Flood Risk Assessment in Surrey

Unda prepares comprehensive flood risk assessments and drainage strategies for residential, commercial and mixed-use sites right across Surrey. A flood risk assessment in Surrey shows your local planning authority that a development will stay safe from flooding for its whole lifetime, and that it will not increase flood risk anywhere else.

Surrey floods from several directions at once. The Thames runs along the county's northern edge through Spelthorne, Runnymede and Elmbridge; the Wey, Mole, Hogsmill and Bourne drain the centre and south; surface water overwhelms drains in the towns after intense rain; and groundwater rises out of the North Downs chalk. We assess every source that matters on your site, then deal directly with the Environment Agency and with Surrey County Council in its role as Lead Local Flood Authority, guiding architects, planning consultants, developers and landowners through to approval.

If you need a quote or want to talk a Surrey site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Surrey at a glance

• Most sites face more than one source. River, surface water and groundwater flooding all occur across Surrey, and Thames-valley and town-centre schemes often face two or three together.
• 340 properties flooded in Egham and Staines during the 2013–14 winter floods, with around 1,170 internal-flooding incidents recorded in Runnymede alone.
• Surrey County Council is your Lead Local Flood Authority, and a statutory consultee on surface water drainage for major development, with its own SuDS design guidance and a Surface Water Drainage Statement pro-forma.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, surface water risk can trigger an assessment even on a Flood Zone 1 site.

Check flood risk anywhere in Surrey

Surrey is outlined below. Screen any postcode in the county against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass, not a substitute for a site-specific assessment.

Flood risk across Surrey: rivers, surface water and groundwater

Surrey faces three main sources of flooding. River flooding dominates the Thames valley in the north and the Wey and Mole catchments through the centre; surface water now threatens almost every built-up area; and groundwater rises out of the North Downs chalk. Most Surrey sites face more than one source, which is why a desktop map rarely satisfies a planning officer on its own.

The northern third of the county drains to the Thames, which forms the boundary through Spelthorne, Runnymede and Elmbridge and floods the low-lying ground around Staines, Egham, Chertsey and Walton when prolonged rain falls on an already-wet catchment upstream. Central and southern Surrey drain through the Wey, which runs through Godalming and Guildford, and the Mole, which rises near Gatwick and cuts through the North Downs at Dorking and Leatherhead. The Hogsmill, Bourne, Eden and Blackwater add a dense network of smaller watercourses. Working out which one drives the risk on a given site, and how it behaves, is the heart of a credible fluvial flood risk assessment.

• Rivers (fluvial). The Thames floods the valley towns of Staines, Egham, Chertsey and Walton; the Wey affects Guildford and Godalming; and the Mole is known for rapid flash flooding through Dorking, Leatherhead and Horley. Land in Flood Zones 2 and 3 along these corridors almost always needs an FRA.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in Surrey's towns; in 2019 and 2021 sudden downpours flooded property and roads in Godalming, Dorking, Guildford and Leatherhead. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even on a Flood Zone 1 site.
• Groundwater. The Chalk of the North Downs raises water tables that flood low-lying ground, roads and basements without ever showing on a river map — the A25 between Reigate and Dorking is a recurring example, where water emerges at the chalk and gault-clay boundary. Critical for any soakaway or below-ground level. See groundwater flooding.
• Ordinary watercourses and sewers. Many Surrey sites sit beside a ditch, brook or culvert classed as an ordinary watercourse, which brings riparian responsibilities and, for works, consent from the County Council rather than the Environment Agency.

Because these sources interact, a single national map is rarely enough. Our notes on the sources of flood risk and on critical drainage areas explain how a Flood Zone 1 site can still need assessment, and why the right starting point is the Environment Agency mapping read alongside the local Strategic Flood Risk Assessment.

A history of flooding in Surrey

These dated events are why authorities here scrutinise development so closely, and why a flood risk assessment in Surrey has to be evidence-led rather than generic. Each one reshaped local policy and the defences a developer now has to design around.

• 2013–14 — the Thames winter floods. The wettest UK winter since records began in 1766 brought two major incidents and 14 severe flood warnings to Surrey in two months. Around 340 properties flooded in Egham and Staines, with roughly 1,170 internal-flooding incidents in Runnymede, about 130 in Spelthorne and around 100 in Elmbridge; over 1,100 people were rescued in six days in February.
• 2019 and 2021 — Mole and Wey flash floods. Sudden, intense downpours sent the Mole and Wey over their banks, flooding property and roads through Godalming, Dorking, Guildford and Leatherhead and showing how quickly central Surrey responds to rainfall.
• March 2024 — Guildford and the Wey. Renewed flooding through central Guildford, the most recent in a sequence stretching back through 2013–14 and 2000 to the major flood of 1968, underlined the persistent fluvial risk that the town's alleviation scheme is intended to address.
• Recurring groundwater flooding. After wet winters, the North Downs chalk drives slow, persistent groundwater flooding across Tandridge, Mole Valley and Reigate & Banstead that can last for weeks after the rain stops.

A pattern runs through all of it: ground that flooded once tends to flood again. A robust assessment treats this recorded history as evidence, not background colour.

Do you need a flood risk assessment in Surrey?

You need a flood risk assessment for most development in Surrey within Flood Zones 2 or 3, for sites of one hectare or more in Flood Zone 1, and increasingly for smaller Flood Zone 1 sites where the Flood Map for Planning shows surface water risk or the local SFRA flags a critical drainage area. If you are unsure, send us the address and we will tell you quickly whether one is required.

The detail expected scales with the application: an outline scheme needs to show the principle is sound, a full application needs design and calculations, and discharge of a condition needs full technical compliance. Our guide to whether you can do your own assessment explains where the line falls, and the difference between the public flood-risk map and the Flood Map for Planning explains why the right dataset matters.

Planning policy and Surrey County Council as your Lead Local Flood Authority

Two kinds of authority set the rules in Surrey. The Environment Agency leads on the main rivers — the Thames, the Wey and the Mole. Surrey County Council is the Lead Local Flood Authority (LLFA), leading on surface water, groundwater and ordinary watercourses, and acting as a statutory consultee on surface water drainage for all major development.

Surrey County Council as Lead Local Flood Authority

For major development — 10 or more dwellings, sites of 0.5 hectares or more, or 1,000 square metres or more of floor space — Surrey County Council is a statutory consultee, and its advice on surface water drainage is a material consideration the borough or district must take into account. The County Council publishes separate SuDS design guidance for major and non-major development and asks developers to submit its Surface Water Drainage Statement pro-forma, which records how a scheme follows the drainage hierarchy and the national SuDS standards. An LLFA objection usually stops the application being determined until it is resolved, so getting the strategy right first time is the surest way to keep a scheme moving.

• The Surrey SuDS pro-forma. The Surface Water Drainage Statement pro-forma is completed for major applications and used by the County Council in its statutory-consultee role; it follows the national non-statutory technical standards and the Defra/EA rainfall runoff guidance.
• Greenfield runoff rates. Surrey expects discharge to be as close to greenfield as possible and no greater than existing rates for all storm events, with the proposed 1-in-100-year-plus-climate-change rate no greater than the existing 1-in-100 rate.
• The drainage hierarchy. Infiltration first, then a watercourse, then a surface water sewer, then a combined sewer — with each step down justified by evidence such as BRE 365 infiltration testing.
• Pre-application advice. Surrey County Council offers a chargeable pre-application SuDS service and can be reached on SuDS matters at SuDS@surreycc.gov.uk; early engagement reduces time at the formal consultation stage.
• The four pillars. Schemes are expected to deliver water quantity, water quality, amenity and biodiversity together, in line with the four pillars of SuDS and the 2025 national standards.

The Surrey Local Flood Risk Management Strategy

The County Council's Local Flood Risk Management Strategy 2017–2032 sets out how the responsible authorities manage flooding from rivers, surface water and groundwater across the county, supported by Surface Water Management Plans in Woking, Guildford and Epsom & Ewell and by Section 19 investigations after significant events. These local documents, alongside the borough Strategic Flood Risk Assessments, are where the specific evidence for your site is most likely to be found.

Borough Strategic Flood Risk Assessments and Local Plans

Surrey is a two-tier county, so the planning authority is your borough or district council — one of Elmbridge, Epsom & Ewell, Guildford, Mole Valley, Reigate & Banstead, Runnymede, Spelthorne, Surrey Heath, Tandridge, Waverley or Woking. Each holds its own Strategic Flood Risk Assessment and adopted Local Plan flood policies, which can be more demanding than national policy. The Guildford Borough Council SFRA, for example, identifies parts of the town centre as Flood Zone 3b functional floodplain — a designation that shaped the Unda project described below.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — as well as this local framework. Where a higher-risk site is involved, the Sequential and Exception Tests apply and the right vulnerability classification has to be argued.

What your flood risk assessment in Surrey must show

• Risk assessed from every credible source, using the Flood Map for Planning and local data.
• An appropriate location, tested against the Sequential Test where policy requires it.
• Safe for its lifetime, once the Environment Agency climate change allowances are applied — generally 100 years for housing.
• No increase in flood risk elsewhere, with attenuation to greenfield rates and, where needed, compensatory storage.
• Residual risk managed, with safe access, egress and finished floor levels, the area where many defended-site applications are won or lost.

Development and flood risk assessment in Surrey

Surrey is one of the most development-pressured counties in the South East, and the kind of site you bring forward shapes the assessment you need. The right scope is the difference between a clean approval and a drainage objection that holds an application up for months.

• Town-centre and brownfield regeneration. Schemes in Guildford, Woking, Staines and Redhill often sit in Flood Zones 2 and 3, needing finished floor levels, safe access and egress, and frequently compensatory storage on tight urban plots.
• Thames-valley sites. Land around Staines, Egham, Chertsey and Walton carries some of the county's highest fluvial risk, where the River Thames Scheme is intended to lower flood levels but has yet to be built — so residual risk must still be assessed.
• Green Belt and edge-of-settlement release. Much of Surrey is Green Belt, so released sites are scrutinised closely and surface water and ordinary watercourses often drive the drainage strategy.
• Rural and chalk-downland schemes. Conversions and new builds across the North Downs and Surrey Hills frequently turn on groundwater and infiltration feasibility rather than a main river — check the geology early with our ground conditions checker.

Climate change is not optional here. A Surrey assessment must apply the Environment Agency's allowances for higher river flows and rainfall over the development's lifetime, which sets floor levels, freeboard and how much attenuation a drainage scheme needs.

Home buyers and businesses come to us too. A flood-risk question can affect house prices and insurance, and a conveyancing search is not the same thing as a planning-grade assessment. Our flood risk survey for property purchase and insurance flood risk assessment are built for exactly those situations.

How Unda helps with flood risk assessment in Surrey

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Surrey County Council as LLFA and national planning policy. We are experts on the county's flooding, not a generic report mill, and we stay involved past the report to answer objections.

A flood risk assessment in Surrey from Unda assesses every credible source — the Thames, Wey or Mole, down to surface water, groundwater, sewers and any ordinary watercourse on the boundary — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report. Where the council needs it, the drainage strategy sits alongside the assessment so the two cases stay consistent and an objection has nowhere to land.

The full write-up is in our project note, Environment Agency objection withdrawn for a mixed-use redevelopment in Flood Zones 2 and 3, which shows how a planning-compliant assessment supported by current modelling moves a scheme from objection to approval.

Our flood risk services for Surrey

• Flood risk assessment for planning. Site-specific reports that satisfy the Environment Agency and your Surrey borough or district council.
• Surface water drainage strategy and SuDS. Designed to Surrey County Council's standards and pro-forma from the first sketch.
• Groundwater flood risk assessment. A defensible design water table for North Downs chalk and basement sites, not a guess.
• Sequential and Exception Test reports. Prepared alongside the assessment where policy requires them.
• Flood modelling. Hydraulic modelling of the Thames, Wey or Mole where national mapping does not reflect local conditions.
• Discharge of drainage conditions. The detailed schemes needed to clear pre-commencement drainage conditions, and preliminary drainage design for outline applications.
• Chartered flood risk consultants. Specialists who sign their advice; read more about Unda.

Surrey flood resources and contacts

The most reliable Surrey flood information comes from the lead authorities. For an immediate threat to life during a flood, always call 999.

• Check for flooding. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk.
• Surrey County Council flooding and SuDS. The county's SuDS planning advice and pre-application service, and the Local Flood Risk Management Strategy.
• Borough Strategic Flood Risk Assessments. Each Surrey council publishes its own, such as the Guildford Borough Council flood risk pages.
• The Guildford Flood Alleviation Scheme. Background and consultation in our note on the Guildford scheme and the River Wey.
• Live river levels. Whether the Thames, Wey or Mole are running high right now via our live river levels map, and a first-pass postcode screen on our flood risk map.

For flooding from blocked or burst sewers, contact Thames Water or your local sewerage undertaker. For broader policy context, our notes on the Environment Agency's planning role, flood zones 1, 2 and 3 and residual flood risk are a good starting point.

Flood risk assessment in Surrey: frequently asked questions