Flood Risk Assessment in Cobham

Unda prepares comprehensive flood risk assessments and drainage strategies for residential, commercial and mixed-use sites in Cobham and across the wider Mole valley. A flood risk assessment in Cobham 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.

Cobham sits on the River Mole, which swings through the town in a pronounced meander past Painshill before heading north to the Thames. The Mole is one of Surrey’s most flood-prone rivers, and there are no flood defences in the town itself, so river levels are felt directly along Downside, Stoke d’Abernon and the streets near the bridge. Add the surface water that collects in the town’s low points after heavy rain, and groundwater rising through the gravels that overlie the London Clay, and most sites here face more than one source at once. We assess every source that matters on your site, then deal directly with the Environment Agency and with Surrey County Council as Lead Local Flood Authority, guiding architects, planning consultants, developers and landowners through to approval.

In 1968 a single storm put much of Cobham under water, and the River Mole overtopped again through the winter of 2013–14 — in a town with no formal flood defences, the case for getting flood risk right is written into its own history.

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

Flood risk assessment in Cobham at a glance

• The River Mole runs through the heart of the town. Cobham lies on the Mole, a river known for rapid, ‘flashy’ flooding, and land along the Stoke d’Abernon, Downside and town-bridge corridors falls within Environment Agency Flood Zones 2 and 3.
• There are no flood defences in Cobham. Unlike the defended towns lower down the Mole, Cobham relies on natural floodplain and finished-floor design rather than built defences, so a site-specific assessment carries the safety case.
• Surrey County Council is your Lead Local Flood Authority. It is a statutory consultee on surface water drainage for major development, with its own SuDS guidance and a Surface Water Drainage Statement pro-forma; Elmbridge Borough Council is the planning authority.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, surface water or groundwater risk can trigger an assessment even on a Cobham site that screens as Flood Zone 1.

Check flood risk anywhere in Cobham

Cobham sits within the Borough of Elmbridge, outlined below. Screen any postcode in and around the town 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.

Flooding in Cobham: the River Mole, surface water and groundwater

Cobham faces three main sources of flooding. River flooding from the Mole dominates the low ground along the valley through Stoke d’Abernon, the town centre and Downside; surface water collects in the town’s natural low points after intense rain; and groundwater rises through the river gravels that sit over the London Clay. Most Cobham sites face more than one of these, which is why a desktop map rarely satisfies a planning officer on its own.

The Mole leaves the chalk of the North Downs near Leatherhead and meanders north across an alluvial plain towards Cobham, where it loops round in a tight ‘axehead’ bend skirting Painshill Landscape Garden. Between Cobham and Esher the river’s old courses have laid gravel over the London Clay, generally two-and-a-half to seven metres deep, and it is this geology — free-draining gravel above an impermeable clay — that governs whether a soakaway will work and how high groundwater can rise. Working out which source drives the risk on a given site, and how it behaves, is the heart of a credible fluvial flood risk assessment.

• Rivers (fluvial). The River Mole floods the valley floor through Stoke d’Abernon, the town bridge and Downside, with Downside Road and Downside Bridge Road among the first places to feel high levels. Land in Flood Zones 2 and 3 along the river almost always needs an FRA.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains and pools in Cobham’s topographic low points; in the 2013–14 floods it was not always possible to separate river water from surface water across the middle Mole. 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 gravels over London Clay raise water tables that flood low-lying ground and basements without ever showing on a river map — a particular issue for Cobham’s many basement and lower-ground-floor schemes. See groundwater flooding.
• Ordinary watercourses and sewers. Many Cobham sites sit beside a ditch, brook or culvert classed as an ordinary watercourse, which brings riparian responsibilities and, for works, consent from Surrey 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 Elmbridge Strategic Flood Risk Assessment.

A history of flooding in Cobham

Cobham has a long, well-documented flood record on the River Mole. These dated events are why the authorities scrutinise development here so closely, and why a flood risk assessment in Cobham has to be evidence-led rather than generic. Each one is a reminder that ground which flooded once tends to flood again.

• 1968 — the great Mole flood. A rare storm of roughly 1-in-200-year severity dropped more than 100mm of rain in 24 hours and pushed the lower Mole past 240 cubic metres a second. Much of Cobham and East Molesey was submerged, thousands of homes were damaged, and new defences were built downstream in response — but not at Cobham.
• 2013–14 — the winter floods. Storm after storm between mid-December 2013 and early January 2014 brought parts of South East England around two-and-a-half times their normal rainfall. The Mole overtopped its banks, and on 13 December 2013 the gauge near the Mole’s confluence with the Thames recorded its highest peak in 16 years; at Esher the event was assessed at around a 1-in-50-year chance.
• Recurring surface-water and groundwater flooding. After prolonged wet weather, water pools in Cobham’s low points and rises through the valley gravels, affecting roads, gardens and basements well away from the river itself.

A pattern runs through all of it: the absence of formal defences in Cobham means the safety of a new scheme rests on where it sits, how high its floor levels are set, and how its drainage behaves — exactly what a robust assessment is built to demonstrate. The town and surrounding valley fall within the Environment Agency’s River Mole at Stoke d’Abernon, Cobham and South Hersham flood warning area.

Do you need a flood risk assessment in Cobham?

You need a flood risk assessment for most development in Cobham 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 Elmbridge SFRA flags a drainage constraint. 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: Elmbridge Borough Council and Surrey County Council as your Lead Local Flood Authority

Two kinds of authority set the rules for a Cobham site. The Environment Agency leads on the River Mole as a main river. 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 major development. Your planning application itself is determined by Elmbridge Borough Council.

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 Elmbridge must take into account. An LLFA objection usually stops the application being determined until it is resolved, so getting the drainage strategy right first time is the surest way to keep a scheme moving.

• The Surrey SuDS pro-forma. Surrey County Council asks developers to complete its Surface Water Drainage Statement pro-forma and design to its SuDS design guidance, recording how a scheme follows the drainage hierarchy and the national standards.
• Greenfield runoff rates. Surrey expects discharge to be as close to greenfield as possible, 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. Cobham’s gravel-over-clay ground makes that testing especially worth doing early.
• 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.

Elmbridge Borough Council, the SFRA and the Local Plan

Elmbridge Borough Council determines planning applications in Cobham and holds the local flood-risk evidence. Its Level 1 Strategic Flood Risk Assessment (2024) is a ‘living’ document covering flood risk from all sources across the borough, with fluvial flood-zone mapping, modelled outlines and surface water mapping, and it is the first place to look for the specific evidence on your site. The borough’s Flood Risk Supplementary Planning Document sets out what the council expects an application to demonstrate. Your assessment must satisfy national policy — the NPPF and its Planning Practice Guidance — as well as this local framework, and where a higher-risk site is involved the Sequential and Exception Tests apply.

What your flood risk assessment in Cobham must show

• Risk assessed from every credible source, using the Flood Map for Planning and the Elmbridge SFRA.
• 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 in the Mole floodplain.
• Residual risk managed, with safe access, egress and finished floor levels — the area where many undefended-site applications are won or lost. Read more on residual flood risk.

Development and flood risk assessment in Cobham

Cobham is one of the most sought-after places to build in Surrey, 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.

• Replacement dwellings and basements. Much of Cobham’s development is the replacement or extension of large homes, often with basements or lower-ground floors — where groundwater in the valley gravels and safe access in a flood become the deciding issues.
• Riverside and town-centre sites. Plots along the Mole through Stoke d’Abernon, the town bridge and Downside sit in Flood Zones 2 and 3, needing finished floor levels, safe access and egress and, on tight plots, compensatory storage.
• Green Belt and edge-of-settlement sites. Much of the land around Cobham, Oxshott and Downside is Green Belt, so released sites are scrutinised closely and surface water and ordinary watercourses often drive the drainage strategy.
• Infiltration-led drainage. The river-terrace gravels can suit soakaways where the London Clay does not, so early permeable paving and infiltration testing can unlock a simpler drainage solution — check the geology first with our ground conditions checker.

Climate change is not optional here. A Cobham 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.

Flood risk assessments for every situation in Cobham

People come to us for a flood risk assessment in Cobham for many reasons — securing planning permission, buying a home, satisfying a mortgage lender or insurer, achieving a BREEAM rating, or designing the drainage for a new scheme. It helps to keep three things separate: a flood risk assessment, a sequential or exception test, and a drainage strategy are three different 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. Groundwater and flood modelling are not separate reports: groundwater is a source every assessment considers, and modelling is a technique that feeds one.

• Flood risk assessment for planning. The site-specific report that satisfies the NPPF and shows your development is safe for its lifetime without increasing risk elsewhere.
• Flood risk survey for property purchase. An independent, property-specific survey for buyers and their advisers, going beyond the conveyancing search to assess the actual building.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover has been restricted, declined or priced on flood risk.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 so a scheme keeps its flood and surface-water credits.
• Sequential and exception tests. The locational justification that a higher-risk site is the most appropriate available, prepared alongside the assessment.
• Groundwater flood risk assessment. A defensible design groundwater level for Cobham’s gravels, tested against floor levels, basements and drainage.
• Flood modelling. 1D and 2D hydraulic modelling of the Mole where national mapping does not reflect how water behaves on the ground.
• Surface-water drainage strategy (SuDS). A scheme that manages runoff to Surrey’s standards, following the drainage hierarchy and the four pillars of SuDS.
• Discharge of drainage conditions. The detailed design that clears a pre-commencement or pre-occupation drainage condition, with preliminary drainage design for outline applications.

How Unda helps with flood risk assessment in Cobham

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 River Mole and how it floods through Cobham, not a generic report mill, and we stay involved past the report to answer objections.

A flood risk assessment in Cobham from Unda assesses every credible source — the Mole, surface water, groundwater in the valley gravels, 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. You can read more about Unda and our chartered flood risk consultants, or browse the wider flood risk assessment in Surrey service that this page sits within.

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Cobham flood resources and contacts

The most reliable Cobham 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, including the River Mole at Stoke d’Abernon, Cobham and South Hersham warning area.
• Surrey County Council flooding and SuDS. The county’s SuDS planning advice and its Local Flood Risk Management Strategy; report flooding from surface water or an ordinary watercourse to the County Council.
• Elmbridge Borough Council. The borough’s Strategic Flood Risk Assessment and Flood Risk SPD set out what a local application must address.
• Floodline. Round-the-clock advice and flood-warning sign-up on 0345 988 1188.
• Sewer flooding. For flooding from blocked or burst sewers, contact Thames Water; for power cuts in a flood, call 105.
• Live river levels. Whether the Mole is running high right now via our live river levels map, and a first-pass postcode screen on our flood risk map or drainage calculators.

Flood risk assessment in Cobham: frequently asked questions

Talk to an experienced flood risk consultant about your Cobham site

We have prepared flood risk assessments and drainage strategies across Cobham and the Mole valley, from Stoke d’Abernon and the town bridge to Downside and Oxshott. Get a free, no-obligation quote and a consultant will call you back within 60 minutes.