Flood Risk Assessment in Dorking

A flood risk assessment in Dorking starts with two watercourses that meet on the town's northern edge. The River Mole runs past Pixham on its way to the gap it cuts through the North Downs, and the Pipp Brook comes down from Westcott, passes north of the High Street and joins it there. Our consultants write flood risk assessments in Dorking to support planning applications, house purchases, mortgage offers and insurance cover, and prepare sequential and exception tests and surface water drainage strategies alongside them.

Dorking on this page is the town the Office for National Statistics maps as its built-up area: the centre, Pixham, the streets climbing towards the Greensand Ridge and the northern end of North Holmwood. Westcott, Mickleham, Westhumble and the foot of Box Hill lie outside that line but share the same rivers, the same council and the RH4 and RH5 postcodes, so they are covered here as well. Mole Valley District Council decides planning applications, Surrey County Council is the Lead Local Flood Authority, and our flood risk assessment in Surrey page sets the town in its county.

On Christmas Eve 2013 the Mole at Dorking reached 2.74 metres, the highest level its gauge has ever recorded, and Surrey County Council found that the river was already so full it could not take the Pipp Brook's flow, so properties in the centre of Dorking flooded.

For a site anywhere in RH4 or RH5, call our flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

What happened in Dorking on Christmas Eve 2013?

Surrey County Council's Section 19 flood investigation for Mole Valley, published on 28 October 2015, records how much rain fell on 24 December 2013, what the river did and which roads closed. The Environment Agency's gauge record adds the height and time of the peak.

1. Forecast and rainfall. About 45 mm of rain was forecast for 24 December; the Mole corridor received 75 mm, and the river rose by at least a foot an hour.
2. 04:47, 24 December. The Environment Agency issued a flood warning for the River Mole at Dorking and Mickleham to 59 registered properties. It later counted 9 of them as flooded.
3. During the day. The Mole reached capacity and could not take the extra water arriving from the Pipp Brook, and properties in the centre of Dorking flooded. Vincent Lane was closed.
4. 15:15, 24 December. The River Mole gauge at Dorking peaked at 2.74 m, still the highest level on its record.
5. Mickleham and Westhumble. The A24 closed in both directions and Chapel Lane in Westhumble was shut. Guests at the Burford Bridge Hotel, below Box Hill, were told to stay inside because the water was moving too fast to evacuate them safely; they were brought out later that day.
6. North Holmwood and Westcott. Glory Mead in North Holmwood, close to Bent's Brook, was closed. In Westcott, White Down Lane and Ranmore Common Road were closed too, although no watercourse runs near either road.
7. Christmas Eve. Mole Valley District Council opened rest centres in Dorking and Leatherhead. The Mole was back to normal about 12 hours after its peak.
8. 17 January and 1 February 2014. Two more warnings went to the same area, for 59 and then 61 properties. No further flooded properties were counted.

Across the whole district that winter there were about 270 incidents of internal flooding; the council gave council tax relief to 181 properties and business rate relief to 13, and handed out 11,400 sandbags. Measured flows at Dorking and Leatherhead pointed to roughly a 1 in 50 year flood, but river levels went beyond what both gauging stations could measure, so the investigation judged the true return period nearer 60 to 90 years. The 2017 Strategic Flood Risk Assessment puts the event in a longer line: September 1968, which it calls the worst in living memory, and February 1990 and December 2000, when flooding was widespread along the Mole. The Dorking gauge can be followed live on our live river levels map.

Where Dorking's flood water comes from

River flooding is the main risk in Dorking, yet in 2013 the investigators could not always tell where river flooding stopped and surface water began. Each source below is listed with the part of town it affects and what the published evidence says about it.

Below Pixham the Mole turns north through the chalk at Westhumble and Mickleham, the gap between Box Hill and the Ranmore slopes, and carries on to Leatherhead, Cobham and the Thames at Molesey. Our River Mole page follows the whole river. West of Westcott the land drains the other way, into the Tillingbourne and on to the Wey.

Checking a postcode in Dorking, Pixham or North Holmwood

Enter a postcode in the map below to see how the Environment Agency's river, surface water and reservoir data read for that spot, or use our full flood risk map for any address in England. The coral outline is the ONS built-up area for Dorking: it runs from Pixham and the Deepdene side in the north-east to the edge of North Holmwood in the south, and stops short of Westcott to the west.

Why the Pipp Brook has its own flood warnings

The Pipp Brook drains about 21 square kilometres of steep woodland and farmland on the northern slopes of Leith Hill, and it responds to heavy rain quickly. The Environment Agency treats it separately from the Mole: its Pipp Brook warning area covers Wotton, Westcott and Dorking, while the River Mole at Dorking and Mickleham and River Mole at Brockham and Pixham areas cover the main river.

In 2017 the Strategic Flood Risk Assessment listed a Dorking flood alleviation scheme, led by the Environment Agency, to protect the town from the Pipp Brook. It was then at appraisal, with Defra money to test natural flood management with the Forestry Commission. In 2019 the Agency put in 30 leaky barriers on the upper brook, part of the first set of natural flood management pilots the government funded, and the University of Surrey has monitored them since.

The University of Surrey's results, presented in March 2026, show how little the barriers change in a large flood. For floods up to about a 1 in 5 year event the barriers cut the peak flow by up to 4% and delayed it by up to 15 minutes. For 10 to 50 year floods the reduction in peak flow was negligible and the delay no more than 5 minutes. Our article on what leaky dams can and cannot do explains why. For an applicant, it means a site beside the Pipp Brook is assessed on the flood risk as mapped; the barriers do not remove it.

Owners of land along the brook have duties of their own. After 2013 the district council, Surrey County Council and the Environment Agency wrote jointly to residents about their responsibilities along the Pipp Brook, which our guide to riparian ownership sets out.

Policy INF3 and the rules for a flood risk assessment in Dorking

Applications in Dorking are decided against the Mole Valley Local Plan 2020-2039, adopted on 15 October 2024 after an examination that ended with the inspectors' report of 18 September 2024. Its flood policy is INF3. The evidence behind it is the Level 1 Strategic Flood Risk Assessment that JBA prepared in December 2017 for Mole Valley, Reigate and Banstead and Tandridge together; for the sites it allocates, the plan also points to site-specific guidance in a Level 2 SFRA.

INF3 reaches smaller schemes than national policy does. Where feasible, SuDS should be built into all major schemes and into any scheme adding three or more homes, or of 0.25 hectares or more, inside a Local Critical Drainage Area shown on the Local Plan Policies Map; they are encouraged in all other development. Greenfield sites, and brownfield sites inside those drainage areas, should discharge at greenfield runoff rates, and every other site as close to greenfield rates as feasible. For smaller schemes inside the 1% surface water flood extent or next to a watercourse, Surrey County Council gives its own advice on non-major drainage. Our summary of the 2025 National Standards for SuDS covers the national side.

Ground conditions change across the town: Surrey's 2015 investigation found chalk to the north, clay to the south and sandstone and mudstone between, and Surrey County Council expects infiltration test results with the application. The SFRA also records groundwater source protection zones centred on Dorking, and in an inner zone anything other than clean roof water needs a hydrogeological risk assessment before it goes into the ground; our explainer on source protection zones covers the detail, and our ground conditions and soakaway map gives a first read of the geology under a plot.

The shadow East Surrey Council is already in place and takes over on 1 April 2027; until then Mole Valley District Council and Surrey County Council keep their roles. Our article on local government reorganisation explains what changes for flood risk and drainage.

Which projects in Dorking need a flood risk assessment?

Proposals in Dorking run from house extensions and conversions to the larger housing schemes the Local Plan brings forward. The table gives the usual position; the plot's flood zone, its surface water risk and the size of the scheme decide the detail.

The Local Plan sets an average of 336 new homes a year across Mole Valley to 2039, and when it was adopted the Guildford Dragon reported that the Dorking area was expected to take about 23% of them, some 1,467 homes. The plan releases Green Belt land at Dorking and sets town centre sites aside for housing-led redevelopment, with the council's refurbishment of Pippbrook House as its flagship project there. The SFRA traces the Pipp Brook through the northern part of the town, so sites on that side of the centre need their flood evidence settled early.

For a household project, our guides on extending a house in a flood zone and on the Environment Agency's standing advice explain when a short assessment is enough.

Reports our consultants write for sites from Pixham to North Holmwood

We write these reports for householders, buyers, landlords, developers and businesses across Dorking and its villages. A town centre conversion might need two of them, a farm building near Westcott only one.

• Sites with more than one source. A plot near Pixham can face the Mole, the Pipp Brook, groundwater and surface water at once, and a flood risk assessment weighs them together.
• Applications to Mole Valley. Extensions, conversions and new homes in Zones 2 and 3 need a flood risk assessment for planning written to Policy INF3 and the 2017 SFRA.
• Buying by the river. Properties in the centre of Dorking flooded in December 2013, so a flood risk survey for property purchase shows a buyer and lender what the river did and could do.
• New homes in a flood zone. INF3 applies the sequential and exception test before the Local Plan's town centre and edge-of-town housing can go in Zones 2 or 3.
• Rainwater from the start. Surrey always asks for rainwater reuse in a drainage strategy, and the chalk, clay and source protection zones under Dorking change what else is possible.
• Larger schemes. Major sites and schemes inside a Local Critical Drainage Area need a surface water drainage strategy that brings runoff to the greenfield rates INF3 sets, or as close to them as the site allows.
• After permission. Surrey County Council's standard drainage conditions, in use since October 2025, ask for infiltration tests, calculations and a verification report, which the discharge of drainage conditions provides.
• Beyond the mapped outlines. River levels in 2013 went beyond what the Dorking gauge could measure, so sites close to it or to the Pipp Brook may need flood modelling where the national data are too coarse.
• Chalk and rising water. Groundwater can emerge where the Pipp Brook meets the Mole and around Westcott, which a groundwater flood risk assessment investigates.
• Cover after 2013. Owners of the 181 Mole Valley properties given council tax relief that winter, and their neighbours, can use a flood risk assessment for insurance when renewals get difficult.
• When the A24 closes. The Mickleham road and the Burford Bridge Hotel were cut off on Christmas Eve 2013, and a flood warning and evacuation plan sets out how people leave before that happens.
• Testing land early. Sites released from the Green Belt around Dorking benefit from a flood risk feasibility study before a layout is fixed.

Who to call about a flood in Dorking, Westcott or Mickleham

Call 999 if floodwater puts anyone in danger, and 105 to report a power cut during a flood. For anything else, the table lists who deals with which part of the problem in and around Dorking.

Before or during heavy rain, the Environment Agency's flood information for Dorking shows warnings in force and the latest river levels. Westcott's lanes and North Holmwood's side roads flooded in 2013 outside every warning area, so surface water there is reported to Surrey County Council rather than the Agency.

Dorking flood questions we hear before an application or a sale

Every report for a Dorking site is checked and signed off by a senior consultant, our flood risk consultants are chartered, and Unda Consulting Limited is a CIWEM Business Partner. Call 01293 214444 or email enquiries@unda.co.uk with the address and what you plan to build there.