustainable design and construction methods.However, the Plan does not contain specific policies in relation to subterraneandevelopment per se.RBKC’s Unitary Development Plan 2002 (a development plan at the local level

for the area covered by RBKC), revised in 2007, contains policy CD24, which isof relevance to subterranean development proposals since it ‘seeks to resistdevelopment in, on, over or under garden squares, in order to protect their specialcharacter; and promote proposals for their enhancement’ (RBKC, 2007, p. 62).However, only one Unitary Development Plan policy refers specifically tosubterranean development, namely policy CD32, which states that the Boroughseeks:

to resist subterranean developments where: (a) the amenity of adjoiningproperties would be adversely affected; or (b) there would be a materialloss of open space; or (c) the structural stability of adjoining or adjacentlisted buildings or unlisted buildings within conservation areas might beput at risk; or (d) a satisfactory scheme of landscaping includingadequate soil depth has not been provided; or (e) there would be a lossof trees of townscape or amenity value; or (f) there would be a loss ofimportant archaeological remains. (RBKC, 2007, p. 68)

This policy was introduced in response to increased numbers of subterraneandevelopment proposals, as part of Unitary Development Plan amendments addedin 2007. However, this brought into the concern of planning the requirement forconsideration of structural factors in terms of the stability of existing buildings.Since planners were not qualified to comment on this, the Leader of the Council(the local councillor [elected representative] who heads the largest political grouprepresented on the Council) required them to request each application forsubterranean development to be accompanied by a structural engineer’s report,which in turn had to be assessed by a qualified consultant engineer (interview,development control planner, RBKC, March 2010).

Subterranean Development Supplementary Planning Document

Prior to 2009, policy in the Unitary Development Plan led to a broad acceptance ofsubterranean residential development proposals, and between 2001 and 2007 only7% of such applications were refused (Arup, 2008). Partly in response to thegrowth in such applications, as indicated above, the Borough commissionedconsultants Ove Arup to consider the real or perceived impacts of subterraneandevelopment, including ‘any development underground, either under existingstructures or under gardens, and in particular large subterranean rooms and spaces;


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new basements and basement extensions’ (RBKC, 2009, p. 6). A subsequentSubterranean Development Scoping Study (Arup, 2008) considered different typesof subterranean development, related construction methods, and associated issuesand impacts, including for instance possible damage to existing structures. Thiswas used as the basis for a ‘mini-consultation’ with interested parties and localpeople in 2008, which, together with the Scoping Study, was used as the basis ofthe Borough’s Subterranean Development Supplementary Planning Document,adopted in May 2009 (RBKC, 2010a).The Supplementary Planning Document supplements Unitary Development

Plan Policy CD32, set out above, and sets out detailed planning guidance forsubterranean development, primarily related to residential use, for instance inrelation to the historic environment; comfort and safety; flooding; environmentalsustainability; structural stability; reducing the impacts of noise, nuisance andtransportation; design; trees and landscaping; and land contamination. It istherefore a significant material planning consideration (namely a considerationrelevant to planning) in determining applications for planning permission, andforms part of the Local Development Framework (a set of documents relevantto spatial planning in the area). It also includes a checklist of requirementsfor other information to be submitted with planning applications for subterraneandevelopment so that compliance with Policy CD32 can be demonstrated(interview, development control planner, RBKC, March 2010).In general terms, the Supplementary Planning Document is in favour of

subterranean residential development in view of its capacity to increase the size ofexisting dwellings without having the same impact on the character andappearance of the Borough as above-ground extensions. Nevertheless, it indicatesthat subterranean extensions under listed buildings should be resisted in view ofthe impact these could have on the hierarchy of, and relationship between, historicfloor levels (RBKC, 2009). Indeed, this policy had been informally in place priorto adoption in the Supplementary Planning Document, and it had proved then tobe effective and supported in appeal decisions. Part of the reason for the concernover historic floor levels and the hierarchy of rooms in historic buildings arisesfrom the number of Georgian and Victorian buildings in RBKC with originalbasements, with the implication that to add basement extensions could havesignificant implications for character as well as stability (interview, developmentcontrol planner, RBKC, March 2010).In relation to sustainability, the Supplementary Planning Document acknowl-

edges that subterranean development can have a negative environmental impact.Hence it states that:

Given the nature of subterranean development and the complexity ofcalculating and assessing CO2 emissions and savings, as a proxy theCouncil will take a pragmatic approach and will therefore require thatthe entire dwelling following the subterranean development meets Level4 of the Code for Sustainable Homes (CfSH) or BREEAM [BREEnvironmental Assessment Method, a widely-used method for assessingthe environmental performance of buildings] ‘Excellent’ rating for non-residential developments. In most circumstances this will secure a


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substantial carbon saving, while not penalising the owners of propertiesthat already have a low carbon footprint. (RBKC, 2009, p. 16)

Essentially, the requirement for the whole dwelling to achieve Level 4 (rather thanjust the subterranean development) is linked to the fact that the dwelling afterdevelopment will be larger, requiring more energy, and so energy efficiencyupgrading of the whole dwelling is needed to offset this. While the enforceabilityof this policy in some cases may be questioned (interview, development controlplanner, RBKC, March 2010), the Supplementary Planning Document indicatesthat exceptions can be made for basement extensions to listed buildings or inexceptional circumstances within conservation areas, and in such cases applicantsare simply required to demonstrate why CfSH Level 4 cannot be achieved. Sinceover 70% of the Borough is covered by conservation areas, there is substantialscope for such exceptions (interview, development control planner, RBKC, March2010).The Supplementary Planning Document addresses the issue of possible damage

to existing structures from subterranean development, though it altered theapproach of the authority to this issue. Essentially, as indicated above, after PolicyCD32 was introduced in 2007, RBKC required each application for subterraneandevelopment to be assessed by independent chartered engineers. However, OveArup concluded in their Scoping Study that this was unnecessary and costly, witheach assessment costing £500 (interview, policy planner, RBKC, February 2010).Consequently, the Supplementary Planning Document in 2009 replaced thisprocess with a requirement for submission of a construction method statement,prepared by a Chartered Civil Engineer or Chartered Structural Engineer, inconjunction with a planning application or application for Listed Building Consentfor subterranean development (interview, development control planner, RBKC,March 2010). Such a statement must:

provide specific details of the excavation, temporary works andconstruction techniques, including details of the potential impact ofthe subterranean development on the existing and neighbouringstructures, based on the specific site characteristics, including the typeof geology and hydrology found in the area. (RBKC, 2009, p. 17)

This is intended to ensure that subterranean development is appropriatelydesigned, taking into account all contextual circumstances.The Supplementary Planning Document acknowledges the potential for

subterranean development to harm the special architectural or historic interest orcharacter of listed buildings, arising for instance from the building’s location,hierarchy of rooms, and original purpose. It therefore specifies that subterraneandevelopment will not be allowed directly below listed buildings. This policy hasbeen tested on appeal, with planning inspectors usually upholding the Borough’sview (interview, policy planner, RBKC, February 2010), and the number ofapplications for subterranean development beneath listed buildings substantiallyreduced after this policy was applied (interview, development control planner,RBKC, March 2010). Concern for the potential cumulative impacts (impacts


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resulting from incremental changes brought about by a series of actions) ofsubterranean development had arisen for instance from the observation that manyearly basement extensions below gardens had been followed by the paving-over ofthe garden, with potentially serious consequences for gardenscapes and drainage(interview, development control planner, RBKC, March 2010). This led to thepolicy in the Supplementary Planning Document of not allowing subterraneandevelopment under more than 85% of a garden (interview, policy planner, RBKC,February 2010). A similar concern for cumulative visual impact led to thestatement in the Supplementary Planning Document that, in conservation areas,the development of light wells (unroofed external spaces within the volume of abuilding, to allow light to penetrate relatively dark areas) visible from the street, orlarge light wells in rear gardens, would be discouraged.The Supplementary Planning Document acknowledges the need to reduce the

impacts of nuisance arising from ‘noise; vibration; dust; odours; impact onpedestrian movement; skips; moving spoil and construction materials andsuspension of parking bays’ (RBKC, 2009, p. 22). Consequently, it indicatesthat as well as supplying appropriate information in relation to duties under theControl of Pollution legislation, RBKC undertake, where appropriate, to requirethe contractor to be a member of the Considerate Constructors’ Scheme, a nationalinitiative set up by the construction industry. It also indicates that, whereappropriate, the authority requires the submission of a Construction TrafficManagement Plan, showing how the routing of construction vehicles and otherfactors can be managed so as to minimize the impact of works.The Supplementary Planning Document also covers policy in relation to

archaeology (requiring pre-application consultation with the Greater LondonArchaeological Advisory Service where appropriate); use, comfort and safety(requiring for instance natural light and ventilation for sleeping accommodation);flooding (requiring for instance applications in certain flood risk zones to beaccompanied by a Flood Risk Assessment); trees and landscaping (requiring forinstance no mature trees be harmed); and land contamination (requiring forinstance affected sites to be investigated and if necessary remediated) (RBKC,2009). In addition, it sets out guidance for neighbours of owners or occupiers whoare considering construction of a subterranean development, together withinformation on relevant legislation such as the Party Wall Act 1996, the Controlof Pollution Act 1974 (which makes provisions for instance for the collection anddisposal of waste, as well as other aspects of pollution), and the Highways Act1980 (which makes provisions for the management and operation of England’sroad network).While it is premature to judge the effectiveness of the Supplementary Planning

Document, it would seem that it has the capacity, for instance, to protect theintegrity of listed buildings (interview, policy planner, RBKC, February 2010).Moreover, the Ladbroke Association (2009)—a non-profit neighbourhoodconservation society—underlines the importance of the Document in addressingthe impact of noise, vibration and dust, potential structural damage, and traffic.Nevertheless, there remain some concerns in relation to the SupplementaryPlanning Document. For instance, there is still confusion over what issues fallwithin the remit of planning and what fall within that of Building Regulations,


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since RBKC continue to receive many objections to applications for subterraneandevelopment from neighbours concerned about structural stability (interview,policy planner, RBKC, February 2010). In addition, as indicated above, it mayprove difficult to upgrade listed buildings or those in conservation areas to CfSHLevel 4 without infringing other planning conditions, so the requirement mayprove unworkable for older properties (interview, development control planner,RBKC, March 2010). While the Supplementary Planning Document takes this intoaccount by allowing exceptions in such circumstances, the prevalence of suchbuildings in RBKC significantly limits the value of the requirement for suchupgrading.

Experience of Other Boroughs

In order to assess the experience of planners in other boroughs included in theinitial research, interviews were held with a development control planner fromthe London Borough of Merton (February 2010), and a development controlplanner from the London Borough of Wandsworth (February 2010), both ofwhom were familiar with applications for subterranean development. Theyacknowledged a recent increase since 2001 in such development, and associatedplanning applications; moreover, in the London Borough of Merton, there hadbeen a specific increase in development involving the creation of undergroundparking accessed via a car lift in the front garden (interview, development controlplanner, London Borough of Merton, February 2010). Partly as a consequence ofthis general increase, the London Borough of Wandsworth’s Unitary Develop-ment Plan contains a policy in relation to subterranean development, and theBorough also applies supplementary guidance in relation to such development(interview, development control planner, London Borough of Wandsworth,February 2010).However, the London Borough of Merton’s Unitary Development Plan does

not include policy in relation to subterranean development and so each suchapplication is treated on its own merits. This is in spite of the increasingevidence of such development in the Borough, particularly within new-buildschemes, and evidence of structural damage such as wall failures due tosubterranean development, although structural engineers’ reports had beenrequired in some cases (interview, development control planner, LondonBorough of Merton, February 2010). In relation to effects on listed buildings,practice in Merton was to consult English Heritage and internal experts onindividual schemes, rather than to have a set policy such as refusingapplications under listed buildings, as in RBKC (interview, developmentcontrol planner, London Borough of Merton, February 2010), and a similarapproach was applied in Wandsworth (interview, development control planner,London Borough of Wandsworth, February 2010). However, the ‘offsetting’policy applied by RBKC, via the requirement for CfSH Level 4, wasacknowledged by planners from Merton and Wandsworth as having potentialfor application elsewhere (interview, London Borough of Merton, February2010; interview, development control planner, London Borough of Wandsworth,February 2010).


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Experience of Developers/Private-sector Agents

A group of private-sector representatives were also interviewed in order to assesstheir experience of subterranean development. These comprised the following(employer anonymized): a representative of Developer A (providing subterraneanresidential schemes) (February 2010); a representative of Agent B (providing arange of real-estate services) (March 2010); a representative of Agent C (charteredsurveyors) (March 2010); and a representative of Agent D (chartered surveyors)(March 2010). All interviewees had experience of subterranean development inLondon, particularly on the western side of the city, with one having suchexperience also in Dublin.In relation to demand for subterranean development, three interviewees

acknowledged a significant increase in recent years (interview, representative ofDeveloper A, February 2010; interview, representative of Agent B, March 2010;interview, representative of Agent D, March 2010). The developer representativesuggested that the crucial factor influencing such demand was the cost of propertyper unit area, since this might imply that it was cheaper to extend a dwelling via abasement than to move to a larger property (interview, representative of DeveloperA, February 2010). Moreover, one interviewee suggested the most commonmotivation for such development was to enhance property value (interview,representative of Agent B, March 2010). This was supported by anotherinterviewee who suggested that creating a basement cost around £300/ft2 (27.9m2), while properties in RBKC (for instance) were worth around £1, 000 – 2, 000/ft2 (93 – 186 m2), so development would seem likely to be profitable in terms ofadded value (interview, representative of Agent D, March 2010).Several interviewees also indicated a significant disparity in the approach of

different local authorities to subterranean development. Hence the developerrepresentative confirmed that RBKC required significantly more work than otherauthorities from developers in connection with subterranean development, arisingparticularly from its requirement for a structural engineer’s design, which alsoincreased overall fee levels, although planning permission was generally easy toobtain (interview, representative of Developer A, February 2010). Moreover,another interviewee highlighted major inconsistencies between authorities in termsof approach, and a particular difference was noted between the approach ofRBKC, where for instance subterranean development was resisted under listedbuildings, and the City of Westminster, which was seen to be much morewelcoming of such development (interview, representative of Agent B, March2010). Nevertheless, one interviewee noted a fairly uniform approach amongstplanning authorities, as long as relevant conditions were complied with (interview,representative of Agent C, March 2010); and another noted a general consistencyin practice, with relative ease of obtaining permission, in spite of frequentobjections from neighbours (interview, representative of Agent D, March 2010).The developer representative suggested that wider application of policy and

guidance with respect to subterranean development could be helpful in enablingrequirements to be taken on board at the design stage, as illustrated by theapproach taken by RBKC, which showed how basements could be designed andconstructed so as not to affect the water table, for instance. This interviewee added


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that RBKC’s approach via the Supplementary Planning Document seemed toenhance internal consistency in its response to applications, as compared, forinstance, with the London Borough of Camden, where such consistency waslacking (interview, representative of Developer A, February 2010). Anotherinterviewee agreed that lack of consistency could be a problem both within andbetween planning authorities, and that guidance at national level might assist inthis respect (interview, representative of Agent B, March 2010). Significantly,another interviewee suggested that the confusing nature of legislation in relation topermitted development was exacerbating problems of inconsistency, sincelegislation was interpreted differently by different authorities (interview,representative of Agent D, March 2010).All of the interviewees indicated that they were aware of the implications of

subterranean development for sustainability, and that they sought to comply withrelevant guidelines for instance in relation to trees and planting, testing ofsoils for contamination and conveyance to landfills, insulation, and assessmentof environmental impacts. However, while RBKC’s requirements for CfSHLevel 4 in certain circumstances was supported by one interviewee (interview,representative of Agent C, March 2010), two others expressed doubts as to thefeasibility of this requirement, particularly with its cost implications (interview,representative of Agent B, March 2010; interview, representative of Agent D,March 2010).

Analysis—Implications for Planning Policy

The research questions that provided the framework for the case study outlinedabove were as follows: to what extent is there a need for wider application ofpolicy for subterranean residential development?, and to what extent does theapplication of such policy in RBKC have more general applicability? To assist inanswering these questions, the main points arising from analysis of the researchresults, and consideration of implications for planning policy, are set out inTable 6.Essentially, in relation to the first question, it seems clear from the research

results that the increasing demands for subterranean residential development,combined with the parallel need for policy application to ensure sustainabledevelopment and a low-carbon future, imply the desirability of wider applicationof policy for subterranean residential development. The research has indicated thatsuch policy application has the potential not only to address relevant issues ofamenity, but also to optimize environmental sustainability. In relation to the latterin particular, the increasing focus of national planning policy on promotingsustainability and a low-carbon future implies the need to more widely highlightand address the planning implications of an emerging development type such assubterranean development, particularly since this seems likely to become moresignificant in the future, in RBKC as well as many other contexts. Consequently, itmay be asserted that the failure to develop the wider application of policy forsubterranean residential development presents a missed opportunity to achieve themaximum benefits (or minimum disbenefits) in terms of quality of life and long-term sustainable development.


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In addition, the research outlined above highlighted a further rationale for widerapplication of policy, in the form of problems of (unjustified) inconsistency ofapproach to subterranean development between planning authorities that can leadto confusion, unpredictability and sub-optimal outcomes. As indicated above, suchinconsistency was noted between RBKC and some other London boroughs, andthis would seem likely to increase in the future, following the application ofRBKC’s Supplementary Planning Document, with implications also for increasedvariation between authorities in the cost of development. In this context, thedevelopers/agents interviewed viewed the possibility of wider use of more specificpolicy or guidance positively.In relation to the second question, it seems clear from the research results that

RBKC has progressed further than any other UK planning authority with respectto policy for subterranean residential development, and there would seem to beconfidence in the authority that the Supplementary Planning Document iscomprehensive in its coverage of the relevant issues, with policy appropriatelydeveloped and applied in view of local needs and circumstances (interview, policyplanner, RBKC, February 2010; interview, development control planner, RBKC,March 2010), although some doubts exist as to the usefulness of the requirementfor CfSH Level 4. There is also an indication from other planning authorities (aswell as from developers and agents involved in subterranean development) thatsuch policy may be potentially helpful in other contexts, either where policy inrelation to subterranean residential development does not exist or where it needsfurther detail. Certainly, similar policy has been developed by the LondonBorough of Wandsworth, indicating that policy can usefully be applied in other

TABLE 6. Summary of analysis—implications for planning policy

Existing problems/issues Research issues highlighted Recommendations

Increasing demands forsubterranean residentialdevelopment

RBKC SupplementaryPlanning Document (andrelated policy) appears topresent good practice in thelocal context, with netbenefits

Additional national/metropolitan/local policyon subterranean residentialdevelopment to supplementexisting targets forsustainability of new homes

Increasing need forapplication of policy for asustainable, low-carbonfuture

While RBKC policy iscontext-specific, it containssome generic lessons forpotential applicability inother contexts

Such additional policy couldapply aspects of RBKCpolicy to optimizeoutcomes for sustainabilitymore widely

Opportunity to ensuresubterranean residentialdevelopment addressesamenity issues andoptimizes outcomes forsustainability

Additional research on theexperience of otherauthorities, monitoring ofthe experience of RBKC,and on the construction andwhole-life energy costs ofdifferent subterraneandevelopment types, toinform policy


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contexts. While in the case of Lambeth the relevant policy is less detailed, thismay arguably be justified by the lower level of relevant applications, the lowerpopulation density and the lower sensitivity of the historic environment.However, to fully address the second question requires further consideration of

whether the policy progressed in RBKC can be seen as good practice for potentialtransfer to other contexts. In other words, is the policy mainly creditable in so faras it is related and applicable to the locality, or are there generic elements thatcould be usefully transferred elsewhere (potentially to any context where there isdemand for subterranean residential development)? On one hand, the very need forpolicy in relation to subterranean residential development in RBKC is a reflectionin part of the relatively high number of applications for subterranean developmentin the Borough, which is in turn related to the wealthy overall composition of thearea, as outlined above. In addition, the nature of such policy, as applied forinstance in the Supplementary Planning Document, clearly reflects the othercharacteristics of the Borough such as the prevalence of conservation areas andlisted buildings, and the high population density, with evident implications for theeffects of subterranean development. It is therefore clear that, while the policy asapplied is appropriate in the local context, (at least some) aspects of the policiesapplied in RBKC may not be suitable in other contexts where different conditionsapply. For instance, while the high number of applications in itself would seem toprovide a justification for policy, such high numbers (and therefore such ajustification) are not evident in many other contexts. Moreover, the prevalence ofbuildings of historic interest in RBKC clearly necessitates an emphasis in localpolicy on protection of historic heritage, but this is not evident in many othercontexts, possibly including some where there is significant demand forsubterranean residential development.On the other hand, there are arguably many generic issues addressed in RBKC’s

policy and guidance (including substantive Unitary Development Plan PoliciesCD24 and CD32, as well as the Supplementary Planning Document) that couldusefully be applied elsewhere, possibly in any context, including in relation toissues such as sustainability, drainage, planting and loss of amenity duringconstruction. While it may be argued that the need to address these issues in policyis more urgent in the context of RBKC (partly in relation to demand and partly inrelation to the nature and sensitivity of the area for instance), it may equally beargued that such issues (and policy) can in many respects be regarded as of genericinterest and applicability (although of course the weighting given to specificaspects in other contexts may differ substantially). If this is accepted, it leads to thesupplemental question of: what is the most desirable form of such policy?It may be argued from the above that a clear implication in terms of the most

desirable form of such policy is the possibility of identification of some policycomponents of relevance wherever subterranean residential development isproposed (if not within a national context, at least within a regional ormetropolitan context). Hence relevant national (or regional/metropolitan) policyor guidance in relation to subterranean development could therefore perhapssupplement existing targets for increasing the sustainability of new homes. Forinstance, this could require all subterranean residential extensions to be of thesame CfSH standard as for above-ground developments (Level 6 by 2016), with


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the parent building requiring to be upgraded accordingly so that the wholebuilding (including the extension) would be more energy efficient as a result of thedevelopment. If such policy was at the national level, it could be paralleled bypolicy at metropolitan (London-wide) level (reflecting London-specific concernsand conditions), and it could of course be accompanied by more detailed localpolicy (reflecting detailed local concerns and conditions). This reflects to a degreethe views of developers/agents interviewed, with some suggestion for nationalpolicy, combined with recognition of the potential need for metropolitan and/orlocal policy.In addition, however, the research highlighted some areas where there was a

need for further research and evidence to inform the form of wider policyapplication. For instance, given the limited geographical focus of this research,further investigation could explore the experience of other authorities in Londonand elsewhere, in relation to subterranean residential development. Moreover, theresearch showed that there was a significant difference between the views ofRBKC planners and those from other boroughs on the relationship of planningpolicy to structural stability in relation to subterranean development. Essentially,RBKC planners suggested that structural stability was an issue with whichplanning policy could and should be concerned, while those from other boroughsindicated that structural stability was primarily a matter for Building Regulations.This may suggest the need for further monitoring of the impacts of RBKC’s policybefore related policy is applied in other contexts.Furthermore, in the longer term, it may be argued that the planning system

could seek to take account of the impact of subterranean development by relatingthis more precisely to the estimated lifespan of the development. Hence the long-term sustainability benefits of such development could be traded-off against theinitial negative impacts. However, it would first seem necessary for furtherquantitative research to be carried out in order to assess the typical constructionand whole-life energy costs of different subterranean development designs andtypes, so as to allow the calculation of the number of lifetime years required toensure that the development had a net positive effect in terms of sustainability.

Conclusions

It is clear that demand for subterranean development is growing in several UKplanning authorities, linked to high property prices, lack of space to extend aboveground, and planning restrictions. Moreover, such development has significantimplications (positive and negative) for issues such as sustainability, amenity andenvironmental quality, in the context of ever-increasing requirements for policythat adequately points the way to a low-carbon future. As a response to suchissues, detailed policy and guidance has been applied in RBKC, which addressesrelevant areas of planning concern as well as related issues of structural stability,and which would seem to have resulted in net benefits in the context ofsubterranean residential development. However, on a wider basis, there isevidence of confusion and inconsistency between (and in some cases, within)planning authorities in their response to applications for subterranean residentialdevelopment.


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Consequently, the potential exists for substantial gains from the broaderapplication of policy for subterranean development. The following suggestionsmay therefore be made for such application. First, at national level, and/or possiblyat metropolitan level (via for instance the London Plan), policy could set out cleargeneric requirements for subterranean residential developments, building on theexperience of policy such as RBKC’s Supplementary Planning Document. Forinstance, it could specify the requirement for all subterranean residentialdevelopment to meet the same CfSH level as for above-ground residentialdevelopment, with the parent building requiring to be upgraded accordingly.Second, at metropolitan and/or local level, there could be more widespreadapplication of such policy that is more sensitive to locality-specific needs andcircumstances, building upon any national policy. Again, the case of RBKC can beconstructive in this context in informing good practice for potential transfer. Third,in order to appropriately inform such possibilities, further research could beundertaken to investigate the experience of a wider range of boroughs with respectto subterranean development, as well as to monitor the effects of relevant policy inRBKC, and to investigate the potential longer-term effects of such development inrelation to energy efficiency in terms of the construction and whole-life energycosts of different subterranean development designs and types. Such conclusions,it may be argued, could also be relevant to non-UK contexts, particularly in viewof the global relevance of subterranean development, and the increasing prioritiesfor sustainable development for instance at the European level.

Acknowledgements

The contribution of three anonymous referees to the development of the finalversion of this article is gratefully acknowledged.

References

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