Flood Risk & Drainage Requirements for Spelthorne Borough Council

If you are bringing a site forward in Spelthorne, two authorities decide whether it can be built and how its water is managed. Spelthorne Borough Council determines the planning application against its Local Plan, and Surrey County Council, as the Lead Local Flood Authority, sets the surface water drainage and SuDS standards. Get either half wrong and the application stalls. We prepare the flood risk assessment and the drainage strategy that answer both.

Spelthorne is one of the most flood-exposed boroughs in England — a low, flat wedge of the Thames valley hemmed in by the river and its tributaries — so the requirements here are stricter and more closely scrutinised than in most of Surrey. Developers, architects and planning consultants come to Unda for a flood risk assessment for a Spelthorne planning application, a surface water drainage strategy, a property purchase or mortgage report, a report for insurance, and for BREEAM, alongside the sequential and exception tests a higher-risk site needs.

To talk a Spelthorne site through or get a quote, contact one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

Between 2015 and 2018, 34% of all new addresses created in Spelthorne were in Flood Zone 3 — the highest-risk category — one of the largest shares of any authority in England. In a borough this exposed, an assessment that does not stand up to Surrey's Lead Local Flood Authority is the quickest way to lose months off a programme.

Flood risk and drainage in Spelthorne at a glance

• Two authorities, two tests. Spelthorne Borough Council is the local planning authority; Surrey County Council is the Lead Local Flood Authority (LLFA) for surface water. A scheme here answers a flood-risk test and a separate drainage test.
• A brand-new Local Plan. The Spelthorne Local Plan 2024–2039/40 was adopted on 17 March 2026, replacing the 2009 Core Strategy. Flood risk and drainage are now judged against Policy E3 (Managing Flood Risk), not the old Policy LO1.
• The Environment Agency leads on the Thames. The non-tidal River Thames is a main river through the borough, so the EA is a statutory consultee on fluvial flood risk alongside the council.
• Surrey wants greenfield runoff rates. Post-development discharge is limited to greenfield Q1, Q30 and Q100 rates, with a completed SuDS pro-forma required on every major application.
• An adopted evidence base. The council's Level 1 and Level 2 Strategic Flood Risk Assessments map the borough's flood zones and functional floodplain and drive the sequential test.

Who decides flood risk and drainage in Spelthorne?

Three bodies share the job. Spelthorne Borough Council determines the planning application and judges flood risk against its Local Plan. Surrey County Council, as the Lead Local Flood Authority, is the statutory consultee on surface water drainage for major development. The Environment Agency leads on the River Thames as a main river. Knowing which body wants what, and satisfying each on its own terms, is the difference between a clean submission and a stalled one.

This split catches applicants out more than any other single point, because the drainage sign-off does not come from the district council you send the application to. Surrey County Council reviews the surface water strategy under its own sustainable drainage guidance, while the borough's planning team weighs the flood risk assessment and the sequential test. Our explainer on the role the LLFA plays and on how LPAs handle flood and SuDS sets out how the pieces fit.

Does a Spelthorne application need a flood risk assessment or a drainage strategy?

Most sites in Spelthorne need at least one of the two, and many need both. A flood risk assessment is required for development in Flood Zones 2, 3a and 3b, and for sites of 0.5 hectares or more in Flood Zone 1. A surface water drainage strategy is expected on all major development, and on smaller schemes where surface water is a concern. The two are separate reports answering different questions, and a Spelthorne application usually has to satisfy both.

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They 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. In a borough where Flood Zone 1 is the exception rather than the rule, the scoping matters — and since surface water risk can now trigger an assessment even on a nominally low-risk site, the Flood Map for Planning is only the starting point.

Getting the vulnerability classification right, and demonstrating safe access and egress over the development's lifetime, is where Spelthorne applications most often turn. Where a site sits in a higher flood zone, the sequential and exception tests apply before the application can be recommended for approval.

Spelthorne's Local Plan flood and drainage policies

Spelthorne judges flood risk and drainage against its adopted Local Plan. Following the adoption of the Spelthorne Local Plan 2024–2039/40 on 17 March 2026, the governing policy is Policy E3 (Managing Flood Risk), which replaced Policy LO1 (Flooding) from the 2009 Core Strategy and Policies Development Plan Document. Both are set out below, with what each obliges an applicant to demonstrate.

What Policy E3 (Managing Flood Risk) requires

Policy E3 in the Spelthorne Local Plan 2024–2039/40 requires development to be located, designed and laid out to ensure it is safe, that flood risk is minimised without being increased elsewhere, and that any residual risk is safely managed. New development is steered to areas of lowest flood risk through the sequential test, and all proposals must demonstrate that land drainage is adequate and will not increase surface water runoff. The wording was strengthened during examination through agreed changes from Surrey County Council as the LLFA and the Environment Agency, so it now carries specific expectations on sustainable drainage and climate resilience. In practice, a Spelthorne application answers Policy E3 with a robust flood risk assessment and a matching drainage strategy that speak to the same evidence.

Policy LO1 (Flooding) — the superseded 2009 policy

Policy LO1 was the flood policy in Spelthorne's 2009 Core Strategy and Policies DPD, supported by a Flooding Supplementary Planning Document. It required a flood risk assessment for all development within Flood Zones 2, 3a and 3b, and for Flood Zone 1 sites of 0.5 hectares, 10 dwellings or 1,000 square metres of non-residential floorspace or more. LO1 has been superseded by Policy E3, but it still appears in older committee reports, consents and appeal decisions, so it is worth recognising when reviewing a site's planning history. The council has also confirmed it will prepare a new Flooding Supplementary Planning Document to sit under the adopted plan.

Surrey County Council's SuDS and drainage requirements

Surrey County Council, as the Lead Local Flood Authority, sets the surface water drainage standards a Spelthorne scheme has to meet, and it is a statutory consultee on all major planning applications. Its guidance expects post-development runoff to be limited to greenfield rates, sustainable drainage to be designed following the discharge hierarchy, and a completed SuDS pro-forma to accompany the application. Miss these and a drainage objection can hold an application for months.

The single most useful thing to know before you design is how Surrey expects runoff to be controlled. Drainage schemes must be limited to the greenfield Q1, Q30 and Q100 rates for the corresponding storm events, with the Q100 rate used for the 1-in-100-year plus climate change event. Brownfield sites must get as close to greenfield rates as reasonably practicable and must never exceed the pre-development rates. Our note on the SuDS hierarchy, the 2025 national SuDS standards and the Surrey SuDS pro-forma explains what the LLFA is looking for.

Where infiltration is proposed, Surrey expects tests carried out to BRE Digest 365 in the winter period, with the base of any soakaway at least a metre above the peak groundwater level and a factor of safety of at least two in the calculations. Much of Spelthorne sits on Thames valley gravels over London Clay, so infiltration is viable in places and unworkable in others — our free geology and ground conditions map and BRE 365 infiltration rate calculator give an indicative read before a trial pit, and the drainage calculators help size the scheme. Surrey applies standard drainage conditions that require the final design before commencement and a verification report before occupation, and any connection to or alteration of an ordinary watercourse needs separate consent under Section 23 of the Land Drainage Act. Designing to these standards from the outset is the surest way to avoid the kind of LLFA drainage objection that stalls an application.

The council's Strategic Flood Risk Assessment

Spelthorne's Strategic Flood Risk Assessment is the evidence base behind every flood decision in the borough. The council holds a Level 1 SFRA, first produced in 2022 and updated in 2024, and a Level 2 SFRA that examines the higher-risk sites in more detail. Together they map the borough's flood zones, define the functional floodplain and drive the sequential test that steers development to lower-risk land.

The SFRA matters to an applicant because of how it defines Flood Zone 3b, the functional floodplain. Spelthorne's SFRA delineates 3b using the 1-in-20-year modelled flood extents from the Lower Thames, River Ash and Lower Colne models, and new development is not permitted there. The council's own analysis found that almost half of the borough's urbanised area would be affected by the 0.1% (1-in-1,000-year) flood, an extent that the 1% (1-in-100-year) flood is expected to mirror by around 2056 once climate change is taken into account. A credible flood risk assessment reads the SFRA against the site rather than relying on the national map alone. You can screen an address on our flood risk map before commissioning work.

What a Spelthorne application must include to be validated

Spelthorne sets its own local list of the documents an application must include to be validated, most recently amended in December 2023. On flood risk and drainage, it requires all development to demonstrate that land drainage will be adequate and that the scheme will not increase surface water runoff, with a flood risk assessment where the site or its scale triggers one. Submit without the right evidence and the application is invalidated before it is ever assessed.

• A flood risk assessment where the site falls in Flood Zone 2, 3a or 3b, or is 0.5 ha or larger in Flood Zone 1, or where surface water risk is mapped.
• A surface water drainage strategy on major development, showing adequate land drainage and no increase in runoff, with the Surrey SuDS pro-forma completed.
• A sequential test where any part of the site lies in a higher flood zone, justifying the location against reasonably available lower-risk alternatives.
• The right supporting evidence — ground investigation and infiltration testing where SuDS rely on soakaways, and a flood exceedance plan for the drainage design.

The most common reason a Spelthorne submission is delayed is a mismatch between the flood risk assessment and the drainage strategy, or a drainage strategy that does not follow Surrey's discharge hierarchy. Our guide on how a planning application gets invalidated for flood risk and drainage sets out how to avoid it.

How national planning policy applies in Spelthorne

National policy sits above Spelthorne's Local Plan and shapes what the council and Surrey can ask for. The National Planning Policy Framework and its Planning Practice Guidance set the flood risk and drainage tests; the borough applies them locally through Policy E3, the SFRA and Surrey's SuDS standards. An assessment has to satisfy both the national framework and the local detail.

Under the NPPF, applications in areas at risk must be supported by a site-specific flood risk assessment, must locate the most vulnerable development in the lowest-risk areas, must be safe for their lifetime, and must incorporate sustainable drainage unless there is clear evidence it would be inappropriate. The Planning Practice Guidance and the recent PPG updates on the sequential test add detail on how those judgements are made, and the national standards for sustainable drainage set the technical baseline that Surrey builds on. Because Spelthorne is so heavily constrained by flood risk, these national tests bite harder here than in most authorities.

Flood risk in Spelthorne's area

Spelthorne occupies a low, flat corner of the Thames valley between Staines-upon-Thames, Ashford, Sunbury and Shepperton, and river flooding dominates. The non-tidal River Thames runs along the borough's southern and western edge, joined by the River Colne, Colne Brook, Wraysbury River and the River Ash — the Colne System — with surface water and groundwater adding to the risk on the valley floor.

The borough has a long flood record, with major Thames events in 1947 and again in the winter of 2013–2014, when hundreds of Spelthorne properties flooded. The Environment Agency's flood risk guidance and the borough's SFRA both treat this history as evidence rather than background. The River Thames Scheme, which includes a new 3.2 km flood relief channel through Spelthorne, is designed to reduce fluvial risk to roughly 11,000 homes and 1,600 businesses between Egham and Teddington — but until it is built and operating, a site cannot rely on protection that is not yet there. For the full picture of how the borough floods, see our area pages on flood risk in Staines-upon-Thames and across Surrey.

How Unda helps with flood risk and drainage in Spelthorne

We act as the accurate translation layer between your scheme and what Spelthorne Borough Council, Surrey County Council and the Environment Agency require. We are experts on this authority's flood risk and drainage regime, not a generic report mill, and we know how closely a borough this exposed scrutinises a submission.

A flood risk assessment for a Spelthorne development from Unda assesses every source that matters here — the Thames, the Colne System and the River Ash, surface water on the valley floor, and groundwater rising through the gravels — compares modelled flood levels against your proposed floor levels, and sets out safe access, egress and mitigation the regulators will accept. Where the council and Surrey need it, a surface water drainage strategy sits alongside as a separate report, so the flood-risk case and the drainage case stay consistent and an objection has nowhere to land. Every report is overseen by a chartered flood risk consultant who stays involved past the report to answer consultee comments.

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

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports that answer different questions. We prepare all three, alongside the right form of assessment for buyers, lenders and BREEAM, for sites throughout the borough.

Flood risk assessments

• Flood risk assessment for planning. The core report, site-specific and prepared to satisfy the NPPF, the Environment Agency and Spelthorne Borough Council. Groundwater and, where needed, flood modelling are assessed within it.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged Thames or surface water risk and you need the actual building assessed, not a broad map.
• 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 report alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for Thames-gravel sites and any basement, assessed as a source within the flood risk assessment.

Sequential and exception tests

The sequential and exception tests are a separate, locational judgement about whether a development should be in a higher-risk location at all. The sequential test defines an area of search, identifies reasonably available lower-risk sites and justifies the proposed one; the exception test is the second gate where a scheme still needs to proceed. In a borough as constrained as Spelthorne, a clear sequential argument is often decisive. The how the tests work guide explains the sequence.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering that calculates existing versus proposed runoff and holds discharge to Surrey's greenfield rates.
• Flood modelling. 1D and 2D hydraulic modelling brought in where the national mapping is not enough to establish accurate Thames or Colne flood levels.

Flood risk and drainage in Spelthorne: frequently asked questions