Flood Risk & Drainage Requirements for Westminster | Unda

Westminster City Council applies City Plan 2019-2040 (2026), adopted on 21 January 2026, and its flood risk policy is now Policy 39, with basements under Policy 50. Policy 39 asks for a site-specific flood risk assessment on every site in a Surface Water Flood Management Zone, even in Flood Zone 1, and the council judges the drainage itself as Lead Local Flood Authority.

That zone trigger reaches well beyond the river: a mews house extension or a new office floor well away from the Thames in the City of Westminster can still need an assessment because of where surface water collects in a storm. This page sets out what Westminster's policies ask for on flood risk and drainage, which documents prove it, and how we meet each one. To talk through a site, call 01293 214444 or email enquiries@unda.co.uk. Reports for applications to the City of Westminster come from Unda Consulting Limited, a CIWEM Business Partner.

Who decides flood risk and drainage on an application to Westminster City Council?

Westminster is one council doing two jobs: it decides the application and, as Lead Local Flood Authority, it is also the body that checks the drainage. There is no county tier and no internal drainage board in the city; the Westminster SFRA lists its partners as the Environment Agency, Thames Water, the Greater London Authority, the Port of London Authority and the Canal & River Trust.

Because one council holds both roles, a drainage objection is not a side issue to be tidied up after the decision. It sits on the same file. Our explainer on what a Lead Local Flood Authority does covers the role in general.

Westminster's flood and drainage policies in the City Plan

Two City Plan policies carry the flood and drainage tests, with London Plan Policy SI 13 behind them. The Partial Review adoption statement confirms that the 2026 plan replaced the 2021 version on 21 January 2026, which renumbered both policies.

Requirements checked November 2026.

Policy 39 (Flood risk): what Westminster asks of every application

Policy 39 requires development in the City of Westminster to be safe from flooding for its lifetime and to manage surface water with SuDS, and it names three triggers for a site-specific flood risk assessment. Part A reads: *"All developments should be safe for their lifetime from the risk of flooding, complying with the council's Strategic Flood Risk Assessment (SFRA), Surface Water Management Plan (SWMP), Local Flood Risk Management Strategy (LFRMS) and the Mayor of London's Regional Flood Risk Appraisal (RFRA)."* Part B asks for an assessment on *"1. Developments of 1 hectare or greater; 2. All developments in Flood Zones 2 and 3; and 3. All developments within a Surface Water Flood Management Zone."* (City Plan 2026, page 160).

The rest of the policy is about use and location. Highly vulnerable uses are not allowed in Flood Zone 3; more vulnerable uses and essential infrastructure there must pass the Exception Test; and *"Within the Rapid Inundation Zone, basement dwellings and basement extensions to existing dwellings will not be acceptable."* Part F expects a Flood Warning and Evacuation Plan for more vulnerable uses in the area at risk of a tidal breach. Part G seeks a 16 m undeveloped buffer around flood defences, and part H stops development limiting the future raising of defences under the Thames Estuary 2100 Plan.

On drainage, part J says: *"New development must incorporate Sustainable Drainage Systems (SuDS) to alleviate and manage surface water flood risk. Development should aim to achieve greenfield run-off rates and demonstrate how all opportunities to minimise site run-off have been taken."* The supporting text (39.3) adds that proposals *"should be accompanied by a signed Flood Risk and Sustainable Drainage pro forma"*, and 39.4 rules out sleeping accommodation below the modelled breach flood level.

Officers check three things the national flood map alone does not answer: the site against the Surface Water Flood Management Zones, the breach depth and time to inundation for anything in Flood Zone 3, and floor levels shown against those depths on the drawings. Our assessment takes the zone, breach and surface water data from the SFRA, sets the floor levels and escape route against them, and states in plain terms which Policy 39 part each finding answers, so the case officer can tick it off.

Policy 50 (Basement developments): the flood tests below ground

Policy 50 governs every new basement and basement extension in Westminster, and its first clause ties the design to the flood evidence: basements should *"Incorporate measures recommended in the structural statement or flood risk assessment to safeguard structural stability, and address surface water and sewerage flooding"* (City Plan 2026, page 203). The size limits sit in part B: no more than 50% of the garden, one storey below the lowest original floor, and at least one metre of soil plus a 200 mm drainage layer above the roof of the basement.

The supporting text sets the paperwork. A structural methodology statement, with self-certification from a suitably qualified engineer, goes with every basement application (50.2). Where the basement sits in Flood Zone 2 or 3, or in an area of surface water flood risk, *"A site-specific Flood Risk Assessment (FRA) and a structural statement will be required"*, and the measures include *"positive pumped devices or equivalent to address sewerage flooding"* (50.3). Permitted development rights for basements were removed across the whole city by the Basement Development (City-wide) Article 4 Direction on 31 July 2016, so every basement needs planning permission.

The SFRA's Appendix B goes further for groundwater. It asks the assessment to look at basements that cut through the gravels into the London Clay, those within 300 mm of the upper aquifer, and those near the historic courses of the Westbourne and Tyburn. Our basement assessment answers each of those points with the site's geology, and sets out the non-return valve or pumped device, the internal stair to a refuge above the design flood level, and the resilience measures. Our note on basement and cellar conversions in a flood zone explains the national vulnerability rules behind them.

London Plan Policy SI 13 (Sustainable drainage) in Westminster

Policy SI 13 is the regional drainage policy every Westminster drainage strategy is measured against, and the council's own SFRA cites it by name. It says *"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"*, through a hierarchy that runs from rainwater use, infiltration and green attenuation down to *"controlled rainwater discharge to a combined sewer"* (London Plan 2021, Policy SI 13). Westminster's public drainage is a combined sewer system owned by Thames Water (SFRA 3.4.1), so a strategy that ends at that last step has to show why each step above it does not fit the site.

The plan position: renumbered policies and a new City Plan in early preparation

Policy 39 replaced Policy 35 (Flood risk) of the 2021 City Plan on 21 January 2026, and Policy 50 replaced Policy 45 (Basement developments); the number 35 now belongs to the waterways policy. The council says the 2021 plan *"should therefore not be used when determining planning applications in Westminster"*. The wording of the flood policy barely moved, but the 2026 text swaps the old Surface Water Flood Risk Hotspots for the Surface Water Flood Management Zones drawn from the 2023 SFRA modelling.

Westminster is preparing a new City Plan, covering the period to 2041. The council gave notice of its intention to commence plan preparation in July 2026 under the Town and Country Planning (Local Planning) (England) Regulations 2026, and its scoping consultation runs from 1 August to 11 December 2026. The council's timetable (V.1, July 2026) puts Gateway 1 on 25 March 2027, examination in public from March to September 2029 and adoption in October 2029. No draft policies have been published, so Policies 39 and 50 of the City Plan 2019-2040 (2026) remain the flood and drainage tests. The Partial Review that produced the 2026 plan was submitted on 28 November 2024, examined in hearings from 29 April to 1 May 2025 and found sound in the Inspector's report of 13 November 2025. Alongside the new City Plan, the draft new London Plan was out for consultation from 16 July to 15 October 2026, with its examination in public due in 2027, and the London Plan 2021 stays in force until it replaces it.

When does a Westminster application need each report?

Policy 39, Policy 50, the flooding and planning application requirements page and the validation list between them decide what goes in with the application. A flood risk assessment, a sequential or exception test and a drainage strategy answer different questions, and in Westminster each has its own triggers.

Westminster's own guidance sets out how it treats the sequential test. The Environment SPD (February 2026) accepts that in a city this dense *"it is impractical to employ the Sequential Test to distinguish between potential development sites"*, so proposals in Flood Zones 2 and 3 are generally treated as sequentially acceptable, subject to the vulnerability table and the Exception Test. The SPD was written before the August 2026 NPPF, so on a scheme where the test is in doubt we check the position against Chapter 18 policy F5 and say so in the report. Our page on testing alternative sites explains the two tests.

Drainage strategy requirements from Westminster as Lead Local Flood Authority

Westminster publishes no separate SuDS design guide; its drainage standards sit in Policy 39J, the SFRA's Appendix B and chapter 5 of the Environment SPD. Major applications *"must be accompanied by a sustainable drainage strategy"*, with the pro forma submitted at validation, according to the council's requirements page (updated 2 June 2026). The council's sustainable drainage page (updated 23 June 2026) adds that SuDS are a material planning consideration on all major applications, and that Westminster has been consulted on them as Lead Local Flood Authority since 15 April 2015.

• Run-off rate. Aim for the greenfield rate; where a tight urban plot cannot reach it, show how every opportunity has been taken to get as close as possible (Policy 39J and 39.10).
• Hierarchy. Work down the London Plan SI 13 hierarchy; Appendix B names rainwater harvesting and recycling, pervious surfaces and green or blue roofs as the SuDS to consider first in Westminster.
• Four pillars. Appendix B expects water quantity, water quality, biodiversity and amenity to be addressed, not storage alone.
• Impermeable area. Reduce it where possible, use pervious surfaces, and where infiltration is not feasible put storage beneath them.
• Surface water modelling. Use the council's own modelling (SFRA Figures A-14 to A-16) for the site alongside the national surface water map.
• Maintenance. The strategy sets out how each SuDS feature is managed for the lifetime of the development.
• Paperwork. The London Sustainable Drainage Proforma for majors and the SFRA Appendix C drainage checklist.

Climate change is built into the evidence: the SFRA's surface water modelling tested a 40% increase in peak rainfall intensity (SFRA 2023, 3.3.15), and every strategy applies the Environment Agency's climate change allowances for the London management catchment. Our drainage calculators give a first estimate of greenfield rates and storage, and our overview of the London drainage proforma explains how the form is completed.

Where permission is granted with a drainage condition, the detail still has to be approved before work starts. We prepare the evidence for clearing the drainage condition against the same Policy 39J tests, so the submission matches what the strategy promised.

Westminster's SFRA and validation list

The Westminster Strategic Flood Risk Assessment was prepared by WSP and issued in November 2023, replacing the 2019 SFRA. It is the evidence behind Policy 39 and the base our assessments start from. It maps what the national flood map does not: Surface Water Flood Management Zones from modelling calibrated using the Thames Water Beckton Sewer model and validated against the 12 July 2021 flood event, tidal breach extents from the Environment Agency's Thames Tidal Upriver Breach Inundation Assessment (May 2017), and Rapid Inundation Zones of 0 to 30 and 31 to 60 minutes. The SFRA puts 17% of Westminster in Flood Zone 3, including Pimlico, Millbank, Victoria and Whitehall. Our explainer on what an SFRA is covers how councils use them.

Westminster's Cabinet Member for Growth and Planning adopted a revised Local List of Planning Application Validation Requirements on 21 July 2026. Item L23 asks for a flood risk assessment on all development of 1 hectare or more, all basement development, all development in Flood Zones 2 and 3, and development in a Surface Water Flood Risk Management Zone where there may be flood risk implications. On non-major schemes, sustainable drainage goes in a separate section of that assessment, and more vulnerable uses in the tidal breach area also need a Flood Warning and Evacuation Plan. Item N9 asks for a Sustainable Drainage (SuDS) Design Strategy on all major development, following paragraph 39.10 and the London Plan drainage hierarchy, with the London Sustainable Drainage Proforma appended. The council's flooding requirements page still links the earlier checklist, agreed in October 2021 and updated in 2024. That 2024 checklist cites Policies 35 and 45, the numbers the policies carried before they were replaced in January 2026. The SFRA also uses the superseded 2021 numbers (Policy 45 at 5.5.1, and Policy 35J in Appendix B), and the July 2026 list cites Policy 39 but still names "Policy 45 Basements", a superseded number. A report written today should cite Policies 39 and 50. The council's requirements page offers three FRA checklists (major, non-major and small scale), and the non-major list includes new homes from one to nine, commercial floorspace over 100 m² and all basement development. A missing or thin assessment is one of the commonest reasons an application is invalidated.

National policy in Westminster: Chapter 18 of the NPPF

Westminster's policies sit on top of the National Planning Policy Framework published on 17 August 2026, whose flood risk policies are in Chapter 18. Policy F4 sets when a site-specific flood risk assessment is needed, F5 the sequential test, F6 the exception test and incompatible uses, F7 the safety of occupants, and F8 sustainable drainage on proposals that could affect drainage. The Planning Practice Guidance on flood risk and coastal change was last updated on 17 September 2025. Several Westminster documents, the Environment SPD among them, were written before the August 2026 Framework and cite the older one, so we reference Chapter 18 directly. Our guide to the NPPF and our summary of the August 2026 changes explain what changed.

Flood risk across the City of Westminster

Westminster's 4.7 km of tidal frontage on the River Thames is defended by the partly listed Embankment wall and the Thames Barrier to a 1 in 1000 year standard, so the tidal risk that matters is a breach, not overtopping. Flood Zone 3 is mapped as though those defences were absent, which is why the SFRA describes tidal and fluvial risk as low and still asks every assessment there to deal with breach depths. Riverside defences west of the Barrier, including Westminster's, will need raising by 2050 under TE2100, according to the Environment SPD.

Surface water and sewers are the everyday risk. The storms of 12 and 25 July 2021 affected about 250 properties in Westminster, most of them on 12 July, when rain overwhelmed the surface water and combined sewer systems (SFRA 3.3.8). Five lost rivers, the Westbourne, Tyburn, Tyburn Brook, Kilbourne and Long Ditch, are now culverted, and the Westbourne still surfaces at the Serpentine and the lakes in St James's Park and Buckingham Palace Gardens. Groundwater risk is rated low, with a chalk aquifer beneath gravels and clay, but it governs deeper basements. Check a site on our flood risk map, and see our London-wide guide for the wider picture and the 2025-30 London surface water strategy.

Flood risk and drainage services for applications to Westminster City Council

Each service below answers a requirement in Westminster's own policies or evidence. A flood risk assessment asks whether the site is safe, a sequential test asks whether it should be there, and a drainage strategy asks where the water goes; a Westminster scheme in Flood Zone 3 with a basement can need all three.

• Flood risk assessments. Policy 39B makes Flood Zones 2 and 3 and the surface water zones automatic triggers, and our flood risk assessment service covers every source the SFRA maps.
• Planning applications. Each flood risk assessment for planning is set out against Policy 39 part by part, with the breach depths and the surface water zone data.
• Buying property. Pimlico, Millbank and the riverside sit in Flood Zone 3 behind the defences, and a flood risk survey for property purchase explains the breach and basement position to a buyer.
• Sequential and Exception Tests. More vulnerable uses in Flood Zone 3 must pass the Exception Test under Policy 39D, which our sequential and exception test reports set out.
• Drainage. Policy 39J puts SuDS on all new development, and our drainage strategy work follows the SI 13 hierarchy to the outfall.
• Surface water. Major schemes need a Sustainable Drainage Strategy with the London proforma, which a surface water drainage strategy provides.
• Conditions. Drainage conditions on a Westminster permission are cleared through our discharge of drainage conditions service.
• Groundwater. Appendix B asks basements near the Westbourne and Tyburn or into the London Clay to assess groundwater, covered by a groundwater flood risk assessment.
• Evacuation plans. Policy 39F generally expects a flood warning and evacuation plan for more vulnerable uses in the tidal breach area.
• Commercial property. Victoria and Whitehall are commercial and government land in Flood Zone 3 (SFRA 1.2.7), and commercial due diligence sets out the breach risk for a purchase or a loan.
• Insurance. About 250 Westminster properties were affected in July 2021, and an insurance flood risk assessment gives underwriters the site evidence.
• National policy. Westminster's documents still cite the pre-August 2026 Framework, and our NPPF guidance page sets out Chapter 18.

Guides to other councils are listed on our local authority pages.

Frequently asked questions