Flood Risk Assessment in Crawley Borough Council

Crawley Borough Council determines planning applications against one of the newest Local Plans in England, and its flood risk and drainage policies are applied as written. Policy EP1 expects a flood risk assessment and a site-specific drainage strategy where they are triggered, West Sussex County Council reviews the drainage case on every major application, and the borough's validation list will stop an application that arrives without the right documents.

This page sets out what Crawley Borough Council requires for flood risk and drainage on a planning application: the Local Plan policies and their codes, the documents the March 2025 validation list asks for, the county's role as Lead Local Flood Authority, and the constraint most applicants never see coming, which is Gatwick Airport's say over open water in a drainage design. We prepare flood risk assessments for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface water drainage strategies, and we take each one through the council's process.

If you want to talk a Crawley site through, contact one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

Crawley adopted its Local Plan in October 2024 and Natural England lifted the water neutrality restriction a year later, so the borough is working through a backlog of held-up applications. The schemes that clear it fastest are the ones whose flood risk and drainage submissions satisfy Policy EP1 first time.

Flood risk and drainage for Crawley Borough Council planning: at a glance

• Crawley Borough Council is the local planning authority. It determines applications in the borough and its own drainage engineer reviews schemes alongside the statutory consultees.
• West Sussex County Council is the Lead Local Flood Authority. As a statutory consultee it reviews the surface water drainage strategy on every major application in Crawley, and expects discharge held to greenfield runoff rates so far as practicable.
• The Crawley Borough Local Plan 2023–2040 was adopted on 16 October 2024. Policy EP1 (Development and Flood Risk) and Policy EP2 (Flood Risk Guidance for Householder Development, Small Non-Residential Extensions) carry the flood and drainage requirements.
• A drainage strategy is required with every major application, and any proposal in Flood Zones 2 or 3 needs a flood risk assessment. The evidence base is the Crawley Borough and Upper Mole Catchment Level 1 Strategic Flood Risk Assessment, updated in November 2023.
• Water neutrality no longer applies. Natural England withdrew its Sussex North position statement on 31 October 2025, and under Strategic Policy SDC4 the water efficiency standard reverts to 110 litres per person per day for new homes.

Who decides flood risk and drainage for a Crawley application?

Crawley is a two-tier area, which catches people out. The borough council registers and determines the application, but the technical flood and drainage consultees sit elsewhere: the county council for surface water, the Environment Agency for the main rivers, and Thames Water for most sewer connections. A submission has to satisfy all of them, and each can hold an application up on its own.

The split between the borough and the county is the one that matters most in practice. Our guide to what a Lead Local Flood Authority does explains why the LLFA's view carries so much weight, and what happens when the LLFA objects to a drainage strategy.

Does Crawley Borough Council require a flood risk assessment or drainage strategy?

Any proposal in Flood Zones 2 or 3 needs a flood risk assessment, as does a site of more than one hectare in Flood Zone 1, and a Flood Zone 1 site where the development could be affected by surface water or another non-fluvial source. A drainage strategy is required with every major application. Crawley's national and local validation list, reviewed in March 2025, sets these triggers out, and an application missing a required document will not be registered.

Invalidation is administrative rather than a refusal, but it stops the clock before it starts. Our notes on why applications get invalidated for flood risk and drainage and when a flood risk assessment is needed cover the traps, including the Flood Zone 1 surface-water one that catches Crawley sites in particular. If you are unsure what your site needs, send us the address and we will tell you.

Crawley's drainage and SuDS requirements: what the LLFA expects

West Sussex County Council reviews the surface water drainage case on every major application in Crawley. It expects the drainage hierarchy followed, discharge held to greenfield runoff rates so far as practicable, attenuation for the 1 in 100 year storm with a climate change allowance, and a maintenance arrangement that will outlive the developer. Following the publication of the updated national standards for SuDS, the county recommends addressing each standard in its own section of the drainage strategy while it prepares new developer guidance.

• The surface water drainage statement. The county uses a surface water drainage statement pro-forma for major applications, completed and signed by a competent drainage engineer. A strategy that skips it invites a holding objection.
• Runoff and attenuation. Existing and proposed runoff are calculated and the scheme is designed so that post-development discharge does not exceed the agreed rate, with storage for extreme events. Crawley's validation list adds that surface water must not drain to the foul sewer and asks how the drainage details will be certified after construction.
• Infiltration first, where the ground allows. Much of Crawley sits on slowly draining Weald Clay, so soakaways often fail. BRE 365 infiltration testing settles the question early, before a layout is fixed around a solution the ground cannot deliver.
• Ordinary watercourses. Works to a ditch, culvert or brook that is not a main river need separate land drainage consent from West Sussex County Council, on top of planning permission.
• Pre-application advice. The county's flood risk management team runs a pre-application advice service, which is worth using on any scheme where the drainage answer is not obvious.

The whole borough sits within 13 kilometres of Gatwick Airport, so any drainage design that creates open water can trigger aerodrome safeguarding consultation over bird-strike risk. Attenuation in Crawley is routinely designed to drain down in hours, not days, and a pond-led strategy drawn up without this in mind can fail late and expensively.

The airport constraint is not theoretical. Crawley's validation list points applicants to the aviation industry's guidance on wildlife hazards around aerodromes, and Gatwick has objected to schemes in the borough's orbit over permanent open water. When the council assessed the West of Ifield proposals on its boundary in 2025, the flood compensation areas were designed to hold water for only three to four hours for exactly this reason. A surface water drainage strategy for a Crawley site has to reconcile the LLFA's storage requirements with the airport's aversion to standing water, which usually means underground attenuation, permeable paving and other SuDS techniques that do the job without a lake. Our SuDS pro-forma guide, drainage calculators, BRE 365 infiltration rate calculator and geology and ground conditions map are free starting points, and our guide to the drainage information planning permission needs explains what the documents must contain.

Crawley Borough Local Plan flood and drainage policies

The Crawley Borough Local Plan 2023–2040, adopted by Full Council on 16 October 2024, carries the borough's flood risk and drainage policies. Policy EP1 does most of the work, Policy EP2 creates a lighter route for householders in the flood zones, and the water policies in the SDC series set the efficiency standards that survived the end of water neutrality. The codes matter: officers cite them in reports, and a submission that addresses each policy by name is easier to approve.

Policy EP1 Crawley: development and flood risk

Policy EP1 of the Crawley Borough Local Plan 2023–2040 requires development to avoid areas exposed to an unacceptable risk of flooding and to ensure flood risk is not increased elsewhere. In practice it is the policy that makes both the flood risk assessment and the site-specific drainage strategy a requirement, and council officers check submissions against it explicitly. An assessment that never engages with EP1 by name has been picked up in consultation responses, so we address it directly in every Crawley report.

Policy EP2 Crawley: the Flood Risk and Resilience Statement

Policy EP2 gives householders and small non-residential schemes in Flood Zones 2, 3a and 3b a proportionate alternative to a full flood risk assessment. The Flood Risk and Resilience Statement must show the development would mitigate any loss of flood storage and that its design is resilient to the flood risk it faces over its lifetime, taking climate change into account. It is a useful route for an extension or outbuilding, and far cheaper than the full assessment it replaces, but it still has to be evidenced.

Strategic Policy SDC4 Crawley: water neutrality after October 2025

Strategic Policy SDC4 was written while Natural England's 2021 position statement held up development across the Sussex North Water Supply Zone, which covers Crawley. That position was withdrawn on 31 October 2025, after Southern Water and the Environment Agency agreed an abstraction licence cap protecting the Arun Valley wetlands, so applications no longer need to demonstrate water neutrality. Under SDC4's own terms the water efficiency standard reverts to 110 litres per person per day for homes, and the council has said clearing the backlog of held applications will take months. Our water neutrality guide covers what the withdrawal means if your scheme was caught by the old restriction.

The Strategic Flood Risk Assessment behind Crawley's decisions

Crawley's flood evidence base is the Crawley Borough and Upper Mole Catchment Level 1 Strategic Flood Risk Assessment, prepared by JBA Consulting jointly for Crawley Borough Council and Horsham District Council and updated in November 2023. It is the document officers reach for when they test a site, so an applicant's own assessment needs to be consistent with it, or explain credibly why it differs.

• It splits Flood Zone 3 into 3a and 3b. The SFRA maps the functional floodplain separately from the wider high-probability zone, which decides whether the sequential and exception tests bite on a given site.
• It carries climate change extents. The 2023 update used the Upper Mole hydraulic model, run with peak flow uplifts of 25, 35 and 70 per cent, so the council can see the flood outline your development must stay safe within for its lifetime. Our guide to climate change allowances explains how these uplifts are chosen.
• It informed which sites were allocated. The council used the SFRA to apply the sequential test to its Local Plan allocations, and an unallocated site in a flood zone will face the same test at application stage.
• It maps more than rivers. Surface water, groundwater, reservoirs and the borough's flood defences, whose standard of protection varies from a 1 in 5 to a 1 in 200 year event, are all in scope, and a site-specific assessment is expected to look at every source.

The Environment Agency's flood map and the SFRA do not always agree. The council has itself noted inconsistencies in the 2025 flood map for planning in parts of the borough, where surface water risk displays as river risk, so checking a Crawley site against both, and against the September 2025 planning practice guidance, is part of getting the assessment right. Our note on the September 2025 PPG update explains how the sequential test now applies to every source of flooding, including surface water.

How national policy applies to a Crawley application

Everything above sits on the national framework. The National Planning Policy Framework directs development to the lowest-risk land available, requires site-specific flood risk assessments in the situations Crawley's validation list reflects, and expects sustainable drainage on major development. The planning practice guidance sets the working detail, and the Environment Agency's standing advice fills in the rest. What the Local Plan adds is Crawley's own layer: EP1's explicit demand for a drainage strategy, EP2's resilience statement route, and the SFRA evidence the council tests against. Our guides to the NPPF and flood risk and to flood risk assessments for planning set out the national rules in full.

Flood risk in Crawley: the short version

Crawley is predominantly a surface water town. The post-war New Town was built at the head of the River Mole with engineered channels shedding rainfall towards the Gatwick Stream, Ifield Brook, Tilgate Brook and Crawters Brook, and when intense rain outruns that network the water ponds in the neighbourhoods. Fluvial risk concentrates along the Mole and Gatwick Stream through Langley Green, Three Bridges and Forge Wood.

• The record bears it out. The floods of September 1968 closed Gatwick Airport for several days; the autumn 2000 event flooded over 70 properties across the area, including 44 in Maidenbower; and in June 2012 surface water flooded homes across Maidenbower, Furnace Green and Ifield Green.
• Defences have improved since. The Environment Agency's Upper Mole flood alleviation scheme, built in stages through the 2010s at Tilgate, Worth Farm and Clay's Lake, reduces flood risk to more than 1,400 homes and businesses in Crawley and Horley and to Gatwick Airport, with Clay's Lake enlarged to store nearly 400,000 cubic metres.
• Defended is not risk-free. Planning decisions still assess residual risk, and the flood map is drawn as if defences were not there.

That is deliberately brief, because this page is about what the council requires. For the full picture of how the borough floods, neighbourhood by neighbourhood, see our flood risk assessment in Crawley page and our history of the 2000 Crawley and Gatwick floods.

How Unda helps with Crawley Borough Council applications

We know this authority's regime in detail: which documents the validation team checks for, how Policy EP1 is applied in officer reports, what West Sussex County Council's drainage engineers expect to see, and how to design attenuation that satisfies the LLFA without troubling Gatwick's safeguarding team. Every report is written to be consistent with the 2023 SFRA, overseen by a chartered flood risk consultant, and we stay involved after submission to answer consultee queries.

Flood risk and drainage services for every situation in Crawley Borough Council's area

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. We prepare all three, alongside the right form of assessment for buyers, insurers and BREEAM assessors.

Flood risk assessments

• Flood risk assessment for planning. Site-specific, prepared to satisfy the NPPF, Policy EP1 and the Environment Agency. Groundwater and, where needed, flood modelling are assessed within it.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged Mole catchment or surface water risk and the actual building needs assessing rather than a postcode.
• Flood risk assessment for insurance. Property-specific evidence for brokers and underwriters where premiums have risen or cover has been restricted.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning assessment alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for clay-and-gravel sites and basements, assessed as a source within the flood risk assessment or standalone on complex sites.

Sequential and exception tests

The sequential and exception tests are a separate, locational report about whether development should be in a higher-risk location at all. In Crawley the council applied the sequential test to its own allocations through the SFRA, so an unallocated flood-zone site should expect the test to be applied rigorously at application stage.

Drainage strategy and SuDS

• Surface water drainage strategy (SuDS). Calculates existing and proposed runoff and designs SuDS that meet the county's requirements and Gatwick's open-water constraint.
• Preliminary drainage design. The early read that flags infiltration and attenuation issues before they fix the site layout.
• Discharge of drainage conditions. The detailed design stage that clears a pre-commencement or pre-occupation drainage condition.
• Flood modelling. 1D and 2D hydraulic modelling where the national mapping or the Upper Mole model outputs are not enough, feeding the assessment or the drainage design.

You can screen any Crawley postcode on our free flood risk map before committing to anything.

Crawley flood risk and drainage contacts for applicants

These are the bodies a Crawley applicant actually deals with, with the documents each one owns. For flooding happening right now, call 999 if there is danger to life, or Floodline on 0345 988 1188.

• Crawley Borough Council planning. The Local Plan, adopted policies map and the validation requirements are on crawley.gov.uk, along with the council's water neutrality update.
• The Crawley and Upper Mole SFRA. The November 2023 Level 1 Strategic Flood Risk Assessment is the evidence base behind the flood policies.
• West Sussex County Council flood risk management. The county's sustainable drainage pages and its pre-application advice service cover the LLFA's requirements and land drainage consent.
• Environment Agency. National flood risk assessment guidance, plus the Upper Mole flood alleviation scheme that protects the town, with live warnings at check-for-flooding.service.gov.uk.
• Gatwick Airport safeguarding. The airport's aerodrome safeguarding pages explain the 13km consultation zone that shapes open-water SuDS in the borough.
• Thames Water. Foul and surface water sewer connections in most of Crawley.

Crawley Borough Council flood risk and drainage: common questions