Sustainable drainage systems (SuDS)

National planning policy

The National Planning Policy Framework (NPPF) explains that sustainable development should not increase flood risk elsewhere, and it gives priority to the use of sustainable drainage systems. Paragraph 182 of the NPPF (December 2024) states:

"Applications which could affect drainage on or around the site should incorporate sustainable drainage systems to control flow rates and reduce volumes of runoff, and which are proportionate to the nature and scale of the proposal. These should provide multifunctional benefits wherever possible, through facilitating improvements in water quality and biodiversity, as well as benefits for amenity. Sustainable drainage systems provided as part of proposals for major development should:

a) take account of advice from the Lead Local Flood Authority;

b) have appropriate proposed minimum operational standards; and

c) have maintenance arrangements in place to ensure an acceptable standard of operation for the lifetime of the development."

The council is the lead local flood authority as defined in part a) of this paragraph. Further national guidance on Sustainable Drainage Systems is provided in the Flood Risk and Coastal Change Planning Practice Guidance.

Regional planning policy

Policy SI 13 of the London Plan 2021 relates to the sustainable management of surface water across London through development.

B. Development proposals should aim to achieve greenfield run-off rates and ensure that surface water run-off is managed as close to its source as possible. There should also be a preference for green over grey features, in line with the following drainage hierarchy:

1) rainwater use as a resource (for example rainwater harvesting, blue roofs for irrigation)

2) rainwater infiltration to ground at or close to source

3) rainwater attenuation in green infrastructure features for gradual release (for example green roofs, rain gardens)

4) rainwater discharge direct to a watercourse (unless not appropriate)

5) controlled rainwater discharge to a surface water sewer or drain

6) controlled rainwater discharge to a combined sewer.

C.  Development proposals for impermeable surfacing should normally be resisted unless they can be shown to be unavoidable, including on small surfaces such as front gardens and driveways.

D. Drainage should be designed and implemented in ways that promote multiple benefits including increased water use efficiency, improved water quality, and enhanced biodiversity, urban greening, amenity and recreation

Local planning policy

The Local Plan 2024 includes requirements on the sustainable management of rainwater through development proposals. Policy GB12 Sustainable Drainage sets out the overall expectations for the management of surface water through development proposals.

GB12: Sustainable drainage

A.  Development must contribute towards a reduction in the rate and volume of surface water run-off into the combined sewer network through measures that promote multifunctional benefits.

Run-off rates

B. Development proposals must aim to achieve greenfield run-off rates. Householder applications must reduce the rate of run-off from the site in a way that is proportionate to the scale of development and reflects the site constraints.

C. Only where it can be clearly demonstrated that the on-site reduction in surface water run-off to greenfield rates is not feasible or appropriate, such as where there is limited capacity for SuDS with multifunctional benefits, may financial contributions be provided by the developer towards the delivery of SuDS in the locality.

SuDS design and details

D. Surface water run-off must be managed as close to its source as possible (following the London Plan SuDS hierarchy), through:

• storing rainwater for later use, such as rainwater harvesting for irrigation or domestic water butts
• an increase of permeable or porous surfaces and green infrastructure, including trees and urban hedgerows, to enhance natural drainage
• the implementation of green/blue roofs on all flat roofs including extensions
• including at least one SuDS element that provides other environmental benefitsPrioritising sustainable, natural green SuDS, over engineered options (underground attenuation tanks or oversized pipes)
• supporting water efficiency measures, the reuse of greywater and water harvesting measures to reduce water demand and sewerage flows

E.  SuDS proposals must be adequately designed, built and maintained for the lifetime of the development.

F.  Impermeable surfaces in gardens and landscaped areas will not be permitted as part of a development.

G. The council supports the retrofitting of SuDS in any development (even if the proposed development will not have drainage implications).

The lead local flood authority at the council is a statutory consultee for all major developments in the borough; developments of 10 homes or more and 1,000m2 or more of commercial development.

Householder or small-scale developments: how to meet our policy

Eligible development requiring SuDS include:

• basement development
• lower ground/ground floor rear or side extensions
• outbuildings in the rear garden
• landscaping requiring planning permission
• replacement or creation of hardstanding requiring planning permission

Development currently exempt from SuDS requirements:

• changes of use with no external alterations
• changes to fenestration
• extensions to upper floors, including roof extensions

Our policy could be met through an increase in permeable surfaces, increase in greening and/or rainwater storage to control the amount and speed that surface water run-off leaves the site:

• accessible rainwater downpipes should have a water butt and/or SuDS planter installed. A SuDS planter takes water from the downpipe to irrigate plants, with an overflow back into the downpipe.
• flat or gently sloping roofs created as part of the development should be formed of green roofs in line with Local Plan Policy GB12 (D.4). This includes rear extensions and outbuildings. Green roofs should be specified at planning stage to ensure that appropriate heritage and planning matters are addressed.
• roof drainage should be directed towards a soakaway wherever possible within site constraints.
• any replacement hardstanding must be made permeable or porous (permeable paving, gravel, resin bound gravel), should drain towards a permeable area, or removed altogether and returned to natural grass or planting bed.

It is expected that submissions take a whole site approach to managing water in a sustainable way, with interventions proportionate to the scale of development. The ongoing maintenance of all SuDS elements is important to ensure that the system continues to function.

A drainage statement should be submitted with a planning application for eligible development and, as a minimum, the following must be included:

• an annotated plan showing the existing and proposed area of impermeable (tile roof, paving, concrete etc.) and permeable (grass, planting beds etc.) surfaces
• an annotated drawing/plan showing the proposed location and dimensions of SuDS elements (green roof, permeable paving, water butt, attenuation tank). This should include the connections to the wider drainage network
• a statement demonstrating that the drainage hierarchy has been followed, including any justification for selecting less sustainable interventions
• a statement confirming the maintenance activities required and the responsibility for the ongoing maintenance of all SuDS elements

If this information is submitted at planning application stage this will reduce the need for pre-commencement conditions attached to the planning permission.

For planning applications which include basements, SuDS shall be provided in the same way as above. It should be noted that where 1 metre of soil is provided on top of the basement, this will only be considered permeable if it is covered by grass, vegetation or permeable surfaces.

One+ new residential units or equivalent (all other developments): how to meet our policy

SuDS should be considered from the outset at a very early stage of the process (masterplanning) to ensure they are fully integrated with the topography, landscaping and conditions of the site. We have produced the following SuDS guide based on the design process and aimed at promoting best-practice.

A drainage strategy should be submitted to support the application, and it is recommended that pre-application advice is sought through the Planning Advice Service. Developers should also fill in the SuDS proforma to ensure that all the relevant points have been addressed. The SuDS strategy should show how the development will comply with policy GB12 and aim to achieve greenfield runoff for all events (factoring in other flows into the sewer such as swimming pool discharges, groundwater and other flows).

The drainage strategy should include the following information:

• greenfield, pre- and post-construction run-off rates
• the proposed SuDS types, their location, landscape integration strategy, attenuation capacity, specification, structural integrity, construction, operation, and access. More sustainable green SuDS should be favoured over attenuation tanks
• section/profile drawings of the SuDS if relevant (rainwater harvesting tanks, green roofs, blue roofs, sub-base attenuation, permeable paving, planters, rain gardens etc.)
• drainage plans to show clearly how surface water run-off will be conveyed to the SuDS and any connections to the sewer system if necessary
• details of surface water management during construction
• details of exceedance flow routes
• foul drainage plan, including any necessary pumps
• a detailed maintenance plan confirming proposed adoption of SuDS (ownership), long-term maintenance regime (including maintenance schedule, costs) and communication to residents
• Records of relevant consultations with other interested parties (Thames Water, Canal and River Trust,  Environment Agency where relevant)
• A phasing plan if the scheme is to be delivered in different phases.
• SuDS pro-forma

While some details can be secured once planning permission has been granted through condition, a srainage strategy must be submitted before permission can be granted to set out the overall strategy for managing surface water in sufficient detail.

In line with Local Plan Policy GB13, the council expects an integrated approach to water management within a development will be undertaken, including reducing water demand and considering the reuse of collected rainwater.

A drainage verification condition will be placed on all permissions for major development where SuDS are proposed. This will ensure that sufficient information regarding the constructed SuDS measures and their ongoing maintenance is received following construction.

There may be sites where constraints mean that greenfield runoff rates cannot be achieved on the site, or where the least sustainable measures such as attenuation tanks in isolation would be implemented. In these circumstances there is provision in Local Plan Policy GB12 for a financial contribution to delivery offsite SuDS. Attenuation on site should be prioritised, and details should be presented in the drainage strategy to set out the volume of attenuation that needs to be delivered offsite. The financial contribution will be considered on a case-by-case basis.

Paving front gardens

The council promotes the removal of paving in front gardens. When considering paving your front garden the council encourages making use of permeable paving and a wide variety of materials are available. You will need planning permission to hard pave your front garden if it is more than 5m2 in area. However, you can pave without the need of planning permission if you:

• use porous materials for paving
• make provision to direct water run-off from the hard surface to a permeable or porous area or surface within your property boundary

If you would still like to hard pave your front garden in non-permeable paving, apply for planning permission; Local Plan Policy GB12 (F) will apply. The policy resists impermeable surfaces in gardens and landscaped areas. New applications for dropped kerbs will not be permitted without consideration of the permeability of the area draining to the highway, as set out in the Transport and Streets SPD.

The Planning Portal has further information on paving front gardens.

The government also developed guidance on the permeable surfacing of front gardens.

Further information

• CIRIA (Construction Industry Research and Information Association) SuDS Manual  which gives detailed advice regarding SuDS. Chapter 10, Designing for urban areas is particularly relevant.
• Susdrain: numerous case studies about SuDS.
• Designing Rain Gardens by Urban Design Learning.
• Livingroofs.org: information on green and brown roofs.
• GRO Code: information on green roofs.

Contact us

If you have any questions, contact the Policy Team [email protected].