Flood Risk Assessment in Croydon

Croydon is unusual among London boroughs in one important respect: it is ranked the fourth settlement in England most susceptible to surface water flooding, with up to 33,614 residential properties identified as at risk across the borough. That single fact shapes almost everything Croydon Council asks of a planning application. Every application, whether or not the site sits in a mapped flood zone, is expected to demonstrate it is not adding to that risk.

Developers, architects and planning consultants come to Unda for flood risk and drainage work in Croydon for a range of reasons, each treated with the same care: a flood risk assessment for a planning application, a survey to support a property purchase, evidence for a mortgage lender or insurer, a report structured for BREEAM credits, and a sustainable drainage strategy (SuDS) to satisfy the borough's Lead Local Flood Authority. We prepare each of these as its own report, built around what Croydon Council specifically requires.

Croydon Council is both the planning authority and the Lead Local Flood Authority for the borough, and its Local Plan requires a flood risk assessment on every application — not only those in a flood zone — so getting the drainage evidence right from the outset matters more here than in most councils.

• Who decides. Croydon Council is a London borough and a unitary authority, so it is both the Local Planning Authority determining the application and the Lead Local Flood Authority (LLFA) consulted on surface water drainage — the same council, not a two-tier split.
• Flood risk assessment. Required for every planning application under the council's Local Validation List, and specifically for major development in Flood Zone 1 and all development in Flood Zones 2 and 3 under Policy DM25.
• Drainage strategy. Expected for "Large Minors and above" development, demonstrating sustainable drainage systems that manage runoff as close to source as possible, in line with the London Plan drainage hierarchy.
• Adopted policy base. The Croydon Local Plan (2018) — Policy SP6 and Policy DM25 — supported by the Level 1 and Level 2 Strategic Flood Risk Assessment (AECOM, September 2021).
• Discharge expectation. Croydon's Local Plan requires surface water drainage to achieve better than greenfield runoff rates, not merely match them — a stricter starting point than many authorities set.

Who decides flood risk and drainage for a Croydon planning application?

Because Croydon is a London borough, it holds both roles that are often split between two different councils elsewhere in England. Croydon Council is the Local Planning Authority, which determines the application, and it is also the Lead Local Flood Authority, the statutory consultee for surface water drainage on major development. There is no separate county-level LLFA to satisfy, as there would be in a two-tier area — one council, two roles, and both need to be satisfied by the same submission.

The Environment Agency remains the relevant authority for the borough's main rivers, including the River Wandle, and for any tidal or reservoir risk, while Croydon Council leads on surface water, groundwater and the borough's ordinary watercourses under the Flood and Water Management Act 2010. Applicants dealing with a site near the Wandle, Norbury Brook or Caterham Bourne should expect both bodies to have an interest, for different reasons.

• Local Planning Authority. Croydon Council — determines the application and applies the Local Plan and Local Validation List.
• Lead Local Flood Authority. Croydon Council — statutory consultee on surface water drainage for major applications since 15 April 2015, and available for non-statutory advice on minor development.
• Environment Agency. Leads on main river, tidal and reservoir flood risk, including the River Wandle corridor.
• Thames Water. Sewerage undertaker for the borough — relevant where a drainage strategy proposes discharge to a public sewer as the lowest-preference option in the drainage hierarchy.

Does a Croydon planning application need a flood risk assessment or drainage strategy?

Under Croydon's Local Validation List, every application requires a flood risk assessment, even where the site is not identified as being at risk of flooding, so the submission can demonstrate it is not contributing to flood risk elsewhere. Householder extensions can often satisfy this with a brief statement referencing the online flood risk maps; anything larger needs a proportionate, site-specific report tied to the council's flood risk assessment for planning approach.

A site within one of Croydon's identified Critical Drainage Areas — including Purley Cross, Brighton Road and South and Central Croydon — can trigger the same expectations even where the Environment Agency's Flood Map for Planning shows Flood Zone 1, because the risk being managed is surface water rather than river or tidal flooding. Our Critical Drainage Areas explainer and guide to when an FRA is required cover this trap in more detail, and our planning application invalidation guide sets out the documents that most commonly get an application rejected before it is even registered.

Croydon's SuDS and drainage requirements

Croydon Council, as the borough's LLFA, recommends that a SuDS or drainage strategy report is submitted alongside the planning application itself rather than left to a condition, and it publishes an Advice to Planning Applicants guidance document specifically to help applicants get this right first time. Surface water drainage is treated as a material planning consideration on every major application the council determines.

Policy DM25.3 sets the technical bar: drainage must manage surface run-off as close to its source as possible, follow the London Plan Sustainable Drainage Hierarchy (store for reuse, infiltrate, attenuate in open features, attenuate in tanks, then discharge to watercourse, surface water sewer or combined sewer as a last resort), and — the point that catches out applicants used to other councils — achieve better than greenfield runoff rates, not simply match them. Where that cannot be achieved, the shortfall has to be justified to both the council and the LLFA, with greenfield rates treated as the minimum fallback in line with the London Plan.

• Discharge rate. Better than greenfield runoff rates required under Policy DM25.3; greenfield rates (typically 2 to 8 litres per second per hectare) are the minimum acceptable fallback.
• Drainage hierarchy. Store rainwater for reuse, then infiltrate, then attenuate in open features, then attenuate in tanks, then discharge to a watercourse, surface water sewer or combined sewer, in that order of preference.
• Infiltration. Where infiltration is proposed, testing to BRE Digest 365 is expected, informed by the borough's underlying chalk and clay geology.
• SuDS proforma. Major applications must submit the checklist in Appendix 2 of the council's Advice to Planning Applicants alongside the London Sustainable Drainage Proforma, which every London borough requires via the Greater London Authority.
• Pre-application advice. Bespoke, chargeable LLFA pre-application advice is available directly from Croydon's flood and water team for developers who want the drainage approach tested before submission.

Our SuDS proforma guide and Four Pillars of SuDS explainer cover the quantity, quality, amenity and biodiversity criteria a Croydon scheme needs to evidence, and our LLFA objection guide sets out the most common reasons a drainage strategy gets flagged — usually a discharge route defaulting to sewer without first ruling out infiltration, or a missing climate change allowance. We prepare surface water drainage strategies and, where a scheme has already secured permission subject to a drainage condition, the detailed design needed to discharge it. For outline applications, a preliminary drainage design flags infiltration and runoff issues before they affect site layout.

Croydon's Local Plan flood and drainage policies

The Croydon Local Plan (2018) carries two policies that govern almost every flood risk and drainage decision the council makes: Policy SP6, the strategic policy on environment and climate change, and Policy DM25, the detailed policy on sustainable drainage systems and reducing flood risk, alongside London Plan (2021) Policies SI 12 and SI 13. Both remain the adopted policy base while the council's Local Plan Review proceeds through examination.

Policy SP6 — flood risk, urban blue corridors and water management

Policy SP6.4 requires the council, acting as the Lead Local Flood Authority, to work with the Environment Agency, water and highways infrastructure providers and developers to reduce flood risk and protect groundwater and aquifers across the borough. It requires the Sequential and Exception Tests to be applied where Policy DM25 calls for them, a site-specific flood risk assessment for major development in Flood Zone 1 and all development in Flood Zones 2 and 3, and sustainable drainage on all development — including refurbishments and conversions, not only new build. It also asks development to account for possible groundwater contamination within Source Protection Zones 1 and 2, which matters in Croydon given that groundwater from the chalk beneath the borough supplies more than 70% of local drinking water.

Policy DM25 — Sustainable Drainage Systems and reducing flood risk

Policy DM25 is Croydon's detailed flood and drainage policy and the one an FRA or drainage strategy has to satisfy clause by clause. DM25.1 requires development to be steered toward lower flood risk land, to apply the Sequential and Exception Tests set out in Table 8.1, and to take account of every source of flooding — fluvial, surface water, groundwater, sewers, reservoirs and ordinary watercourses — with the most vulnerable uses placed on the lowest-risk parts of a site. DM25.2 requires development in at-risk areas to be safe for its lifetime and to build in flood resilience and resistance measures. DM25.3 is the drainage clause: sustainable drainage systems are required on all development, must manage runoff as close to source as possible, must follow the London Plan drainage hierarchy, must achieve better than greenfield runoff rates, must be multifunctional and support amenity and biodiversity, must improve water quality through a management train, and must be designed with future maintenance in mind.

The Croydon Strategic Flood Risk Assessment

Croydon's evidence base is the Level 1 and Level 2 Strategic Flood Risk Assessment prepared by AECOM, updated in September 2021 following Environment Agency comments. The Level 1 SFRA originally covered the four London boroughs of Croydon, Merton, Sutton and Wandsworth before being issued as a standalone Croydon document; the Level 2 SFRA provides the detailed, site-specific assessment for locations the council has identified as needing the Exception Test.

The SFRA identifies stretches of the River Wandle, including through Wandle Park, and sections of the Norbury Brook as Flood Zone 3b functional floodplain, where more vulnerable and highly vulnerable development is not permitted. It also confirms Croydon's 2011 Surface Water Management Plan identification of sixteen Critical Drainage Areas across the borough, with the most frequent recorded surface water flooding at the Purley Cross roundabout and along Brighton Road, and further episodes at Kenley station, Hamsey Green, Purley Oaks Road, Norbury and Thornton Heath.

For strategic planning and the Sequential Test, Croydon's SFRA advises using the 2070 to 2115 development lifetime, which for more vulnerable residential development means testing a climate change allowance impact of between 35% and 70% on the 1% annual probability (1 in 100 year) flood level. A residential scheme is expected to be assessed against a minimum 100-year lifetime; other development types should justify their own. Our guide to Strategic Flood Risk Assessments and flood zones explainer cover how this evidence base feeds into an individual site's FRA.

Croydon's validation list and required documents

Croydon's Local Validation List, most recently updated in May 2026, is explicit that a flood risk assessment must accompany every application — even where the site is not identified as being at risk — so the council can see that a proposal is not contributing to flood risk. For basement development in an area at risk of groundwater flooding, the FRA must include a Basement Impact Assessment covering the site's relationship to aquifers and watercourses, its impacts on flooding and drainage, and how ongoing drainage and maintenance will be managed. For "Large Minors and above" development, the validation list separately requires a drainage strategy showing how sustainable drainage will manage runoff as close to source as possible.

• Every application. A flood risk assessment, proportionate to the scale, nature and location of the development, referencing the SFRA and the online flood risk maps.
• Basement development. A Basement Impact Assessment within the FRA where groundwater flood risk is identified.
• Large Minors and above. A drainage strategy demonstrating the London Plan drainage hierarchy and the borough's better-than-greenfield runoff expectation.
• Supporting checklist. Appendix 2 of the council's Advice to Planning Applicants and the London Sustainable Drainage Proforma, submitted alongside the drainage strategy for major applications.

Missing or incomplete flood risk and drainage documents are among the most common reasons a Croydon application is invalidated before it is even registered, stopping the determination clock before it starts. Our guide to invalidated planning applications and drainage information guide set out exactly what is commonly missing and how to avoid the delay.

How national flood risk policy applies to a Croydon application

Croydon's Local Plan sits within the framework set by the National Planning Policy Framework and its Planning Practice Guidance, which direct development away from the highest flood risk areas and require a sequential, risk-based approach wherever it cannot be avoided. The September 2025 update to the Planning Practice Guidance confirmed that this sequential, risk-based approach applies to surface water and groundwater as well as rivers and the sea — directly relevant in a borough where surface water, not river flooding, is the dominant risk. Where a site falls in Flood Zone 2 or 3, or in one of Croydon's Critical Drainage Areas within Flood Zone 1, the Sequential Test and, where required, the Exception Test have to be addressed as part of the FRA, following the same national tests explained in our Sequential and Exception Test guide and applied locally through Croydon's own Table 8.1.

The Environment Agency remains the statutory consultee for main river and tidal risk in the borough, and its role, standing advice and finished floor level expectations are set out in our Environment Agency explainer; Croydon's own LLFA role is covered in our guide to Lead Local Flood Authorities. Applicants preparing for a Croydon Planning Inspectorate appeal should also note that flood risk and drainage evidence has to be complete and robust at submission — our 2026 planning appeals guide explains why there is now far less scope to fix gaps after refusal.

The flood risk picture across Croydon's area

Surface water is the dominant flood risk across the borough, driven by Croydon's position on the North Downs dip slope and its historically culverted watercourses. The River Wandle runs through the borough in a largely culverted channel; the Norbury Brook crosses Thornton Heath and Norbury; and the Caterham Bourne, a winterbourne that flows only intermittently, caused severe flooding through Kenley, Purley and Waddon in 2014, associated with exceptionally high groundwater levels. Croydon Council has since published Section 19 flood investigation reports covering the Caterham Bourne (2014 and 2017) and Merstham Bourne (2015) events, and its Local Flood Risk Management Strategy 2023-2028 records notable surface water flooding in July 2007, over the winter of 2014 to 2015, in August 2015, June 2016 and June 2019.

This compact summary is deliberately brief — a fuller picture of flood risk across the borough, including its river catchments and flood history in more depth, belongs on Unda's area guide for Croydon, which we would be glad to build out alongside this page. Our areas hub covers the same local-knowledge approach across England and Wales.

How Unda helps with flood risk and drainage in Croydon

We are experts in the flood risk and drainage regime Croydon Council applies, from the borough's better-than-greenfield discharge expectation to the specific Local Plan policies and SFRA evidence a reviewing officer will check an FRA against. A Senior Flood Risk Consultant signs off every report we prepare, and we reply to every quote request within 60 minutes, so a Croydon application does not lose momentum waiting on a first response.

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

We prepare flood risk assessments for a Croydon planning application, a property purchase, a mortgage or insurance query, and BREEAM certification, alongside sequential and exception tests, groundwater assessments and surface water drainage strategies. Each is its own report, built around what the specific audience or policy framework requires rather than a single generic template.

• Flood risk assessment for planning. The core FRA for planning report required on every Croydon application.
• Flood risk survey for property purchase. An independent homebuyer flood risk survey where conveyancing searches flag risk.
• Flood risk assessment for insurance. Property-specific evidence for brokers and underwriters — see our FRA for insurance service.
• Flood risk assessment for BREEAM. An FRA structured for BREEAM Pol 03 credits from the outset.
• Sequential and Exception Test reports. Prepared alongside the FRA where Table 8.1 requires them.
• Groundwater flood risk assessment. Specialist hydrogeological input, relevant given Croydon's chalk aquifer — see our groundwater assessment service.
• Flood modelling. Site-specific hydraulic modelling where standard mapping is not sufficient, including around the Wandle corridor.
• NPPF guidance. Our NPPF flood risk guidance page sets out how national policy underpins Croydon's own tests.

Free tools that help scope a Croydon site before you commission a report: the Flood Risk Map for an indicative postcode check, the Live River Levels Map for current conditions on the Wandle and nearby gauges, the Geology & Ground Conditions Checker for an early read on infiltration potential given the borough's chalk and clay geology, our drainage calculators for a first-pass sizing check, and the BRE 365 infiltration rate calculator once trial pit results are in. Broader context on how national and London-wide policy is evolving is covered in our 2025 National Standards for SuDS and London Surface Water Strategy articles, our wider guide to how Local Planning Authorities handle flood risk and SuDS, and our analysis of urban surface water risk across England, which places Croydon's own ranking in national context.

Frequently asked questions