Flood Risk & Drainage Requirements for Crawley Borough Council

Crawley Borough Council asks every major scheme for a flood risk assessment, even on land the flood map shows in Flood Zone 1. It also wants the finished drainage certified by an independent professional who did not design it. Both rules sit in Policy EP1 (Development and Flood Risk) of the Crawley Borough Local Plan 2023 to 2040, adopted on 16 October 2024, and they run alongside West Sussex County Council's August 2026 SuDS guidance and Gatwick Airport's safeguarding of open water.

Below are the Crawley flood risk and drainage requirements that Crawley Borough Council (CBC), West Sussex County Council and the Environment Agency apply to a planning application. Each is set out policy by policy, with what our reports put in front of each body. To talk through a site in the borough, call 01293 214444 or email enquiries@unda.co.uk.

Crawley Borough Local Plan flood and drainage policies, and how we answer each one

Six adopted policies decide the flood and drainage case on a Crawley application. EP1 carries most of it and EP2 gives householders a lighter document. DD5 brings Gatwick into any scheme with open water, SDC3 and SDC4 set the water efficiency standard, and EC4 adds a site-specific test at Gatwick Green. Two supplementary planning documents adopted in summer 2026 explain how the council applies them.

Requirements checked November 2026.

Crawley Policy EP1 (Development and Flood Risk): what an application has to show

Policy EP1 asks for three things national policy does not ask for in the same form. Every major scheme in Flood Zone 1 needs a flood risk assessment, every major application needs a site-specific drainage strategy, and the drainage has to be certified independently once it is built. The policy opens: "Development must avoid areas which are exposed to an unacceptable risk from flooding, and must not increase the risk of flooding elsewhere" (Local Plan, October 2024).

Its six clauses run in order. Clause i applies the sequential test and, if needed, the exception test. Clause ii asks for a flood risk assessment where a site lies in Flood Zone 2 or 3, "and for all major development in Flood Zone 1 or where otherwise required by the NPPF". Clause iii expects SuDS to reduce both peak runoff rates and annual runoff volumes unless that is not technically feasible or financially viable. Clause iv sends surface water to the ground, a watercourse or a surface water sewer, never the foul sewer, and asks for re-use to be considered. Clause v keeps development 8 metres back from the bank of any Main River or Ordinary Watercourse and 3 metres from any sewer without prior consent. Clause vi asks for certification of the drainage works "from a third party professional. This should not be the consultant who designed the drainage features".

The supporting text explains why foul connections matter so much here: surface water reaching the foul sewer is described as "the major contributor to sewer flooding". It also says the drainage information West Sussex County Council asks for is required for all development except the EP2 types. A minor scheme therefore still has to show where its water goes, even though the county will not normally read it.

Our flood risk assessment for an EP1 case screens the site against the SFRA flood zones, its climate change extents and the surface water mapping, and it checks the smaller watercourses the SFRA did not map, which on Crawley's culverted brooks can matter as much as the Main Rivers. The drainage strategy works down the hierarchy, giving the reason each higher option was ruled out, and quantifies the cut in both peak rate and volume. It also says who will certify the works after construction, so clause vi is settled before the condition stage instead of surfacing at it.

Policy EP2 (Flood Risk Guidance for Householder Development, Small Non-Residential Extensions) and the Flood Risk and Resilience Statement

Policy EP2 swaps the detailed flood risk assessment for a shorter Flood Risk and Resilience Statement on three kinds of work in Flood Zones 2, 3a and 3b: householder development, non-residential extensions with a footprint under 250 m² and minor alterations. In the policy's words, "Instead, a Flood Risk and Resilience Statement must be provided". On a proportionate basis it shows how lost flood storage will be mitigated and how the development stays resilient over its lifetime, allowing for climate change (Local Plan, October 2024).

The Householder Development SPD (adopted 1 June 2026) widens the trigger. Applicants start from the Environment Agency's Flood Map for Planning, then check any river risk against the SFRA's own flood zone maps, which in parts of the borough show lower risk than the national map because the SFRA is based on local modelling and its Flood Zone 3b allows for defences. It adds that a statement "will also be required if the development site is found to be at risk of pluvial (surface water) flooding" on the Flood Map for Planning. The SPD also records that the council and the Environment Agency agreed the Local Plan carries greater weight than the Agency's standing advice where the two conflict.

EP2 does not cover a new dwelling in a garden or a house split into flats. Nor does it cover a change of use that moves a building into a more vulnerable class; those need a full assessment. For an EP2 case we prepare the statement itself. It includes the SFRA check, the floor and storage figures, the compensation proposed, and resilience measures sized to the flood level, such as raised electrics and water-resistant finishes.

Policy DD5 (Aerodrome Safeguarding): why SuDS ponds draw Gatwick into a Crawley scheme

Policy DD5 opens: "Development will only be supported if it is consistent with the continued safe operation of Gatwick Airport." The Local Plan says the whole borough sits inside the aerodrome safeguarded area. Its supporting text lists "the creation of new water bodies & SuDS" and large flat or shallow-pitched roofs among the features that can attract birds hazardous to aircraft. Where safeguarding applies, the council has to consult Gatwick Airport Limited, and proposals that cannot be mitigated to the satisfaction of the statutory consultees are refused (Local Plan, October 2024).

The council's Sustainable Design and Construction SPD (adopted 29 June 2026) advises early contact with Gatwick's safeguarding team (gal.safeguarding@gatwickairport.com) on large schemes with surface ponds, swales, wetlands or wide green roofs. The validation list points drainage strategies to the CAST advice note on wildlife hazards around aerodromes. In practice this pulls storage below ground, into tanks, crates and permeable paving, or into open basins and swales that drain down quickly instead of holding water birds can use. Our drainage strategies for Crawley sites say how each open feature drains down and why it will not hold standing water. One document then answers both the safeguarding team and the LLFA.

Policy SDC3 (Tackling Water Stress) and Policy SDC4 (Water Neutrality) after October 2025

Crawley lies in Southern Water's Sussex North Water Resource Zone, so Policy SDC4 was written to make development water neutral. On 31 October 2025 Natural England withdrew its September 2021 position statement, and the council's water neutrality page confirms there is no longer a requirement to show neutrality. SDC4 saw this coming. Its clause 7 says "Should the need to demonstrate water neutrality no longer be required, new residential development must be designed to utilise no more than 110 litres of mains supplied water per person per day", with BREEAM Excellent for water on non-domestic buildings. SDC3 sets the same figures for land outside the zone.

Water efficiency is not a drainage test, but the two meet at rainwater harvesting. Collecting water for re-use tops the national drainage hierarchy, and West Sussex County Council now expects major schemes to consider it from outline stage. Our guide to water neutrality in planning covers the withdrawal for schemes that were held under the old position.

Strategic Policy EC4 (Strategic Employment Location) at Gatwick Green

Gatwick Green, the industrial allocation east of Balcombe Road, carries its own flood clause. Strategic Policy EC4 asks applicants to "Demonstrate, through a Flood Risk Assessment and Surface Water Drainage Strategy, how drainage and flood risk (including cumulative flood risk) will be taken into account" in the layout (Local Plan, October 2024). The site drains to the Burstow Stream, which the plan identifies as at high risk of cumulative flood impacts. The supporting text therefore asks for a local area flood risk assessment and drainage study covering peak rates, volumes, duration and the timing of flood peaks on the receiving watercourses, with betterment downstream where it can be achieved.

On adoption in October 2024 the new plan replaced every policy of the Crawley Borough Local Plan 2015 to 2030. EP1 and SDC3 now cover the ground of the replaced Policy ENV8 (Development and Flood Risk) and the replaced water stress policy, ENV9. The superseded ENV8 in the 2015 plan asked for an assessment on sites of 1 hectare or more, where EP1 ties the Flood Zone 1 trigger to major development.

Where the Local Plan stands in November 2026

No replacement plan is in preparation. The council's Local Development Scheme (updated February 2026) schedules a review of the Local Plan to begin with Regulation 18 engagement in mid 2027. A Regulation 19 consultation follows in autumn 2028 and adoption in October 2029. No notice of intention to start a new-style plan appears on the council's Local Plan pages. The Local Plan map was reissued in September 2026, so the policies above are the ones an application is judged against.

Local government reorganisation is still undecided. West Sussex County Council's 30 October 2026 update says no final decision has been made, expects one by early 2027, and puts any new councils in charge from April 2029. Until then Crawley Borough Council remains the planning authority and West Sussex County Council the Lead Local Flood Authority.

Borough, county, Environment Agency or airport: who decides what

No internal drainage board covers Crawley. The only internal drainage district in West Sussex is the River Arun IDD, run by the Environment Agency. Kilnwood Vale and the land west of Ifield sit just over the boundary in Horsham District, so those schemes go to Horsham District Council, under the same county drainage standard.

The split between borough and county matters most on smaller sites. West Sussex County Council's pre-application advice page says "We do not normally assess minor developments", which leaves the borough's officers to judge the drainage on anything below the major threshold. Our explainer on what a Lead Local Flood Authority does sets out the county's statutory role. Our note on an LLFA objection to a planning application covers what happens when the drainage case is challenged.

Works in or over an ordinary watercourse need land drainage consent from the county under section 23 of the Land Drainage Act 1991, at £50 per structure. The county's consent page says planning permission does not replace it. The Local Plan's supporting text gives the borough as the body to consent work near ordinary watercourses, but the county page and the council's own June 2026 SPD both send applicants to West Sussex. Works near a Main River are regulated by the Environment Agency.

When Crawley asks for a flood risk assessment, a drainage strategy or a resilience statement

The council's National and Local List of Validation Requirements, adopted on 3 November 2025, decides what an application needs at the door; Google still surfaces the March 2025 list it replaced. The list and EP1 set different Flood Zone 1 triggers. The list asks for an assessment on major development on sites over 1 hectare; the policy asks for one on all major development. We work to the policy's wider wording.

The list still refers to the Planning and Climate Change SPD of 2016 and the non-statutory technical standards for SuDS. Both have been replaced, by the council's Sustainable Design and Construction SPD in June 2026 and by the national standards for sustainable drainage published in 2025. The tests have not moved. An application missing one of these documents will not be registered, and our article on planning applications invalidated for flood risk and drainage lists the gaps that most often cause it.

The December 2023 SFRA for Crawley and the Upper Mole

Crawley's evidence base is the Gatwick Sub-Region Level 1 Strategic Flood Risk Assessment for Crawley Borough and the Upper Mole catchment. JBA Consulting prepared it for Crawley and Horsham, and the final updated report is dated December 2023. It is the version the council's environmental evidence page links; Google still ranks the older SFRAs above it. The site screening table (Appendix K) and the guide to using the technical data were reposted in March 2026. No Level 2 SFRA has been published for the borough.

• Its own flood zones. The SFRA says its flood zones should be used for decisions and in places update the Environment Agency's. That is why the Householder SPD sends applicants from the national map to the SFRA maps.
• Flood Zone 3b. Mapped from the 2% annual chance defended outline where detailed modelling exists, an approach agreed with the Environment Agency. Where that output is missing, the 1% annual chance defended extent stands in, and a site-specific assessment has to confirm the true line.
• Small watercourses. Generalised flood zones were drawn only for watercourses with catchments over 3 km², so a site beside a smaller stream or field drain can show as Flood Zone 1 when it is not.
• Surface water and sewers. The county's draft flood strategy classed Crawley as a wet spot, with 9,000 homes and business buildings at surface water risk. The SFRA finds no critical drainage areas. It warns that Crawley's sewage treatment works is unlikely to meet demand to 2050 without investment, and advises a sewer capacity check on each site.
• Groundwater. Mostly negligible on the clay, with low and moderate areas around Gatwick Airport, Three Bridges, Forge Wood, Northgate and Langley Green.

An assessment that reads only the Flood Map for Planning misses much of this. Our assessments start from the SFRA's flood zone and climate change appendices, add the surface water and groundwater layers, and test any unmapped watercourse on the ground. Our explainer on what a strategic flood risk assessment is shows how the SFRA feeds a site report. Our flood risk map gives a first look at the zones and surface water extents for any postcode.

West Sussex County Council's August 2026 SuDS standard on Crawley schemes

For major development the county applies Complying with the National Standards for Sustainable Drainage Systems (SuDS): Guidance for Developers, dated August 2026. Its SuDS page, last updated on 28 August 2026, says the county "expects all major planning applications to comply with the standards". It will not accept high density or lack of space as a reason to leave out source control. These are the figures a Crawley drainage strategy is checked against.

• Discharge. Greenfield runoff rates, even where Thames Water would permit a higher rate into its sewer.
• First 5 mm. The first 5 mm of everyday rainfall fully intercepted on site.
• Storms. The 1% annual chance event plus climate change managed on site, with discharge volumes calculated from the 1 in 100 six-hour storm, as the national standards set out, and 10% added for urban creep. Infiltration features have to half-empty within 24 hours after a 3.33% annual chance event plus climate change.
• Ground. BRE 365 or an approved method, three tests per pit, and the lowest rate used. Groundwater is monitored through the winter, with at least 1 m between its highest level and the base of any infiltration feature.
• Exceedance and easements. An exceedance plan for the 1% annual chance event plus climate change with no off-site flooding; 3 m kept clear of ordinary watercourses and culverts and 8 m of Main Rivers.
• Outline stage. At least one viable drainage route, the outfalls and easements on a layout, winter groundwater and infiltration results, and an early look at rainwater harvesting.

The Local Plan and the validation list still cite the county's 2018 Policy for the Management of Surface Water. The county's current pages point to the August 2026 guidance instead, and its proforma and validation checklist are being revised to match the national standards. Clay covers much of the borough and infiltration often fails on it, so on many Crawley sites the strategy turns on whether a watercourse can be reached before a sewer is considered. Our BRE 365 infiltration rate calculator, drainage calculators and geology and ground conditions map give an early read. The county's figures follow the national standards, which our 2025 national standards for sustainable drainage article sets out.

NPPF Chapter 18 behind Crawley's flood policies

The national layer is Chapter 18 (Managing flood risk and coastal change) of the August 2026 National Planning Policy Framework. Policy F4 sets when a site-specific flood risk assessment is needed, including Flood Zone 1 sites of a hectare or more and land shown at risk from any source. F5 sets the sequential test, with exemptions for allocated sites, householder work, small extensions and most changes of use. F6 is the exception test, F7 requires development to be safe for its lifetime without raising flood risk elsewhere, and F8 asks for sustainable drainage built to the national standards and maintained. The flood risk and coastal change guidance was last revised on 17 September 2025, and the Environment Agency's climate change allowances date from 27 May 2022.

The Sustainable Design and Construction SPD and the validation list still cite earlier Frameworks. EP1's tests line up with Chapter 18 regardless, and go further on major sites in Flood Zone 1. Our summary of the August 2026 NPPF changes covers what moved, and our guide to the sequential and exception tests sets out the national tests the SPD's Table 9.1 applies.

How Crawley floods, in brief

The whole borough lies in the upper reaches of the River Mole catchment. The Local Plan describes Crawley as at the highest risk of surface water flooding in West Sussex, a product of clay soils and dense urban development. The headwaters of the River Mole gather in the borough and flow north past Gatwick. The Gatwick Stream runs through Maidenbower and Three Bridges, the Ifield Brook drains the west of the town through Ifield, and the Tilgate Brook flows from Tilgate through Furnace Green. The SFRA lists Langley Green, Three Bridges and Forge Wood among the neighbourhoods most exposed to river flooding.

The SFRA records major floods in 1968, 2000 and 2012. Its detailed record puts 44 properties in Maidenbower, 20 in Furnace Green and 14 in Ifield Green among those flooded in autumn 2000, and the June 2012 storm reaching a small number of properties in Southgate, West Green, Rusper and Broadfield. Our account of the 2000 Crawley and Gatwick floods tells that story, and our page on flood risk assessment in Crawley covers the town neighbourhood by neighbourhood. For current readings on the Mole and its tributaries, see the live river levels map.

Our reports for an application to Crawley Borough Council

A flood risk assessment asks whether a Crawley site is safe. The sequential test asks whether the scheme should be on that land at all, and a drainage strategy shows where the water goes and how fast it leaves. Each is a separate report. One application can need all three. We have prepared reports for applications to the borough, and each service below is written to the council's own rules.

• Every report. Each Crawley report is written to EP1, the December 2023 SFRA and, on major schemes, the county's August 2026 standard, starting from a flood risk assessment.
• Planning applications. A flood risk assessment for planning for sites in Flood Zones 2 and 3, for every major scheme in Flood Zone 1 under EP1, and where the SFRA shows surface water or unmapped watercourse risk.
• Buying a property. A flood risk survey for property purchase where a search flags Mole catchment or surface water risk on a home or commercial unit in the borough.
• Testing alternative sites. A sequential and exception test for unallocated sites in Flood Zones 2 and 3, using the SFRA's own flood zones and site screening.
• Drainage at application stage. A drainage strategy for every major application, as EP1 clause iv and the validation list require, with the foul sewer kept out of the surface water route.
• Surface water on larger schemes. A surface water drainage strategy set to greenfield rates, the 5 mm interception rule and the half-drain test on infiltration features, with the Gatwick bird-strike question answered.
• After permission. Discharge of drainage conditions against the approved strategy, with the information an independent certifier will need under EP1 clause vi.
• Groundwater. A groundwater assessment where the county's winter monitoring and 1 m clearance rule decide whether infiltration is possible, or on sites in the SFRA's low and moderate groundwater areas near Gatwick and Three Bridges.
• Unmapped streams. Flood modelling where a site sits beside one of the small watercourses the SFRA did not map, so the true flood extent is known before the sequential test is run.
• Foul water and sewer capacity. A foul drainage strategy where the SFRA's advice on Crawley sewage treatment works capacity calls for a site-specific capacity check.
• National policy questions. NPPF guidance and advice on how Chapter 18 sits beside EP1, EP2 and the validation list on a borough scheme.

West Sussex County Council's standard applies in the neighbouring districts too. See our pages on Horsham, which shares Crawley's SFRA, Mid Sussex, Arun and Adur & Worthing, our West Sussex overview, or the full list of council requirements pages.

Crawley flood risk and drainage questions

For anything on this page, call 01293 214444 or email enquiries@unda.co.uk. Unda Consulting Limited: flood risk assessments, sequential tests and drainage strategies for planning. Reports for applications to Crawley Borough Council come from Unda Consulting Limited, a CIWEM Business Partner.