Flood Risk Assessment in the City of Westminster

If you are preparing a planning application in the City of Westminster, two questions decide how much flood risk and drainage work you need to do before you submit: does your site need a flood risk assessment, and does it need a sustainable drainage strategy? Westminster answers both itself. The council is the local planning authority that determines your application, and it is also the Lead Local Flood Authority that judges your surface water drainage. That combination, together with the tidal Thames, a dense network of buried rivers and one of the most closely policed basement regimes in the country, makes Westminster one of the more demanding authorities in England to satisfy on flood risk and drainage.

Our flood risk consultants prepare the flood risk assessments and drainage strategies that Westminster planning officers and the Environment Agency accept. To talk through a specific site, call 01293 214444, email enquiries@unda.co.uk, or start a quote and one of our experienced flood risk consultants will get back to you within 60 minutes.

Westminster asks for a site-specific flood risk assessment on far more schemes than most authorities: every development in Flood Zones 2 and 3 that alters a building's footprint or levels needs one, and so does every basement, wherever it sits.

Who decides flood risk and drainage for a Westminster application?

Three bodies shape what your application must show, and Westminster City Council performs two of the three roles itself.

• Westminster City Council as local planning authority. The council determines your application against its adopted City Plan 2019–2040, the London Plan and national policy. Its flood and drainage requirements are set by Policy 39 of the City Plan.
• Westminster City Council as Lead Local Flood Authority (LLFA). Every London borough is its own LLFA, so Westminster — not a separate county authority — reviews your surface water drainage and is the statutory consultee on major applications. You are dealing with one organisation wearing two hats. Our explainer on what a Lead Local Flood Authority does sets out that role.
• The Environment Agency and Thames Water. The Environment Agency advises on flood risk from the tidal Thames and its defences; Thames Water owns the sewers that much of Westminster's surface water ultimately reaches. Westminster's own guidance stresses that consultation with both is essential to obtain up-to-date flood risk information.

Because the council is both decision-maker and drainage authority, a weak drainage strategy is not a separate technical matter that gets tidied up later — it lands on the desk of the same authority deciding your application. A Lead Local Flood Authority objection to your drainage can hold up the whole permission.

Does Westminster require a flood risk assessment?

Westminster requires a site-specific flood risk assessment for a wide range of proposals, and its trigger points are stricter than the national defaults. The council sets them out in Appendix B of its Strategic Flood Risk Assessment, keyed to which flood zone your site sits in.

Alongside the flood zone test, Westminster's validation guidance sets thresholds by application type. A flood risk assessment is expected with non-major applications that include new development of one to nine dwellings, new commercial development over 100 square metres, extensions or alterations that change the footprint by more than 100 square metres, any change of use to a more vulnerable class, and — without exception — all basement development. If you are unsure whether your scheme crosses a threshold, our guide to when a flood risk assessment is required works through the tests, and a missing assessment is one of the most common reasons a planning application is invalidated.

Westminster's drainage and SuDS requirements

Because Westminster is its own Lead Local Flood Authority, its drainage expectations carry real weight and its Appendix B is unusually direct: a drainage strategy is required for all developments, in every flood zone. Surface water is the source that flooded homes across the borough in July 2021, so the council treats drainage as seriously as fluvial and tidal risk.

For major applications the drainage strategy must be accompanied by the London Sustainable Drainage Proforma at the point of validation. The strategy has to demonstrate that surface water runoff will be managed in line with Policy 39 of the City Plan and Policy SI 13 of the London Plan, evidenced against the CIRIA SuDS Manual and the national non-statutory technical standards. Our overview of the London drainage proforma explains how it is completed.

• Follow the drainage hierarchy. Manage runoff as high up the hierarchy as the ground allows. Across most of Westminster, dense development and shallow made ground make infiltration difficult, which is why the council points developers towards rainwater harvesting and recycling, pervious surfaces and green or blue roofs.
• Aim for greenfield runoff. Developers should target the greenfield runoff rate wherever possible; where that is not achievable on a tight urban plot, the strategy must show how every opportunity to reduce runoff has been taken to get it as close to greenfield as it can.
• Deliver all four pillars of SuDS. A scheme has to address water quantity, water quality, biodiversity and amenity — not simply store water out of sight.
• Reduce impermeable area and store what is left. Cut hardstanding where you can, use pervious surfaces, and where infiltration is not feasible underlay them with attenuation storage. Our drainage calculators and BRE 365 infiltration calculator give a defensible first pass at sizing.
• Set out maintenance. The strategy must say how the SuDS features will be managed and maintained so the system keeps working for the lifetime of the development.

Where a drainage condition is attached to your permission, the detailed scheme still has to be approved before you build. We prepare both the strategy and the detailed design to discharge drainage conditions, and a preliminary drainage design at outline stage where it helps shape the layout. For the wider picture, our guide to what drainage information planning permission needs and the 2025–30 London surface water strategy set the London context.

Basement development: Westminster's toughest flood test

Basements are where Westminster's flood and drainage requirements bite hardest, and no other part of the country scrutinises them as closely. The council's own basement policy already limits a basement to a single storey below the lowest original floor and to no more than half the garden area; on top of that, flood risk adds its own rules, and every basement needs a flood risk assessment.

The July 2021 storms showed why. On 12 July a month's rain fell in roughly an hour, sewers surcharged, and floodwater rose into basements across Maida Vale — three feet deep in some homes — affecting around 500 properties. Westminster's requirements are written against exactly that failure mode.

• Basement dwellings are "highly vulnerable". Under national vulnerability rules a self-contained basement dwelling is highly vulnerable and is not permitted in Flood Zone 3. In Flood Zone 3 other basements are restricted to less vulnerable or water-compatible uses.
• Emergency egress is mandatory. Basements must have internal stair access to a floor above the design flood level for use in an emergency. Where a Flood Zone 1 basement is at surface water risk, that safe access must sit at least 300mm above the design flood level, with evidence to support it.
• Fit a one-way valve. Because basements are especially exposed to sewer surcharge, every drainage connection from a basement to a sewer must, as a minimum, have a one-way (non-return) valve to stop drains flooding the basement when they back up.
• Assess groundwater and the buried rivers. A basement that extends into or near the water table, or sits close to the historic courses of the Westbourne or Tyburn, needs a site-specific FRA and a geotechnical assessment. Our groundwater flood risk assessment covers this source that standard maps miss.
• Provide a structural methodology statement. Self-contained basements, and any basement with evidence of surface water, groundwater or sewer flooding, must be supported by a structural methodology statement and self-certification from a suitably qualified engineer.

The Mayor of London has repeatedly warned about basement flash flooding, as our note on the basement flash flood warning records. For an applicant, the practical message is simple: in Westminster a basement is never a small job on the flood risk and drainage side.

Westminster's Local Plan flood and drainage policies

Two policies do most of the work in a Westminster flood risk and drainage submission, backed by the London Plan. Name them in your assessment and show, point by point, how your scheme complies.

What does Policy 39 require?

Policy 39 is Westminster's flood risk policy in the City Plan 2019–2040 adopted in January 2026. It requires development to manage flood risk from every source, to avoid increasing risk elsewhere, to incorporate sustainable drainage, and to remain safe over its planned lifetime with climate change taken into account. In practice, satisfying Policy 39 means submitting a flood risk assessment and drainage strategy that do those things and evidence them against the council's Strategic Flood Risk Assessment.

What does Policy 45 require for basements?

Policy 45 governs basement development. Beyond its size limits — one storey deep, no more than half the garden — it requires that a basement does not increase flood risk to the site or its neighbours. Coupled with the flood rules in Appendix B, that means a flood risk assessment, a structural methodology statement, safe emergency access above the design flood level, and non-return valves on drainage connections.

Westminster's Strategic Flood Risk Assessment

Westminster's Strategic Flood Risk Assessment (SFRA), published in 2023, is the evidence base behind Policy 39 and the reference your site-specific assessment should build on. It maps flood risk from all sources across the borough, including the impact of climate change, and its Appendix B is the practical checklist of what an FRA and drainage strategy must contain here.

• Surface water management zones. The SFRA maps zones derived from Westminster's own surface water modelling. A site inside one triggers a flood risk assessment even in Flood Zone 1, and the modelling should be used to assess surface water at your site.
• Tidal breach and rapid inundation. Because the borough sits behind the Thames tidal defences, the SFRA asks assessments to consider what happens if those defences are breached — the depth and the time to inundation — not just day-to-day risk.
• Estuary Edges by the river. Development next to the Thames is asked to follow the Estuary Edges design principles for softening the riverside edge.

How national flood risk policy applies in Westminster

Westminster's local rules sit on top of national planning policy, and both are assessed together. The National Planning Policy Framework and its Planning Practice Guidance set the Sequential Test, which steers development to lower-risk land, and the Exception Test for higher-risk sites. With a substantial part of Westminster in Flood Zone 3, the Sequential and Exception Tests are a live issue on many central sites.

Since the September 2025 update to the Planning Practice Guidance, the Sequential Test must weigh surface water, groundwater and sewer flooding, not only rivers and the sea — a change that matters in a borough where surface water is the dominant everyday risk. Our summary of the September 2025 PPG update explains it, and every assessment must apply the Environment Agency's climate change allowances over the lifetime of the development.

Flood risk across Westminster: the local picture

Westminster's flood risk is defined by the tidal Thames, a legacy of buried rivers, and intense surface water flooding — and much of the borough sits in a high flood zone on paper precisely because it is defended.

• Tidal Thames and the Thames Barrier. The borough is protected to a high standard by the Thames Barrier and the tidal defences, but Flood Zones are mapped ignoring those defences — so a substantial part of Westminster, roughly 17% including Pimlico and the riverside frontage past Whitehall and Millbank, falls in Flood Zone 3.
• Five hidden rivers. The Westbourne, Tyburn, Tyburn Brook, Kilbourne and Long Ditch were culverted long ago and now run largely within the sewer network; the Serpentine and the lake in St James's Park are where the Westbourne still surfaces. Their historic routes shape groundwater and basement risk.
• Surface water and sewers. The July 2021 flooding — a month's rain in an hour, sewers surcharging into Maida Vale basements — is the risk most Westminster schemes have to design against. The Thames Tideway Tunnel has since eased pressure on the combined sewers, but intense storms still overwhelm local drainage.
• Groundwater in the London Clay. Perched water in the gravels above the London Clay is a real constraint for deeper basements. Our explainer on the sources of flood risk and our flood risk map help you screen a site before you commission.

Flood risk and drainage services for any Westminster scheme

Applicants come to us for a flood risk assessment or drainage strategy in Westminster for many reasons, and we prepare the right report for each. It helps to keep three things separate: a flood risk assessment, a sequential or exception test, and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions — is the site safe, should the development be here, and where does the surface water go.

• Flood risk assessments for planning. Site-specific FRAs for planning applications across every flood zone in the borough.
• Surface water drainage and SuDS. Drainage strategies and detailed design that meet the LLFA and London Plan requirements.
• Sequential and Exception Tests. The policy tests that higher-risk central sites often need.
• Flood modelling. Hydraulic modelling where mapping does not capture a site's tidal or surface water behaviour.
• Property purchase and insurance. Independent assessments for buying a property and for insurance.
• BREEAM. Assessments structured for BREEAM credit achievement.

You can read more about how our flood risk consultants work, browse the flood risk assessment service and our local area guides, or check ground conditions with the geology and soakaway checker and current river levels.

Frequently asked questions