Flood Risk Assessment in Mole Valley

Mole Valley's flood risk and drainage requirements split between two different bodies, and knowing which one to satisfy for which part of an application is the first thing to get right. Mole Valley District Council decides your planning application, but it is Surrey County Council, not the district council, that signs off your drainage strategy as the Lead Local Flood Authority (LLFA). We prepare flood risk assessments and drainage strategies for sites across Mole Valley for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS), and we deal directly with both the council and Surrey County Council on your behalf.

If you want to talk a Mole Valley site through, contact one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

Mole Valley's Local Plan was adopted on 15 October 2024 with a dedicated flood risk policy, Policy INF3, and a climate-change policy that separately requires sustainable drainage, so an application here has to satisfy two distinct district-level policies as well as Surrey County Council's own drainage standards before it will validate.

Flood risk and drainage in Mole Valley at a glance

• Two authorities, two jobs. Mole Valley District Council is the local planning authority (LPA) that determines your application; Surrey County Council is the Lead Local Flood Authority (LLFA) and the statutory consultee on surface water drainage for major development.
• The Local Plan is current. The Mole Valley Local Plan 2020–2039 was adopted 15 October 2024. Policy INF3 (Flood Risk) and Policy S2 (Combatting the Climate Emergency, which requires sustainable drainage systems) are the flagship codes.
• Surrey's discharge approach is staged, not a single number. Surrey County Council expects surface water held to greenfield Q1, Q30 and Q100 rates in turn, generally matched as closely as practicable even on brownfield sites, with a 1-in-100-year-plus-climate-change design event.
• New Local Validation Requirements apply from 10 November 2025. Mole Valley introduced its own Local Validation Requirements and Biodiversity Net Gain Requirements alongside the national list, so check what a submission needs before you lodge it.

Who decides flood risk and drainage for a Mole Valley planning application?

Mole Valley is a two-tier area, and that structure is the single most common source of confusion for applicants. Mole Valley District Council is the local planning authority: it registers, assesses and determines your application, and its planning officers apply the Local Plan. Surrey County Council sits above it as the county authority and, separately, as the Lead Local Flood Authority under the Flood and Water Management Act 2010 (the body responsible for surface water, groundwater and ordinary watercourse flood risk, and the statutory consultee on drainage for major development). The Environment Agency remains responsible for the River Mole and Mole Valley's other main rivers. In practice, a scheme in Flood Zone 2 or 3, or with a surface-water drainage strategy to agree, is answering to all three at once.

Does a Mole Valley application need a flood risk assessment or a drainage strategy?

Mole Valley applies the national triggers, and Surrey County Council layers its own drainage expectations on top. A flood risk assessment is generally needed for development in Flood Zone 2 or 3, for sites of one hectare or more in Flood Zone 1, and increasingly for smaller Flood Zone 1 sites the Flood Map for Planning shows to be at risk of surface water. A drainage strategy sits alongside it wherever a scheme could affect drainage on or around the site, and Surrey County Council's Surface Water Drainage Pro-Forma is expected as supporting evidence on major applications.

Missing the document a Mole Valley application actually needs is one of the most common reasons a submission does not validate. Our guide on planning applications invalidated for flood risk or drainage sets out the documents that most often catch applicants out, including the Flood Zone 1 trap.

Mole Valley's SuDS and drainage requirements

Surrey County Council, as the LLFA, expects a Surface Water Drainage Strategy on every major Mole Valley application, following the drainage hierarchy from at-source reduction and reuse through infiltration, attenuated discharge to a watercourse, and only then to sewer as a last resort. The scheme has to match greenfield runoff rates as closely as practicable, keep the site free of above-ground flooding up to the 1-in-30-year event, safely contain the 1-in-100-year-plus-climate-change event on site, and route anything beyond that without increasing flood risk off site.

• Discharge rates are staged. Surrey County Council expects surface water discharged using a staged approach limited to the greenfield Q1, Q30 and Q100 rates for the equivalent storm events, or the greenfield QBar rural rate as a single fixed rate for positively drained areas.
• Brownfield sites aim for the same target. Where a genuinely greenfield rate cannot be achieved, Mole Valley applicants need clear written evidence (storage comparisons, cost analysis or level constraints) for why not, assessed case by case.
• Infiltration testing follows BRE Digest 365. Where infiltration is proposed, tests are carried out in winter at the depth of the proposed structure, with at least a 1m gap to the seasonal high groundwater level and a minimum factor of safety of 2.
• Climate change and urban creep are built in. Residential schemes design attenuation to the upper-end 1-in-100-year climate change allowance, plus an urban creep allowance of up to 10% of impermeable area on lower-density housing.
• A Surface Water Drainage Pro-Forma is expected. Surrey County Council's own pro-forma summarises the strategy for a Mole Valley reviewing officer; see our explainer on what a SuDS proforma is and when one is required.

How much evidence a Mole Valley application needs to submit up front, versus what can be conditioned, depends on the stage: a Flood Risk Assessment and an outline drainage layout are expected even at pre-application and outline stage, while full hydraulic calculations, detailed drainage drawings and a maintenance plan are expected at full application and reserved matters. Our guide on what drainage information is needed for planning permission explains what can safely wait for a condition and what cannot.

Mole Valley's Local Plan flood and drainage policies

The Mole Valley Local Plan 2020–2039 was formally adopted on 15 October 2024, following submission to the Secretary of State in February 2022 and an Examination in Public held across June, September and October 2022. Flood risk and drainage are addressed through two policies working together rather than one: Policy INF3 sets the flood risk test, and Policy S2 separately requires sustainable drainage as part of the Plan's climate change response.

Policy INF3 — flood risk

Policy INF3 is Mole Valley's dedicated flood risk policy within the adopted Local Plan's infrastructure chapter, and it sits alongside Policy S2 among the policies the Council names as helping the district adapt to climate change. Where a site allocation carries flood risk, the Plan's policies direct development to avoid built development within Flood Zones 2 and 3 where possible and to incorporate sustainable drainage measures to address and mitigate fluvial flooding risk, in accordance with Policy INF3 and any site-specific guidance in the Level 2 Strategic Flood Risk Assessment. A Mole Valley flood risk assessment needs to show how the proposal meets that test for the site in question.

Policy S2 — sustainable drainage and the climate emergency

Policy S2 requires Mole Valley development to be future-proofed for the changing climate, and its climate change adaptation limb explicitly instructs applicants, where practicable, to incorporate sustainable drainage systems alongside passive design principles, minimised resource use and retained soft landscaping. It is a separate test from Policy INF3's flood risk assessment: INF3 asks whether the site is safe from flooding, while S2 asks whether the drainage design pulls its weight on climate adaptation. Both are cited among the Local Plan's climate change measures, alongside Policy H10 on water efficiency and Policy EN13 on climate change standards and targets.

The Mole Valley Strategic Flood Risk Assessment

Mole Valley's evidence base includes a Level 1 Strategic Flood Risk Assessment, a joint study with Reigate & Banstead Borough Council and Tandridge District Council, produced by JBA Consulting and finalised in December 2017. It maps flood zones, watercourses and the risk of flooding from groundwater across the district, and it underpins the site allocation and Sequential Test evidence in the Local Plan. Site allocation policies in the Plan also point applicants to a Level 2 Strategic Flood Risk Assessment for site-specific guidance where a location carries a more detailed flood risk. Treat the SFRA as background evidence a site-specific flood risk assessment must engage with, not a substitute for one. Our guide on what a Lead Local Flood Authority does explains how the LLFA uses this kind of evidence base alongside its own consultee role.

Local validation requirements: how Mole Valley applications get invalidated

Mole Valley District Council introduced new Local Validation Requirements (LVR), alongside Biodiversity Net Gain Requirements, for planning applications submitted on or after 10 November 2025. These sit on top of the Government's national list and set out what has to accompany a Mole Valley application before it will be registered, following public consultation. Getting a validation refusal is different from an objection or a refusal on the merits: it happens before your application is even registered, and the determination clock never starts until it is put right.

• Check before you submit. Consult Mole Valley's Local Validation Requirements alongside the national list before lodging an application, so a missing flood risk assessment or drainage strategy does not stop the clock.
• The Flood Zone 1 trap applies here too. A site showing as low risk on the Flood Map for Planning can still need a flood risk assessment because of its size or a mapped surface water constraint.
• Drainage documents are commonly missing. A surface water drainage strategy, the completed Surrey County Council pro-forma, or foul drainage details are among the documents that most often hold a Mole Valley submission up.

Our guide on the flood risk and drainage documents applicants miss works through each of these in more detail, and if Surrey County Council has already raised a drainage objection on a live application, our note on how to fix an LLFA objection sets out how to respond.

How national policy applies to a Mole Valley application

Underneath Mole Valley's own policies, the same national framework applies as everywhere else in England. The NPPF and its Planning Practice Guidance steer development to the lowest-risk land through the Sequential Test, and the Exception Test applies where a higher-risk site still needs to come forward. The September 2025 PPG update confirmed that this risk-based, sequential approach applies to all sources of flooding, not river and sea alone, which is part of why a Flood Zone 1 site in Mole Valley can still attract scrutiny over surface water. The 2025 National Standards for Sustainable Drainage Systems set the baseline Surrey County Council's own guidance builds on. See our guides on the NPPF explained and flood zones 1, 2 and 3 in England for the national picture behind Mole Valley's local requirements.

Flood risk in Mole Valley's area: a compact picture

During the winter of 2013/14, Surrey County Council recorded around 270 incidents of internal property flooding across Mole Valley, and the district ultimately granted council tax relief to 181 flooded properties. That scale of event still shapes how carefully the council and the LLFA scrutinise development today.

Mole Valley's flood risk is dominated by the River Mole, a main river the Environment Agency manages, together with tributaries including the Pipp Brook, Rye Brook, Bents Brook, Whithey Brook and Dolby Brook, which fall to Surrey County Council as ordinary watercourses. Surface water compounds the picture in the district's towns and villages. The winter of 2013/14 remains the benchmark event: unprecedented rainfall (275% of an average winter) caused fluvial and surface water flooding across the district, Mole Valley District Council opened rest centres in Dorking and Leatherhead on Christmas Eve 2013, and Surrey County Council has since carried out works including a reinstated culvert at Wonham Lane, Betchworth, new soakaways on Dorking Road, Bookham, and ongoing assessments of flood attenuation options across Bookham and around Brockham and Strood Green.

This is deliberately a brief picture. For the full local flood-risk narrative on specific Mole Valley towns, see our area guides to flood risk in Dorking, where the Pipp Brook meets the River Mole at Pixham, and flood risk in Leatherhead, where the Mole leaves the North Downs chalk and crosses onto London Clay.

How Mole Valley District Council and Surrey County Council behave in practice

Surrey County Council offers a chargeable pre-application advice service for Sustainable Drainage Systems, and for anything hydrologically complex, a significant site, or a proposal that departs from its published guidance, it strongly advises using it before submission rather than finding out at validation stage. Since the winter 2013/14 flooding, Surrey County Council officers have remained active in Mole Valley's flood forums and the wider Surrey Flood Risk Partnership, and both authorities take part in the Surrey Flood Recovery Co-ordination Group. That institutional memory means a Mole Valley drainage strategy that skips the local discharge hierarchy or arrives without infiltration evidence tends to draw a swift, well-informed objection rather than a quiet pass.

How Unda helps with flood risk and drainage in Mole Valley

We read Mole Valley's Local Plan policies, its Strategic Flood Risk Assessment and Surrey County Council's drainage guidance together, so a flood risk assessment and a drainage strategy tell the same story rather than arriving as two documents that do not quite agree. We are a CIWEM Business Partner with chartered members, and a Senior Flood Risk Consultant signs off every report before it goes to the council or Surrey County Council.

Flood risk and drainage services for every situation in Mole Valley's area

A flood risk assessment, a Sequential or Exception Test and a drainage strategy are three distinct reports. They often travel together in a Mole Valley planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We prepare flood risk assessments in Mole Valley for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies.

Flood risk assessments

• Flood risk assessment for planning. The core report, prepared to satisfy the NPPF, the Environment Agency and Mole Valley District Council. Groundwater and, where needed, flood modelling are assessed within it.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged Mole Valley flood risk and you need the actual building assessed, not a broad map.
• Flood risk assessment for insurance. Property-specific evidence for brokers and underwriters where premiums have risen or cover has been restricted.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning FRA alone rarely satisfies it.
• Groundwater flood risk assessment. A defensible design groundwater level for Mole Valley sites and any basement, assessed as a source within the FRA, or as a standalone hydrogeological assessment on complex sites.

Sequential and Exception Tests

The Sequential and Exception Tests are a separate, locational report about whether a development should be in a higher-risk part of Mole Valley at all. The Sequential Test defines an area of search, identifies reasonably available lower-risk sites and justifies the proposed location; the Exception Test is the second gate where development still needs to proceed. A clean site-specific flood risk assessment showing the development is not actually within an area at risk can remove the need for the test at application stage.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering that calculates existing versus proposed runoff and proposes SuDS to hold discharge within Surrey County Council's staged limits.
• Preliminary drainage design. The early, outline read that flags infiltration issues before they affect layout and land value.
• Discharge of drainage conditions. The detailed stage to discharge a pre-commencement or pre-occupation drainage condition once permission is granted.
• Flood modelling. 1D and 2D hydraulic modelling that feeds the assessment, Sequential Test or drainage design where national mapping is not enough for a Mole Valley site.

Early in a drainage strategy it helps to know whether the ground will take water: our free geology and ground conditions map and BRE 365 infiltration rate calculator give an indicative read before a trial pit, and the drainage calculators help size the scheme. You can screen a postcode on our flood risk map or check current conditions on the live river levels map, and every report is overseen by a chartered flood risk consultant who stays involved past the report to answer objections. For the national context behind local requirements like Mole Valley's, our guides to how Local Planning Authorities handle flood risk and SuDS and the Environment Agency's planning role are a good starting point, and our note on the April 2026 planning appeals changes explains why getting the flood risk and drainage case right first time now matters more than ever.

Mole Valley flood risk and drainage: official bodies and contacts

For an immediate threat to life during a flood, always call 999; for a power cut during flooding, call 105.

For the council's own flood advice and sandbag locations, see Mole Valley District Council's flooding page, and for Surrey County Council's design guidance in full, see its Sustainable Drainage System Design Guidance.

Flood risk and drainage in Mole Valley: frequently asked questions