Flood Risk Assessment in Surrey

A flood risk assessment in Surrey begins with the river a site drains to. The north of the county lies on the Thames floodplain from Staines-upon-Thames to Thames Ditton, while the Wey and the Mole run north across the county and break through the North Downs at Guildford and Dorking. Chalk springs, clay and overloaded drains add groundwater and surface water almost everywhere else. Our consultants prepare flood risk assessments in Surrey for planning applications, house purchases, lenders and insurers, together with the sequential tests, exception tests and surface water drainage strategies that often travel with them.

Surrey County Council is the Lead Local Flood Authority for the whole county and reviews the drainage on major schemes, while eleven district and borough councils decide planning applications under their own local plans. That arrangement ends on 1 April 2027, when West Surrey Council and East Surrey Council take over. Reigate and Redhill, Caterham and Farnham have no page of their own on this site, so they are covered here; Woking, Camberley and Epsom have pages of their own, and Horley has its own page.

Surrey County Council counts more than 85,000 properties in the county at high or medium risk from surface water and river flooding. The winter floods of 2013 to 2014 and Storm Henk in January 2024 flooded the inside of more than 2,300 of them.

To talk through a site anywhere in Surrey with one of our consultants, call 01293 214444 or email enquiries@unda.co.uk.

Thames, Wey or Mole: which river system drains a Surrey site?

Almost all of Surrey drains to the Thames, by very different routes. Along the northern edge the Thames itself, joined by the Colne, the Ash and the Bournes, rises slowly and stays high for days. The Wey and the Mole respond much faster, because much of their water comes off clay, and in the east the Hogsmill and the Caterham Bourne are fed by chalk springs. Around Lingfield, in the south-east corner, the Upper Medway Internal Drainage Board is one of the bodies managing the water.

Our river pages cover the River Thames, the River Wey, the River Mole, the Hogsmill River, the River Tillingbourne, Cranleigh Waters, Littlemead Brook, the Chertsey Bourne, the River Colne, the Wraysbury River, the Burstow Stream, the Salfords Stream, the Gatwick Stream and the River Lox on the West Sussex border. Gauges on most of them feed our live river levels map, which shows where Surrey's rivers stand today.

Screening a Surrey postcode before anything else

Enter a postcode below for the Environment Agency's flood zones, surface water extents and reservoir layers at that point, or open our full flood risk map for anywhere in England. Used as a Surrey flood map, it gives a first look at any address, but it reads national data only, so it cannot show a Caterham Bourne in flow, a dry island cut off by the Thames, or a soakaway sitting too close to the winter water table.

Flood Zone 1 does not settle the question here. Under its non-major guidance, Surrey County Council comments on smaller schemes inside the 1% annual chance surface water extent, at any depth, and on those next to a recorded watercourse, and several district plans add triggers of their own. The difference between the zones is set out in our guide to flood zones in England.

Who decides a Surrey application, and under which local plan?

Planning applications in Surrey go to the district or borough council for the site. Surrey County Council, as Lead Local Flood Authority, is the statutory consultee on surface water for major development, which in general means ten or more homes, 1,000 square metres of floor space or a site of a hectare. Each council reads the assessment against its own plan, and those plans stand at very different points: one was withdrawn in 2025, one was adopted in 2024, another in March 2026, and two more are still before an Inspector.

Our council pages go further on Spelthorne, Elmbridge, Mole Valley and Waverley, and across the London border the Royal Borough of Kingston upon Thames is its own lead local flood authority. Each Strategic Flood Risk Assessment maps flooding in more detail than the national map. Guildford's P4 and Woking's CS9 both restrict new building in the functional floodplain, Flood Zone 3b.

Surrey County Council's drainage strategy requirements

Since 6 October 2025, Surrey County Council has assessed every new consultation against the 2025 national standards for sustainable drainage. Its guidance for major development, last reviewed in September 2026, adds the county's own reading of those standards. One rule comes before all the others. Surrey is classed as a water-stressed area, so collecting rainwater for reuse is always required, whatever else the scheme does with its runoff.

• Rainwater reuse first. Collection for non-potable use is the first priority under the national standards and is always required in Surrey, usually with an outfall to the ground or off site as well. Our note on rainwater harvesting within SuDS explains how the two work together.
• Discharge at the 1 in 2 year rate. Where a scheme adds to the volume of runoff, the allowable discharge is usually limited to the 50% annual exceedance probability rate. Brownfield sites get a relaxation only in exceptional circumstances, and a lower maximum quoted by the sewerage company takes precedence.
• Infiltration evidence up front. Test results should come with the application, or a reason and a desk study where a working site cannot be dug. The standard condition then asks for BRE 365 tests, confirmed groundwater levels and at least 1 m of unsaturated ground beneath the base of any soakaway.
• Nothing into highway drains. New private surface water may not discharge to the highway drainage system or onto the road. A site that already connects to it has to agree the arrangement with the county's highways team.
• One section per standard. A major scheme's strategy should take each national standard in turn, with a non-technical summary, a plan of the areas draining to each SuDS feature, an exceedance routing plan showing levels and flow paths, and 10% added for urban creep.

The county sells pre-application advice at three levels, and its standard conditions end with a verification report before anyone moves in. Our guide to why lead local flood authorities object sets out the commonest grounds and how they are resolved.

West Surrey and East Surrey councils from April 2027

Surrey's twelve councils are being replaced by two unitary authorities. Councillors for West Surrey Council and East Surrey Council were elected on 7 May 2026 and sit as shadow authorities until vesting day, 1 April 2027, when the new councils take on every local service. Until then an application still goes to the existing district or borough council, and the drainage still goes to Surrey County Council.

• West Surrey Council. Guildford, Runnymede, Spelthorne, Surrey Heath, Waverley and Woking.
• East Surrey Council. Elmbridge, Epsom and Ewell, Mole Valley, Reigate and Banstead, and Tandridge.
• Planning and drainage together. Under the Flood and Water Management Act 2010 a unitary council is the lead local flood authority for its own area, so from vesting each new council will both decide applications and review surface water drainage.
• Schemes that straddle the date. An application submitted in early 2027 may be decided by the new council, which is why the plan and policy references in a report should match the table above and be checked again at submission.

Our article on local government reorganisation sets out what moves to a unitary council and what stays with the Environment Agency, and the councils' reorganisation hub publishes the transition timetable.

Surrey's flood record, from the county's own Section 19 reports

Surrey County Council investigates any flood that crosses its thresholds, such as ten or more properties flooded internally in one catchment, and publishes a Section 19 report. Read together, they show flooding in Surrey arriving in two kinds of event: long winter floods on the Thames and the Mole, and short, intense storms that overwhelm drains in places the flood zones never reach.

1. September 1968. The largest floods on record in the Wey catchment, after 110 mm of rain fell on the Upper Wey in 48 hours.
2. November 2000. The Wey at Guildford rose higher than it would at Christmas 2013, and at Byfleet nothing as bad came again until that Christmas.
3. Winter 2013 to 2014. About 1,170 incidents of internal flooding in Runnymede, 270 in Mole Valley, 180 in Tandridge, 160 in Guildford, around 130 properties in Spelthorne and 120 in Waverley, with fewer in Elmbridge, Reigate and Banstead, Woking, Surrey Heath and Epsom and Ewell.
4. May and June 2016. A flash flood on the Rive Ditch flooded 45 properties in Woking on 11 May and closed three schools; on 7 June a two-hour downpour flooded more than 86 homes and flats in Caterham on the Hill.
5. July 2021. Surface water reached homes in Nork and the Tattenhams, near Banstead, on 12 July, a year after they flooded in August 2020, and heavy rain on 25 July flooded more than 35 properties internally in Worcester Park.
6. November 2022. At least 25 properties were affected at Smallfield and Burstow, east of Horley, on 17 November.
7. January 2024, Storm Henk. Surface water came first, then groundwater, then the Thames. The county's Thames report logged 129 internal and 354 external flooding reports, and 17 homes flooded internally at Worplesdon.
8. August to November 2024. About 54 mm fell over Lingfield in two and a half hours on 1 August, flooding 54 properties; Guildford town centre flooded three times between August and October; and Storm Conall flooded about 39 properties internally at Wood Street Village on 27 November.

None of these counts appears on the national flood map. They are in the reports on the county's investigations page, and an assessment for a site near any of them should start there.

The Thames corridor, the River Thames Scheme and dry islands

Spelthorne, Runnymede and Elmbridge share the Thames between Staines-upon-Thames and Molesey, part of what Spelthorne Borough Council describes as the largest area of undefended, developed floodplain in England. About 1,000 homes in the River Thames Scheme area flooded from the river in 2014, the council records. The county's Storm Henk report places much of Egham, Staines-upon-Thames and Chertsey in Flood Zone 3, and Upper Halliford and Sunbury wholly within Flood Zones 2 and 3.

The River Thames Scheme, led by the Environment Agency and Surrey County Council, would cut a new flood channel in two sections, through Runnymede from Egham Hythe to Chertsey and through Spelthorne from Littleton North lake to the Desborough Cut, and add capacity at the Desborough Cut and the Sunbury, Molesey and Teddington weirs. Runnymede's policy EE13 safeguards its route. It has no development consent yet, and our article on why the River Thames Scheme is delayed follows its progress. Until the channel exists, an assessment here describes the risk without it.

EE13 also requires an assessment for development in a dry island, ground outside the flood zones but surrounded by them; only minor development is excluded. On those sites, and on any that need the exception test, the council weighs how fast flooding would arrive, how long it would last and whether people could be evacuated. Our note on dry island sites explains why, and the Staines, Egham, Chertsey, Weybridge, Walton-on-Thames, Molesey and Thames Ditton pages take each stretch of the river in turn.

Upstream on the Wey and the Mole

Away from the Thames the rivers move faster. The Wey drains 904 square kilometres of Surrey, Hampshire and West Sussex, and before Guildford flooded at Christmas 2013 flows on the Tillingbourne at Shalford and Cranleigh Waters at Bramley rose sharply together. The Mole drains Weald and London Clay and the hard surfaces of Gatwick and Crawley, so it can go from a quiet channel to a bank-full river quickly, through Horley, Dorking and Leatherhead.

The Environment Agency, Guildford Borough Council and Surrey County Council are developing a flood alleviation scheme for the centre of Guildford, and our article on the Guildford scheme explains where it stands. A defended property can stay in Flood Zone 2 or 3, as our note on still being in a flood zone after new defences explains, so a scheme does not remove the need for an assessment. Guildford, Godalming and Cranleigh on the Wey side, and Dorking, Leatherhead, Cobham and Esher on the Mole, each have their own page.

Groundwater on the North Downs chalk

Groundwater floods in Surrey where the chalk of the North Downs fills after a long wet spell, and in places on the Weald where springs break out at the junction of sandstone and clay. The Caterham Bourne is the clearest case. Dry most of the time, it is fed by springs once groundwater reaches the surface, and Tandridge's Section 19 report records heavy flows about once every seven years along the A22 through Whyteleafe. Before the Thames rose in January 2024, groundwater was already high across the region.

Surrey County Council is not a statutory consultee on groundwater, but its guidance expects applicants to allow for it, and the 1 m clearance below a soakaway depends on knowing the seasonal high level. Surrey Heath's January 2025 Level 1 SFRA includes groundwater emergence mapping, and the county accepts deep bore soakaways in some areas, subject to Environment Agency approval. Our guides to groundwater flooding, groundwater monitoring for planning and BRE 365 infiltration testing cover the evidence, and the ground conditions checker gives a first view of the geology under a site.

Is a flood risk assessment in Surrey required for my proposal?

Nationally, chapter 18 of the August 2026 NPPF sets the triggers in policy F4: development in Flood Zones 2 and 3, sites of a hectare or more in Flood Zone 1, and land the flood map or the council's SFRA shows at risk from another source. Several Surrey councils add rules of their own, so a scheme that needs no report in one borough can need one in the next.

A site whose only risk is surface water may not need the sequential test if the assessment shows the development will be safe; our note on the surface water exemption explains policy F5, and our guide to the August 2026 NPPF changes covers the rest of chapter 18.

What we prepare for Surrey sites, from Staines-upon-Thames to Lingfield

The right report depends on what floods the site and who will read the result: a case officer, the county's drainage team, a lender or an insurer. Every report goes out under a senior consultant's sign-off; our consultants are chartered and Unda is a CIWEM Business Partner.

• Every source together. The Thames, a Wey tributary, a chalk spring and an overloaded drain can all reach one Surrey plot, and a flood risk assessment weighs them in a single report.
• Planning applications. From a Spelthorne replacement dwelling to a Mole Valley allocation, a flood risk assessment for planning answers the borough's flood policy and chapter 18 of the NPPF.
• Buying a home. With more than 85,000 Surrey properties at medium or high risk, a flood risk survey for property purchase explains what a search or a lender has flagged before exchange.
• Lower-risk alternatives. On a Surrey site in Flood Zone 2 or 3, the sequential and exception test shows whether other land is reasonably available and, where it is needed, what the scheme gives back.
• Drainage evidence. Major schemes go to Surrey County Council for comment, and a drainage strategy shows how the site meets the national standards and the county's reading of them.
• Runoff from the site. A surface water drainage strategy puts rainwater reuse first, then infiltration or a controlled discharge, held to the 1 in 2 year rate on most sites, with the calculations the county expects.
• After permission. The county's standard pre-commencement condition asks for test results, groundwater levels and full calculations; discharge of drainage conditions supplies them, and the verification report follows before occupation.
• Insurance. After the 2013 to 2014 floods and Storm Henk flooded more than 2,300 Surrey properties internally, a flood risk assessment for insurance gives an underwriter evidence about the building itself.
• Town-centre premises. Premises around Friary Street and the High Street in Guildford flooded at Christmas 2013, and a business continuity flood risk assessment shows an occupier what the next flood would mean for trading.
• Chalk and springs. On the North Downs chalk and along the Caterham Bourne, a groundwater flood risk assessment establishes the seasonal high level that a basement or soakaway has to respect.
• Site-level flood levels. Where national data cannot show how the Thames, the Wey or the Mole behaves at a particular site, flood modelling produces the levels the assessment relies on.
• Getting people out. Runnymede's EE13 asks whether people on dry-island and exception-test sites could be evacuated, and Guildford's P4 asks for warning and evacuation plans where relevant: a flood warning and evacuation plan answers both.
• National policy. Our NPPF guidance sets out chapter 18 and its policies F4 to F8, which every Surrey council applies alongside its own plan.

Three Surrey jobs show how this works in practice: an Environment Agency objection withdrawn on a mixed-use scheme beside the Wey in central Guildford, padel courts approved at a Weybridge health club with four recorded floods, and a new-build purchase completed in a Surrey village whose 2013 to 2014 flood history did not reach the plot.

More Surrey towns and their flood evidence

Several of Surrey's larger towns are covered here rather than on a page of their own, and Camberley now has one. This is what their own flood evidence shows:

• Woking. The Rive Ditch flash flood of 11 May 2016 flooded 45 properties around Maybury and closed three schools, and at Christmas 2013 Byfleet had its worst flooding since 2000. Core Strategy policy CS9 asks for an assessment next to areas at risk of surface water flooding.
• Epsom and Ewell. The Hogsmill rises from springs at Bourne Hall in Ewell. Policy DM19 gives medium or high risk from surface water, groundwater or any other source the same weight as river risk when deciding whether the sequential approach and a site assessment apply.
• Reigate, Redhill and Banstead. The Redhill Brook and the Earlswood Brook feed the Salfords Stream, along which the Environment Agency runs several flood warning areas, and in Banstead homes in Nork and the Tattenhams flooded from surface water in 2020 and 2021. Policy CCF2 asks for an assessment on critical drainage sites as well as in the flood zones.
• Caterham, Oxted and Lingfield. Tandridge District Council, with its offices at Oxted, covers the Caterham Bourne and the June 2016 flood in Caterham on the Hill. Lingfield flooded on 1 August 2024, with the Upper Medway Internal Drainage Board and Southern Water among the bodies involved. Policy DP21 holds surface water discharge to pre-development rates.
• Farnham and Haslemere. Both are in Waverley, where policy CC4 applies and the council's validation list asks for a surface water drainage strategy at every scale where surface water is created or affected. The north branch of the Wey runs through Farnham.
• Camberley. Surrey Heath's January 2025 Level 1 SFRA and its 2025 Level 2 assessment support the Local Plan 2019 to 2038, which is waiting for the Inspector's report; until it is adopted, policy DM10 of the 2012 Core Strategy applies.

Surrey's town, borough and council pages

Each of these pages goes further into one place's rivers, flood history and council rules:

• The Thames. Staines, Egham, Chertsey, Addlestone and the wider borough of Runnymede, then Weybridge, Walton-on-Thames, Molesey and Thames Ditton.
• The Mole. Dorking, Leatherhead, Cobham and Esher, with Oxshott between them.
• The Wey and its tributaries. Guildford, Godalming and Cranleigh.
• Councils. Spelthorne, Elmbridge, Mole Valley and Waverley, and Kingston upon Thames over the London border.
• Neighbouring counties. Berkshire upstream on the Thames, Hampshire to the west, West Sussex to the south and Kent to the east; every county and town we cover is on our areas list.

Reporting a flood in Surrey: who to contact

In Surrey, as anywhere, a flood that threatens life is a 999 call, and electricity cuts go to 105. Which other body to call turns on the source: a river, a road drain, a sewer or water rising out of the ground.

The county's Local Flood Risk Management Strategy, refreshed in 2025, explains how these bodies divide up Surrey flooding work through the Surrey Flood Risk Partnership Board.

Questions about flood risk assessments in Surrey

Our consultants assess flood risk, run sequential and exception tests and prepare drainage strategies for sites across Surrey, from Staines-upon-Thames to Lingfield and from Farnham to Epsom. To discuss one, call 01293 214444 or email enquiries@unda.co.uk.