29 fernshaw road sustainable drainage systems (suds

29 Fernshaw Road

Sustainable Drainage Systems (SuDS) Report for Planning

For

Mrs G Taylor and Mrs K Howes 29 Fernshaw Road

London SW10

16 February 2015

Revision 3

6515

6515 29 Fernshaw Rd SuDS Report for Planning

16 February 2015 Rev. 3

Executive Summary This report has been prepared by Ferguson Brown Sustainable Engineering to support the planning application for a subterranean extension to the existing property at 29 Fernshaw Road, Royal Borough of Kensington and Chelsea, London, SW10. The existing property is a detached, two-storey building with a mansard roof extension. It is currently used as office space. An appeal was allowed and planning permission granted on 14 March 2014 for the change of use of the building from B1 business use to Class C3 residential use (Application Ref: PP/12/04671 and appeal ref: APP/K5600/A/13/2202066). The proposed development includes a single-storey subterranean extension, rear ground level extension, and refurbishment of the whole building into a three bed residential property. The proposed development does not lead to an increase in impermeable surfaces and surface water run-off, as determined by measurements summarised in sections 2.2 and 3.2 of this report. The inclusion of at least 1 metre of topsoil over the subterranean development under the front courtyard, and the use of porous paving proposed for the front courtyard and side passageways creates an increase in permeable area of 67.56 sqm, reducing the surface water run-off rate from 3.3 l/s to 1.76 l/s, a decrease of 1.54 l/s It is also noted that the change of use of the site from an office building to a dwelling increases the foul water discharge from 1.1 l/s to 1.97 l/s, an increase of 0.87 l/s. To further minimise the consumption of potable water in sanitary applications, and subsequent foul water discharge rate, the use of low water use fittings will be incorporated at detailed design stage to achieve a calculated water consumption of 107 to < 118litres/ person /day. This is in line with the BREEAM Domestic Refurbishment Excellent rating Minimum Standard requirements for issue Wat 01 Internal Water Use. It is therefore concluded, in accordance with the Royal Borough of Kensington and Chelsea (RBKC) planning policy outlined in section 1.1 of this report, that there is not a need to install any additional SuDS solution as part of the proposed development.



Contents

1.0 Introduction 1

1.1 Royal Borough of Kensington and Chelsea Planning Policy 1

2.0 Existing Site 2

2.1 Existing Layout 3

2.2 Existing Permeable and Impermeable Areas (sqm) 4

2.3 Existing Surface Water Run-off 4

2.4 Existing Foul Water Discharge 4

3.0 Proposed Development 5

3.1 Proposed Layout 6

3.2 Proposed Permeable and Impermeable Areas (sqm) 7

3.3 Proposed Surface Water Run-off 7

3.4 Proposed Foul Water Discharge 7

3.5 Minimised Consumption of Potable Water 8

4.0 RBKC Small Scale SuDS Tool 9

5.0 Conclusion 10



This report has been checked in accordance with Ferguson Brown Sustainable Engineering’s internal Quality Assurance procedures.

Issue Rev Prepared by Checked by Description Issued 1 2 Clara Palmer Damian Poyzer SuDS Report for Planning 16/02/2015


16 February 2015 Rev. 3 1

1.0 Introduction This report has been prepared by Ferguson Brown Sustainable Engineering to support the planning application for a subterranean extension to the existing property at 29 Fernshaw Road Chelsea, London SW10 0TG. It describes how the proposed development addresses the Royal Borough of Kensington and Chelsea (RBKC) planning policy regarding Sustainable Drainage Systems (SuDS).

1.1 Royal Borough of Kensington and Chelsea Planning Policy

1.1.1 Core Strategy Policy CL7 Basements

Policy CL7: Basements was adopted in a Full Council meeting on 21 January 2015. Criteria i and j of this policy are considered applicable to this report, they state the Council will require all basement development to: “ i. include a sustainable drainage system (SuDS), to be retained thereafter; j. include a minimum of one metre of soil above any part of the basement beneath a garden; ”

1.1.2 Core Strategy Policy CE2 Flooding

The Council will require development to adapt to fluvial flooding and mitigate the effects of, and adapt to, surface water and sewer flooding. To deliver this the Council will: e. require sustainable urban drainage (SUDs), or other measures, to reduce both the volume and the speed of water run-off to the drainage system ensuring that surface water run-off is managed as close to its source as possible in line with the hierarchy in the London Plan. In particular, major development must make a significant reduction in the current volume and speed of water run-off to the drainage system; f. resist impermeable surfaces in front gardens; Policy CE 2 of the Core Strategy requires surface water run-off to be managed as close to its source as possible. A minimum of one metre of suitably drained permeable soil above any part of a basement within a garden provides for both reducing the amount and speed of water runoff to the drainage system and the long term future of shrub and other garden planting. Care should be taken that the original garden level is maintained and the 1m of permeable soil is connected to the unaffected part of the garden. Other SuDS measures may also be required.

1.1.3 SuDS for Small Developments

For the implementation of the above policy for small developments (up to a maximum of 10 dwellings or 1,000 sqm of non-residential property) RBKC have developed an on-line SuDS assessment tool, and also state the following exemptions.

If the development will not lead to an increase in impermeable surfaces and surface water run-

off, then SuDS are not required.

Basements: the SuDS Tool only applies to basement developments when: the basement extends beyond the above ground structure into the garden and there is less than 1m of permeable ground cover (soil) above the basement, or when the ground above the basement is altered from permeable to impermeable.



2.0 Existing Site The property is a two storey detached building with a mansard roof extension. It is currently in use as an office. The property is within the Sloane Stanley Conservation Area, the building and those adjacent are not listed. An appeal was allowed and planning permission granted on 14 March 2014 for the change of use of the building from B1 business use to Class C3 residential use (Application Ref: PP/12/04671 and appeal ref: APP/K5600/A/13/2202066). Existing site drawings and photos confirm the external areas of the site consist of a front courtyard paved with concrete sets, and rear and side access paths covered by concrete slabs and ornamental stone. All these areas are considered to be impermeable hard landscaping. All roof areas are considered to be impermeable hard landscape, covering the building footprint of 65.4 sqm. The 29 Fernshaw Road Structural Engineering Planning Report, 1445/01/001, PHI Design confirms the site to be fully paved and impermeable (clause 3.3) and that it is within a Flood Risk Zone 1 area (clause 2.1). Section 2.1 shows the existing layout of external areas (with impermeable areas shaded grey), with the sqm of the permeable and impermeable areas given in section 2.2.



2.1 Existing Layout

Impermeable

Impermeable



2.2 Existing Permeable and Impermeable Areas (sqm)

Area Description Permeable Area (sqm) Impermeable Area (sqm) Front Courtyard (Edith Terrace side) 0 51.24 North-West facing pathway 0 14.96 North-East facing side pathway 0 19.42 Building Footprint 0 65.40

Total 0 151.02

All measurements are taken from drawings 13008–110 Garden Coverage Plan – Basement, Goater Jones LLP, 21.01.2015, and 13008–112 Plans Proposed – Ground, Goater Jones LLP, 27.01.2015.

2.3 Existing Surface Water Run-off In accordance with the modified rational method, the existing run-off from the site is calculated from the formula: Qp = 2.78 x Ap x i Where: Qp = peak discharge (l/s); Ap = impermeable area (ha); i = rainfall intensity (mm/hr) Existing impermeable area = 0.015 ha Average rainfall intensity = 79.2 mm/hr. Qp = 2.78 x Ap x i Qp = 2.78 x 0.015 x 79.2 = 3.3 l/s Existing surface water run-off = 3.3 litres / sec

2.4 Existing Foul Water Discharge In accordance with BS EN 12056-2:2000 Gravity drainage systems inside buildings Part 2: Sanitary pipe work, layout and calculation, the discharge from domestic sanitary appliances of the existing office building is calculated from the formula: Qww = K √∑DU Where: Qww = Waste water flow rate (l/s); K = Frequency factor; ∑DU = Sum of discharge units K = intermittent use, e.g. in dwelling, guesthouse, office = 0.5

Appliance Quantity DU (l/s) Total DU (l/s) WC with 6 litre cistern 2 1.8 3.6 Wash basin 2 0.3 0.6 Kitchen sink 1 0.6 0.6

Total 4.8 Qww = 0.5 √4.8 = 1.1 l/s Existing foul water discharge = 1.1 litres / sec



3.0 Proposed Development The proposed development works will involve the construction of a one storey subterranean extension under the front courtyard area, and rear and side passageways. The extension will extend underneath 50% of the existing front courtyard area and 43% of the side passageways. The extension under the front courtyard will retain at least 1 metre of topsoil over the subterranean development below. However, the extension under the rear and side passageways will form an impermeable surface at ground level. The ground floor of the building is also being extended at the rear, covering a portion of the North West passage. The use of porous paving is proposed for the front courtyard and side passageways. It is considered that this will create a permeable area over the entire front courtyard area and the portion of the proposed side passages that do not have the subterranean extension beneath. Section 3.1 shows the existing layout of external areas, with the sqm of the permeable and impermeable areas given in section 3.2.



3.1 Proposed Layout



3.2 Proposed Permeable and Impermeable Areas (sqm)

Area Description Permeable Area (sqm) Impermeable Area (sqm) Front Courtyard (Edith Terrace side) 51.24 0 North-West and North- East facing passage 16.32 12.31 Building Footprint 0 71.15 Total 67.56 83.46

All measurements are taken from drawings 13008–110 Garden Coverage Plan – Basement, Goater Jones LLP, 21.01.2015, and 13008–112 Plans Proposed – Ground, Goater Jones LLP, 27.01.2015.

3.3 Proposed Surface Water Run-off In accordance with the modified rational method, the existing run-off from the site is calculated form the formula: Qp = 2.78 x Ap x i where Qp = peak discharge (l/s) Ap = impermeable area (ha) i = rainfall intensity (mm/hr) Proposed impermeable area = 0.008 ha Average rainfall intensity = 79.2 mm/hr. Qp = 2.78 x Ap x i Qp = 2.78 x 0.008 x 79.2 = 1.76 l/s Proposed surface water run-off = 1.76 litres / sec

3.4 Proposed Foul Water Discharge In accordance with BS EN 12056-2:2000 Gravity drainage systems inside buildings Part 2: Sanitary pipe work, layout and calculation, the discharge from domestic sanitary appliances of the proposed residential building is calculated from the formula: Qww = K √∑DU Where: Qww = Waste water flow rate (l/s); K = Frequency factor; ∑DU = Sum of discharge units K = intermittent use, e.g. in dwelling, guesthouse, office = 0.5

Appliance Quantity DU (l/s) Total DU (l/s) WC with 4 litre cistern 5 1.8 9 Wash basin 6 0.3 1.8 Shower 3 0.4 1.2 Bath 2 0.6 1.2 Kitchen sink 2 0.6 1.2 Washing machine 1 0.6 0.6 Dish washer 1 0.6 0.6

Total 15.6 Qww = 0.5 √15.6 = 1.97 l/s Proposed foul water discharge = 1.97 litres / sec



3.5 Minimised Consumption of Potable Water To further minimise the consumption of potable water in sanitary applications, and subsequent foul water discharge rate, the use of low water use fittings will be incorporated at detailed design stage to achieve a calculated water consumption of 107 to < 118litres/ person /day. This is in line with the BREEAM Domestic Refurbishment Excellent rating Minimum Standard requirements for issue Wat 01 Internal Water Use.



4.0 RBKC Small Scale SuDS Tool The proposed development does not lead to an increase in impermeable surfaces and surface water run-off, as determined by measurements summarised in sections 2.2 and 3.2 of this report. The online SuDS Assessment Tool has not been completed as there is no impermeable increase figure to input. The screenshot below of the Tool demonstrates this.



5.0 Conclusion The proposed development does not lead to an increase in impermeable surfaces and surface water run-off, as determined by measurements summarised in sections 2.2 and 3.2 of this report. The inclusion of at least 1 metre of topsoil over the subterranean development under the front courtyard, and the use of porous paving proposed for the front courtyard and side passageways creates an increase in permeable area of 67.56 sqm, reducing the surface water run-off rate from 3.3 l/s to 1.76 l/s, a decrease of 1.54 l/s. It is therefore concluded, in accordance with the Royal Borough of Kensington and Chelsea (RBKC) planning policy outlined in section 1.1 of this report, that there is not a need to install any additional SuDS solution as part of the proposed development. The online SuDS Assessment Tool has not been completed as there is no impermeable increase figure to input. It is also noted that the change of use of the site from an office building to a dwelling increases the foul water discharge from 1.1 l/s to 1.97 l/s, an increase of 0.87 l/s. To further minimise the consumption of potable water in sanitary applications, and subsequent foul water discharge rate, the use of low water use fittings will be incorporated at detailed design stage to achieve a calculated water consumption of 107 to < 118litres/ person /day. This is in line with the BREEAM Domestic Refurbishment Excellent rating Minimum Standard requirements for issue Wat 01 Internal Water Use.

29 fernshaw road sustainable drainage systems (suds

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