Flood Risk Assessment in Leatherhead

A flood risk assessment in Leatherhead is a site-specific technical report that establishes how flooding could affect a development beside the River Mole and shows the scheme can be made safe for its lifetime without increasing risk to anyone else. Unda prepares flood risk assessments in Leatherhead for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS). Our flood risk consultants know the Mole catchment and Surrey planning policy well.

Leatherhead sits where the River Mole leaves the chalk of the North Downs and crosses onto London Clay, and that single fact shapes almost every flood question the town raises. The Mole is a main river with a long, well-recorded history of flooding through the town and neighbouring Fetcham. The chalk beneath it behaves unusually, swallowing flow underground and returning it through springs, and the built-up valley floor adds surface-water and sewer flooding on top. A report that treats Leatherhead as just another line on a flood map misses what the Environment Agency and Surrey County Council actually ask about.

Leatherhead spans all three flood zones: land along the River Mole and through Fetcham falls in Flood Zones 2 and 3, while the higher North Downs ground sits in Flood Zone 1.

Check flood risk anywhere in Leatherhead and the Mole Valley

Leatherhead and the surrounding Mole Valley are outlined below. Screen any postcode 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, useful for orientation, and is not a substitute for a site-specific flood risk assessment. For day-to-day river conditions you can also use our live river levels map and our free flood risk map.

Flood risk in Leatherhead and the River Mole

Flooding in Leatherhead comes mainly from the River Mole and from surface water, with groundwater a genuine third factor because of the chalk. The Mole is a main river running north through the town and Fetcham, so low-lying land and roads close to the channel flood first. In the most severe events, river, surface water and sewer flooding combine.

The River Mole rises near Gatwick and flows north through the Upper Mole catchment before reaching Leatherhead, where it has carved the Mole Gap through the North Downs between Dorking and Leatherhead. Through that gap the river runs over chalk and behaves in a way few English rivers do. A series of swallow holes take a large share of the flow underground, historically estimated at up to half the river in dry conditions, into the chalk aquifer. That water re-emerges downstream at springs south of Leatherhead and at the Fetcham springs feeding Fetcham Mill Pond, which have never been known to run dry; an 1883 survey put their output at around 3.6 million gallons a day.

At Leatherhead the river leaves the chalk and moves onto London Clay. The water table rises at that transition, much of the water that drained into the chalk returns to the channel through springs in the riverbed, and the ground stops absorbing rainfall the way the chalk does. The result is a valley floor through the town and Fetcham where fluvial, surface-water and groundwater risk overlap, which is why a flood risk assessment in Leatherhead has to look at every source rather than the river alone. We explain how groundwater flood risk is screened, and where a site sits in a critical drainage area, separately.

A history of flooding in Leatherhead and Fetcham

Leatherhead and Fetcham have flooded repeatedly, notably in 1947, 1968, 1990, 2000 and, most seriously in recent memory, over Christmas 2013. That record is why the Environment Agency runs a dedicated River Mole at Leatherhead and Fetcham flood warning area, and why a credible local flood risk assessment is expected to engage with the town's flood history rather than ignore it.

The standout recent event was the night of 24 December 2013. Around 45mm of rain had been forecast; about 75mm fell, the Mole corridor filled overnight and the river rose at least a foot an hour. Surrey County Council's subsequent Section 19 flood investigation for Mole Valley, published in October 2015, examined 81 flooded sites across 12 sub-areas and estimated the Leatherhead event at roughly a 1-in-9-year annual chance. That is sobering given the damage, because it was far from the worst the catchment can produce. In Fetcham the flooding came from surface water, the river and the foul sewer acting together, the kind of multi-source mechanism a desktop map never shows.

The autumn 2000 floods, England's wettest autumn in nearly 300 years, hit the wider Mole catchment hard, with the rapid runoff of the urbanised Upper Mole around Crawley and Gatwick a major factor. Nationally that event flooded more than 10,000 properties and helped drive the reforms that created Lead Local Flood Authorities and the Flood and Water Management Act 2010. We covered it in our piece on the 2000 Crawley and Gatwick floods.

• Christmas Eve 2013. About 75mm of rain against a roughly 45mm forecast, with the River Mole rising at least a foot an hour.
• Section 19 investigation (Mole Valley, October 2015). 81 flooded sites recorded across 12 sub-areas.
• A long record. Significant flooding noted in 1947, 1960, 1968, 1974, 1990, 2000, 2008 and 2013/14.
• A named warning area. The Environment Agency operates the "River Mole at Leatherhead and Fetcham" warning area, with a live river gauge in the town.
• Property Flood Resilience scheme. 170 Leatherhead and Fetcham properties were identified as eligible and 85 taken forward, with the Environment Agency funding up to about £7,500 of resilience measures per home.

Planning and drainage policy for a flood risk assessment in Leatherhead

In Leatherhead the Environment Agency is the statutory consultee for main-river flooding from the Mole, and Surrey County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses. A flood risk assessment here must satisfy the National Planning Policy Framework and its Planning Practice Guidance, Mole Valley's adopted Local Plan, and the surface-water expectations set by Surrey's drainage team and the 2025 SuDS National Standards.

A planning flood risk assessment is normally required for development in Flood Zone 2 or 3, on sites of one hectare or more even in Flood Zone 1, and increasingly on smaller sites where the Flood Map for Planning or the local Strategic Flood Risk Assessment shows surface-water risk, which is common on the Leatherhead valley floor. Since the September 2025 update to Planning Practice Guidance, the sequential, risk-based approach explicitly covers all sources of flooding, so a site sitting in Flood Zone 1 by the river can still attract scrutiny over surface water or groundwater.

The evidence base a Leatherhead report should engage with includes Mole Valley District Council's adopted Local Plan (adopted October 2024) and the joint Level 1 Strategic Flood Risk Assessment it shares with Reigate & Banstead and Tandridge, alongside the separate Crawley and Upper Mole catchment SFRA covering the river's headwaters. Surface-water drainage is judged against the drainage hierarchy and the 2025 National Standards for SuDS. On Leatherhead's chalk, infiltration is sometimes viable and sometimes defeated by a high water table, which is exactly the question that needs BRE 365 infiltration testing rather than assumption; our free BRE 365 calculator and ground conditions checker give an early read.

Local development and what it means for flood risk

Most Leatherhead development sits in or near the Mole valley floor, so flood risk and drainage tend to drive the brief. Riverside extensions and conversions, town-centre regeneration under the Transform Leatherhead programme, and basements cut into the chalk each raise specific questions about finished floor levels, safe access, compensatory storage and groundwater that the right report resolves early.

Householder extensions and conversions close to the Mole are common, and even a modest scheme on the valley floor can need a flood risk assessment if the Flood Map for Planning or the SFRA flags surface-water risk. Town-centre regeneration puts more vulnerable uses onto constrained brownfield land, where the sequential approach, finished floor levels and surface-water drainage all have to be argued carefully. Because the chalk lies close to the surface on the valley sides, basements and below-ground structures raise groundwater questions that national flood maps do not answer; here a dedicated groundwater assessment, often with a season of monitoring, earns its place. Where national mapping is too coarse for a riverside site, flood modelling can define levels and flow routes more precisely.

How Unda helps with a flood risk assessment in Leatherhead

Unda are experts on flooding and planning policy in Leatherhead and the wider Mole catchment, acting as the accurate translation layer between your scheme and what the Environment Agency, Surrey County Council and Mole Valley District Council expect. A Senior Flood Risk Consultant oversees and signs off every report, and we stay involved past submission to answer consultee objections.

We have prepared flood risk assessments and drainage strategies across Surrey and the Mole catchment, and we know how the river behaves: the swallow holes, the springs, and the way the channel and the sewers interact in Fetcham, rather than reading it off a national dataset. Since 2014 Unda has completed more than 5,000 flood risk assessments, is a CIWEM Business Partner with chartered consultants, and is rated 5.0 on Google.

Flood risk assessments for every situation in Leatherhead

Whatever the reason you need one, Unda prepares it. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct 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.

• Flood Risk Assessment for planning. The report submitted with a planning application to show a Leatherhead site is safe for its lifetime and does not increase risk elsewhere.
• Flood risk survey for property purchase. An independent, property-specific survey for buyers and their advisers when a search or lender flags flood risk.
• Flood Risk Assessment for insurance. Property-level evidence for brokers and underwriters where cover or premiums are in question.
• Flood Risk Assessment for BREEAM. Structured around BREEAM Pol 03 for project teams pursuing certification.
• Sequential and Exception Test reports. The locational justification when a site falls in a higher-risk flood zone.
• Sustainable drainage strategy and design (SuDS). How runoff is managed to meet the drainage hierarchy, with discharge of conditions and preliminary drainage design where needed.

Groundwater and flood modelling sit within the flood risk assessment rather than alongside it: groundwater is a source every assessment considers, and modelling is a technique that feeds one. Our drainage calculators help with an early sketch of the numbers.

Local flood risk management and emergency contacts for Leatherhead

Several bodies manage flood risk around Leatherhead. The Environment Agency covers the River Mole as a main river; Surrey County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses; Mole Valley District Council handles local drainage and emergency planning; and Thames Water is responsible for public sewers. In a flood emergency, call 999.

Frequently asked questions about a flood risk assessment in Leatherhead

Flood risk assessments across Surrey and the Mole catchment

Leatherhead is one of the Surrey towns we cover. For the wider county and neighbouring towns, see our pages on flood risk assessment in Surrey, Guildford and Godalming. Useful background reading includes how flood risk affects house prices, residual flood risk behind defences, and the Guildford flood alleviation scheme on the neighbouring River Wey.