Flood Risk Assessment in Cobham

A flood risk assessment in Cobham starts with the River Mole, which loops through the town past Downside Bridge and Cobham Mill with no flood defences on either bank. Mill Road was shut by floodwater in the winter of 2013/14 and flooded again in December 2019, and the High Street was cordoned off in February 2020. Our consultants prepare flood risk assessments in Cobham for planning, for buyers and their lenders and for insurers, together with sequential and exception tests and surface water drainage strategies.

Cobham, on this page, is the town as the Office for National Statistics maps it inside Cobham and Downside ward: the High Street and the streets around it, Church Cobham by the river, Cobham Mill and part of Fairmile beside the A3. The same built-up area continues east through Stoke D'Abernon to Oxshott, which our flood risk assessment in Oxshott page covers. Downside village lies south of the outline but shares the KT11 postcode, the river and the council, so it is covered here as well. Elmbridge Borough Council decides planning applications, Surrey County Council is the Lead Local Flood Authority, and the wider county is set out on our flood risk assessment in Surrey page.

During the 2013/14 floods the Environment Agency looked at putting temporary barriers up in Cobham and found the ground unsuitable. With no permanent defences either, a Cobham site is kept safe by its own floor levels, escape route and drainage.

For a view on any KT11 address, ring our flood risk consultants on 01293 214444 or write to enquiries@unda.co.uk.

Mill Road, the High Street and Downside Bridge: how Cobham has flooded

Elmbridge's Level 1 Strategic Flood Risk Assessment (AECOM, updated 2024) records that the residential areas of Cobham and Esher have a history of repeated flooding from the Middle Mole, and lists the events below. The 2013/14 detail comes from Surrey County Council's Section 19 investigation for Elmbridge, published in October 2015, and the river level from the Environment Agency.

1. March 1947. Heavy rain fell on the snow of a bitter winter, and the flooding that followed was worst in the towns along the Thames.
2. September 1968. Widely accepted as the worst flood recorded in the area, after the wettest September on record and torrential rain over the weekend of 14 and 15 September. Downside Bridge at Cobham was one of several bridges destroyed.
3. December 1994. An overnight downpour raised the Mole by 3 metres and flooded Mill Road.
4. Autumn 2000. The highest Mole flows since 1968: 115 m³/s at the Esher gauge, with large areas of farmland in Elmbridge under water.
5. December 2013 to February 2014. The Mole burst its banks at Cobham and flooded the floodplain and the properties next to it. Surrey County Council closed Mill Road, Stoke Road, Plough Lane, Old Common Road and Portsmouth Road, and the Environment Agency sent warnings to 190 registered owners in December, 196 in January and 199 in February. On 24 December 2013 the river reached 3.46 metres at Downside Bridge, the highest level recorded there.
6. December 2019 and February 2020. Flooding in the town centre and along Mill Road, followed two months later by the High Street being cordoned off and the A245 under water.
7. November 2023 and January 2024. The Mole overtopped at Cobham during Storm Ciarán and again that January, when Painshill Landscape Garden had to close.

The Section 19 report puts the 2013/14 flood at about a 1 in 50 year event at Esher, with river levels above those of 1968 but a smaller flooded area than in 1968 or 2000. The gauge to watch in Cobham is the Environment Agency's River Mole at Downside Bridge: above 2.70 metres low-lying land may flood, and above 3.30 metres property flooding is possible. Our live river levels map shows it alongside the other Mole gauges, and the town sits in the Agency's River Mole at Stoke D'Abernon, Cobham and South Hersham warning area.

Why the Mole runs through Cobham without flood defences

Below Esher, the Lower Mole Flood Alleviation Scheme, built in the years after 1968, confines the river between embankments and walls all the way to the Thames. Nothing like it was built at Cobham. The SFRA describes the Middle Mole here as not formally defended, with only the higher ground on either side of the channel, and the Section 19 report says in as many words that Cobham has no flood defences.

What the town does have is the Environment Agency's sluice gates at Cobham Mill. In 2013/14 the Agency began operating them before any property flooded and kept them under watch for the rest of the winter, and the same report records its finding that temporary barriers could not be used effectively on Cobham's ground.

Two pieces of work are open. The Agency has looked at how the Mole floods through the Cobham and Stoke D'Abernon catchment in an initial assessment, but has published nothing further, and Elmbridge is looking at attenuation options to add capacity to the existing system at Cobham Park, on the floodplain where the SFRA notes surface water already ponds. Neither is a scheme an applicant can rely on, so a flood risk assessment in Cobham works from today's modelled levels plus the climate change allowance. Our River Mole page follows the river from its Sussex headwaters to the Thames.

Check a KT11 address on the map

Enter a postcode below to see the Environment Agency's river, surface water and reservoir data for that spot; our full flood risk map does the same anywhere in England. The coral line traces the part of the ONS built-up area for Cobham that falls in Cobham and Downside ward: the High Street, Church Cobham and the streets around Cobham Mill, with a separate piece of Fairmile by the A3. The river channel, Painshill, Downside village and the floodplain fields sit just outside it, because a built-up area stops where the buildings do.

River, ditches, rain and gravels: the sources of flooding in Cobham

The Mole is the main source, but the SFRA records every other kind of flooding here too. It profiles Cobham together with Oxshott, Stoke D'Abernon and Downside as one 30.6 km² settlement area: 84% of it lies in Flood Zone 1, and 3.2 km² (11%) is functional floodplain, mostly open land in the Mole's wide valley.

Some of these ditches fall outside the river model: the SFRA notes that the Mole modelling does not include every tributary, so a plot beside one of the Fairmile or Downside watercourses will often need its own modelling before anyone can say what level it floods to. Our guide to main rivers and ordinary watercourses explains who regulates which, and why works in a ditch need Surrey's consent rather than the Environment Agency's.

What a Level 2 test of a Mill Road site shows

While Elmbridge was preparing the Local Plan it later withdrew, AECOM's Level 2 SFRA tested Cedar House on Mill Road (site COS1: 0.27 hectares, seven homes proposed). The site is not allocated today, but its site sheet sets out the questions a flood risk assessment in Cobham has to answer next to the river.

• Mostly Flood Zone 2. 69% of the site is in Zone 2 and 31% in Zone 1, with the Mole about 16 metres to the south.
• Three recorded floods. The Environment Agency's recorded outlines for September 1968, February 1990 and December 2013 all cover the site.
• A road that cannot be the way out. In the design flood (1% annual chance plus a 25% climate change allowance) Mill Road and River Hill to the west reach a significant to extreme hazard rating, so dry or low-hazard access has to come from Stoke Road, the A245, to the south and east.
• Floors above the flood. Homes need finished floor levels at least 300 millimetres above the design flood level, which reaches about 20.3 m AOD at the south of the site.
• Storage put back. Building inside the modelled extent along the southern edge means replacing lost floodplain storage level for level, volume for volume.
• A plan for the warning. An emergency plan for occupants, because the site is in the Cobham warning area, plus checks on groundwater and on reservoir flooding, which reaches 8% of the site when the river is already in flood.

The sheet also confirms that homes in Flood Zone 2 do not need the exception test. A planning application for new homes on a site like this would still have to pass the sequential test, because the site is not allocated.

Policy CS26 and the local rules for a flood risk assessment in Cobham

Elmbridge withdrew its Local Plan 2037 in February 2025, which left the Core Strategy of July 2011 in force. Its Policy CS26 is the flood policy a Cobham application is judged against, together with the Development Management Plan (2015) and the Flood Risk Supplementary Planning Document (May 2016). The council has begun work on a new Local Plan, but nothing in it carries weight yet.

The SFRA says the vast majority of development in this part of the borough is residential, so the householder guide is often the first document a Cobham applicant meets. It asks for more than national guidance does: it says the Environment Agency's standing advice table does not give enough detail on its own, and it warns that an application lacking the information may be refused without a chance to supply more during determination.

From 1 April 2027 East Surrey Council takes over both the planning and the flood authority roles for Cobham, with Epsom and Ewell, Mole Valley, Reigate and Banstead and Tandridge in the same new council. The existing councils run as normal until then, and our explainer on local government reorganisation covers what may change for applicants.

When does a project need a flood risk assessment in Cobham?

You need one for development in Flood Zones 2 or 3, which in Cobham means the land along the Mole, for any site of a hectare or more, and for a Flood Zone 1 site at risk from surface water, groundwater or another source. Elmbridge also asks for one with extensions in Zone 2 or in areas of medium to high surface water risk, which brings in houses on the wetspot roads.

Our article on rebuilding a house in a flood zone explains the rebuild rule, including a November 2025 appeal dismissed on the sequential test alone after the Environment Agency's floor-level objections had been resolved. Where a site's only risk is surface water and the assessment shows it stays safe for its lifetime, NPPF Policy F5 can lift that test; our note on the surface water sequential test exemption explains the conditions.

Drainage on Cobham's gravels and clay

The ground under Cobham changes within a few hundred metres. Surrey's Section 19 report maps Boyn Hill Gravel to the east, Taplow Gravel in the centre and Lynch Hill Gravel to the west, over the Bagshot sands, the Claygate Member and London Clay. The SFRA draws the practical conclusion: the main built-up area around Cobham is likely to suit infiltration well, while land beside the Mole carries very significant constraints. Our ground conditions checker gives a first read of the geology under any plot.

• Test before relying on a soakaway. Surrey expects infiltration results with both full and outline applications, or a justification for their absence, and BRE 365 testing is the usual method.
• A metre of dry ground. A soakaway needs 1 metre of unsaturated ground above the highest seasonal groundwater, which can rule one out on the floodplain gravels.
• Rainwater reuse comes first. Surrey is classed as water stressed, so collecting rainwater for non-potable use is required on every scheme, normally with a lower outfall behind it.
• The 1 in 2 year limit. Where a scheme adds runoff and cannot infiltrate it, Surrey holds most sites to the 50% annual chance (1 in 2 year) rate and adds 10% for urban creep.
• Ditches and road drains. Discharging to an ordinary watercourse needs evidence of where the channel goes and what state it is in, and Surrey will not accept new private surface water into highway drainage.

Surrey's advice on smaller schemes reaches below the major threshold: it gives a bespoke response on non-major applications inside the 1% surface water extent or next to a recorded watercourse, which can catch a plot on one of the wetspot roads or beside a Downside ditch. Its drainage guidance for major development sets the full standard.

From Mill Road to Fairmile: what we prepare for Cobham sites

People commission our Cobham reports before a planning application, a purchase, a remortgage or an insurance renewal. The three main reports answer different questions: whether a site is safe, whether it should be built on at all, and where its rainwater goes.

• One report, every source. A flood risk assessment in Cobham weighs the Mole, the ditches from Fairmile and Downside, surface water, groundwater and the KT11 sewers together.
• Applications to Elmbridge. Extensions, rebuilds and new homes need a flood risk assessment for planning written to Policy CS26 and the 2023 householder guide.
• Buying near the river. Mill Road and the High Street flooded in December 2019 and February 2020, so a flood risk survey for property purchase checks a house before exchange.
• Rebuilds and new homes. A replacement house or new flats in Zone 2 or 3 brings in the sequential and exception test, prepared alongside the assessment.
• Gravel or clay. A drainage strategy in Cobham turns on whether the ground drains, which differs between the town centre and the floodplain.
• Surrey's discharge limits. A surface water drainage strategy sets out rainwater reuse, the infiltration evidence and the 1 in 2 year rate Surrey applies.
• After permission. Pre-commencement drainage conditions on Cobham consents are cleared through our discharge of drainage conditions service, including the verification Surrey wants before occupation.
• Streams outside the model. Flood modelling fixes a level where the SFRA notes the Mole model leaves out a tributary, or where Zone 3b follows the 1 in 30 year outline.
• No barriers to wait for. With no defences and temporary barriers ruled out, a flood warning and evacuation plan sets out what occupants do when the Downside Bridge gauge climbs.
• Water in the gravels. Basements and soakaways on the gravels beside the Mole at Cobham need a groundwater flood risk assessment, because the SFRA shows groundwater can reach the surface along the floodplain.
• Questions at renewal. Owners on Mill Road and near the Mole, where the river overtopped in 2013, 2019, 2023 and 2024, can use a flood risk assessment for insurance to put property-level evidence in front of an insurer.
• Shops on a closed street. The town centre flooded in December 2019 and the High Street was shut in February 2020, which a business continuity flood risk assessment plans around.

Warnings, sewers and emergencies: who to contact in Cobham

If floodwater puts anyone in danger, ring 999; a power cut goes to 105. For anything else, each body below looks after one part of flood risk in Cobham.

The Cobham warning area stretches from Stoke D'Abernon, upstream of the town, to South Hersham. Our pages on flood risk assessment in Esher and flood risk assessment in Walton-on-Thames follow the river north, and flood risk assessment in Leatherhead covers the reach upstream.

What Cobham owners and applicants ask about flood risk

To talk through a site in Cobham, Church Cobham, Fairmile or Downside, call 01293 214444 or email enquiries@unda.co.uk. Our consultants are chartered, Unda Consulting Limited is a CIWEM Business Partner, and every report is signed off by a senior consultant before it reaches you.