Flood Risk & Drainage Requirements for Kensington and Chelsea

Kensington and Chelsea is a unitary London borough, so the Royal Borough of Kensington and Chelsea (RBKC) is both the local planning authority that decides your application and the Lead Local Flood Authority (LLFA) that must be satisfied on surface water drainage. Since RBKC adopted its new Local Plan on 24 July 2024, flood risk and sustainable drainage sit in two linked policies, Policy GB11 and Policy GB12, and both now reach further into ordinary development than the 2019 rules they replaced. A flood risk assessment, a drainage strategy or both are typically needed for major schemes, sites in a Critical Drainage Area, anything in Flood Zone 2 or 3, and, under the current Local Plan, every basement development in the borough regardless of flood zone. We prepare flood risk assessments and drainage strategies for planning, property purchase, mortgage and insurance, and BREEAM across Kensington and Chelsea, alongside sequential and exception test reports and surface-water drainage strategies.

Basement conversions are one of the most common forms of development in this borough, and under Policy GB11 they now carry a flood-risk obligation that catches almost every one of them: a site-specific flood risk assessment is required for all basement development, in any flood zone, and no sleeping accommodation is permitted below ground floor level anywhere in Flood Zone 3.

• Decision-maker: the Royal Borough of Kensington and Chelsea acts as both the local planning authority and the Lead Local Flood Authority for the borough.
• Adopted Local Plan: the New Local Plan, adopted 24 July 2024, superseding the 2019 Local Plan Partial Review.
• Flagship policies: Policy GB11 (Flood Risk), Policy GB12 (Sustainable Drainage), Policy GB13 (Water and Wastewater Infrastructure) and Policy CD11 (Basements).
• SFRA: the Strategic Flood Risk Assessment, updated October 2022 by JBA Consulting, underpins the current policies.
• Drainage standard: development must aim for greenfield run-off rates, following the London Plan's SuDS hierarchy, with the London Sustainable Drainage Proforma required for major applications.
• Local twist: four designated Critical Drainage Areas — Kensington, Holland Park, Sloane Square and North Kensington — trigger a flood risk assessment even in nominal Flood Zone 1.

Who decides flood risk and drainage in Kensington and Chelsea

RBKC does not split planning and drainage responsibility between two councils the way a district and county authority would. As a single unitary body, it determines your planning application and, wearing its LLFA hat, is also the statutory consultee for surface water drainage on major development — schemes of 10 or more homes or 1,000m² or more of commercial floorspace. The Environment Agency remains the responsible authority for the tidal River Thames, which forms the borough's southern boundary and sits within the EA's Thames river basin district, and Thames Water is the statutory sewerage undertaker for the borough's combined and surface water sewers. No Internal Drainage Board covers any part of Kensington and Chelsea. For an applicant this means one planning contact and one drainage consultee within the same council, which simplifies pre-application discussions compared with the two-tier split found elsewhere, provided both roles are addressed in the same submission.

The council's Local Flood Risk Management Strategy 2024–2030 sets out how RBKC coordinates this dual role with the Environment Agency and Thames Water, following the lessons of the borough's most significant recent flood event. We explain who is responsible for managing flood risk in England and how the Flood and Water Management Act 2010 created the LLFA role in more detail, and cover the routes for challenging or resolving an LLFA objection to a planning application if drainage evidence is queried at consultation.

Does a Kensington and Chelsea application need a flood risk assessment or drainage strategy?

Whether you need a flood risk assessment, a drainage strategy, or both, depends on the site and the scale of the works, not on which document sounds more familiar. A flood risk assessment asks whether the site is safe from flooding and whether the development would increase risk elsewhere; a drainage strategy asks where the surface water goes and at what rate. Under RBKC's local validation requirements, a flood risk assessment is required for any development of one hectare or more in Flood Zone 1, all development in Flood Zones 2 and 3, and any development in a Critical Drainage Area — and, since the 2024 Local Plan, for all basement development anywhere in the borough. A drainage strategy is required for major development and, in the council's own words, for "applications with drainage implications," a deliberately wide net that catches many smaller schemes too.

Getting the trigger wrong is one of the most common reasons a Kensington and Chelsea application stalls before it is even registered. We've set out the wider pattern in why planning applications get invalidated for flood risk and drainage, and in when you need a flood risk assessment more generally — both worth reading alongside RBKC's own local list, because the local twists (the Critical Drainage Areas and the basement rule) sit on top of, not instead of, the national thresholds.

Kensington and Chelsea's Local Plan flood and drainage policies

Policy GB11 and Policy GB12, adopted 24 July 2024 in RBKC's New Local Plan, are the two policies that now govern flood risk and sustainable drainage across the borough, replacing the single Policy CE2 that covered both subjects under the 2019 Local Plan Partial Review. They sit within the plan's "Green-Blue Future" chapter alongside Policy GB13 on water and wastewater infrastructure, and are cross-referenced by Policy CD11 on basements. Every one of these is a genuine, current obligation on a Kensington and Chelsea application, not background reading.

Policy GB11 — Flood Risk

Policy GB11 requires that highly vulnerable development, including self-contained basement dwellings, is not permitted in Flood Zone 3 or in a Critical Drainage Area, and that no sleeping accommodation is placed below ground floor level anywhere in Flood Zone 3. Crucially, it extends the trigger for a site-specific flood risk assessment to all basement development in the borough, whatever the flood zone — a significant broadening from the 2019 approach, which focused on Flood Zones 2, 3, Critical Drainage Areas and sites over one hectare. For riverside sites, GB11 also formalises a 16-metre set-back from the Thames or Chelsea Creek flood defence, including tie rods and support structures, to protect the future upgrade of defences under the Thames Estuary 2100 plan.

Policy GB12 — Sustainable Drainage

Policy GB12 requires that "development must contribute towards a reduction in the rate and volume of surface water run-off into the combined sewer network," with proposals expected to achieve greenfield run-off rates and householder schemes reducing run-off proportionately to their scale. Surface water must be managed as close to its source as possible, following the London Plan's drainage hierarchy, with green or blue roofs expected on flat roofs and impermeable surfacing in gardens and landscaped areas resisted. Financial contributions towards off-site SuDS are only accepted where an on-site greenfield-rate reduction is clearly not feasible — on-site attenuation has to be considered first.

Policy GB13 — Water and Wastewater Infrastructure

Policy GB13 addresses the borough's wider water and wastewater capacity, requiring an integrated approach to water management, a 110 litre per person per day water efficiency standard, and — on Opportunity Area sites — an Integrated Water Management Strategy. It also references the BREEAM Wat 01 credit, relevant to flood risk assessments prepared for BREEAM assessments in the borough.

Policy CD11 — Basements

Policy CD11 is RBKC's dedicated basements policy, covering structural, heritage, amenity and garden-coverage impacts, and it explicitly states that "a specific policy requirement for basements is also contained in Policy GB11: Flood Risk." In practice this means a basement application in Kensington and Chelsea needs its flood risk evidence addressed alongside, not folded into, the structural and heritage case — two different tests answered by two connected pieces of evidence.

The borough's Strategic Flood Risk Assessment

RBKC's evidence base for these policies is its Strategic Flood Risk Assessment, updated in October 2022 by JBA Consulting to support the New Local Plan. The SFRA confirms that no part of the borough falls within the functional floodplain, Flood Zone 3b — a direct result of the standard of protection provided by the Thames defences — and maps Flood Zone 3 concentrated in the south around Chelsea, with a small area of Flood Zone 2 in the west around South Kensington. It also carries the borough's most detailed groundwater mapping, showing high risk of groundwater emerging at the surface around Kensington Olympia, Earls Court, Gloucester Road, Brompton and South Kensington, alongside a strip of elevated risk to underground assets running roughly 400 metres north of the Thames. This detail matters at application stage because a compliant flood risk assessment for a Kensington and Chelsea site needs to engage with the SFRA's local classification, not just the national Flood Map for Planning — we explain what a Strategic Flood Risk Assessment is and how it feeds a site-specific report in more detail.

Kensington and Chelsea's local validation list and required documents

RBKC's local validation requirements (last consolidated January 2022) confirm the flood risk assessment triggers set out above, and add the borough's drainage documentation expectations. Major applications must be accompanied by a drainage strategy and the London Sustainable Drainage Proforma, the standard four-part summary — site information, proposed discharge arrangement, drainage strategy figures and supporting cross-references — that every one of London's 33 Lead Local Flood Authorities has required since April 2019. Kensington and Chelsea's own version of the proforma is published directly by the Greater London Authority. Basement applications instead trigger a Construction Method Statement and Construction Traffic Management Plan, with groundwater considerations addressed through the Basements Supplementary Planning Document rather than a stand-alone groundwater assessment document.

One point worth flagging directly: some of RBKC's own web pages have not yet caught up with the July 2024 Local Plan renumbering, and still refer to the superseded Policy CE2 rather than the current GB11 and GB12. It is a reminder that a compliant submission needs to be checked against the adopted plan itself, not just whichever council web page comes up first in a search. We explain the wider pattern of missing or mismatched validation documents and what a SuDS proforma needs to contain, and our own page on the London Sustainable Drainage Proforma lists the current proforma template for every London borough, Kensington and Chelsea included.

Kensington and Chelsea's SuDS and drainage requirements

Drainage is not a secondary concern behind flood risk in Kensington and Chelsea; Policy GB12 makes it a requirement of its own, and in a dense, heavily paved, largely Victorian-sewered borough it is often the harder half of a submission to satisfy. The starting expectation is a greenfield run-off rate, achieved by following the London Plan's SuDS hierarchy in order: rainwater used as a resource first, then infiltration at or close to source, then attenuation within green infrastructure, then a controlled discharge to a watercourse, then a surface water sewer, with discharge to a combined sewer as the last resort. Green or blue roofs are expected on all flat roofs, including extensions, and impermeable surfacing in gardens and landscaped areas will not be permitted as part of a development. Where a basement is built under one metre of soil, that covering only counts as "permeable" if it is grass, planting or a genuinely permeable surface — an easy detail to miss on a compact site.

Because much of the borough sits on London Clay, infiltration is often less straightforward than the SuDS hierarchy would prefer, and an infiltration test using the BRE 365 method is usually the first practical step before a discharge route is agreed — our geology and ground conditions checker gives an early read on the likely ground conditions for a specific site. We prepare sustainable drainage strategies and preliminary drainage designs for Kensington and Chelsea sites at every stage, and our drainage calculators help scope the likely attenuation volume before a full strategy is commissioned. Where a drainage condition needs discharging after permission is granted, our discharge of drainage planning conditions service covers the detailed design stage. For the wider national picture, our guides to the four pillars of SuDS, the 2025 National Standards for Sustainable Drainage Systems and foul, surface and combined sewers set Kensington and Chelsea's local rules in their national context.

How national planning policy applies in Kensington and Chelsea

Kensington and Chelsea's local policies sit on top of the same national framework every English authority applies. The National Planning Policy Framework steers development towards the lowest-risk land through the sequential and, where needed, exception test, and expects sustainable drainage on major development. Since the Planning Practice Guidance update in September 2025, the risk-based approach and sequential test apply explicitly to all sources of flooding, including surface water and groundwater — a change that matters in a borough where those two sources, not the Thames, do most of the damage. We explain how the NPPF treats flood risk, what the sequential and exception tests actually test, and how the September 2025 PPG update changed the sequential test's scope, in more depth. Our NPPF flood risk guidance service applies this national framework directly to a Kensington and Chelsea site alongside the borough's own policies.

Flood risk in Kensington and Chelsea's area

Kensington and Chelsea's flood risk is dominated by surface water and groundwater, not the Thames. The tidal river along the borough's southern edge, at Chelsea Embankment and Cheyne Walk, is defended to a 1-in-1000-year standard by the Thames Barrier and the wider Thames Estuary 2100 defences, and the Environment Agency has recorded no fluvial or tidal flooding within the borough since those defences were built. The real pressure comes from surface water flooding, widespread across the borough's dense, heavily paved streets, and from groundwater flooding, which has a long history around Gloucester Road station and in Notting Hill and South Kensington. The clearest evidence is the flooding of 12 July 2021, when 76 streets and more than 500 properties flooded and Holland Park, Notting Hill Gate and Sloane Square Underground stations all closed — significant enough that RBKC opened a formal Section 19 flood investigation, and it now shapes the council's Local Flood Risk Management Strategy 2024–2030. A 2024 High Court case even turned partly on flood-related concealment during the sale of a Notting Hill basement conversion, a reminder that flood and drainage evidence follows a property well beyond the planning stage. For context on how the Thames defences themselves are managed, see our explainer on the Thames Barrier, and on the borough-wide picture, the 2025–30 London Surface Water Strategy and the Mayor of London's basement flash-flood warning, both directly relevant to a borough with as many basements as Kensington and Chelsea.

How Unda helps in Kensington and Chelsea

We have extensive experience of flood risk and drainage planning policy across London's boroughs, including the particular way Kensington and Chelsea links its basement policy directly to flood risk evidence. That means we know to build the flood risk assessment and drainage strategy around what RBKC's planning officers are actually checking for — the Critical Drainage Area maps, the SFRA's groundwater classification, the London Sustainable Drainage Proforma, and the 16-metre Thames set-back where it applies — rather than a generic national template that misses the borough's own rules. Where a council web page has not yet caught up with the 2024 Local Plan, we work from the adopted policy itself, so your evidence is never built on a superseded requirement.

Flood risk and drainage services for every situation in Kensington and Chelsea's area

Planning applications are one reason to need this evidence, but far from the only one. We prepare flood risk assessments for property purchase, where a buyer or their lender wants a site-specific view beyond a standard conveyancing search; for insurance, where cover has been restricted or a broker needs supporting evidence; and for BREEAM assessments, which apply a different, credit-based test to the same underlying risk. Alongside these, we prepare sequential and exception test reports, dedicated groundwater flood risk assessments, and flood modelling where national mapping isn't detailed enough for a Kensington and Chelsea site. A flood risk assessment, a sequential or exception test and a drainage strategy answer different questions — is the site safe, should the development be here, and where does the surface water go — and a Kensington and Chelsea submission often needs more than one of them addressed together. Our Flood Risk Map and Live River Levels Map are free starting points for checking a site's exposure before commissioning a full report, and where a Thames-adjacent or Flood Zone 3 site needs floodable ground floor or void design, our guide to flood return periods and annual probability and to residual flood risk explains what "1-in-1000-year protection" actually means for a site behind the Thames Barrier.