Flood Risk Assessment in Horsham District Council

Horsham District sits across two river catchments — the Arun and the Adur — and, since the spring of 2026, across a live Local Plan examination that is actively rewriting its flood and drainage policy. A development here answers to three different bodies at once: Horsham District Council as the planning authority, West Sussex County Council as the Lead Local Flood Authority (LLFA) for surface water, and the Environment Agency for the main rivers. Getting a flood risk assessment or drainage strategy through Horsham's system means satisfying all three, not just one.

We prepare flood risk assessments and drainage strategies for sites across Horsham District for planning, property purchase, mortgage and insurance, and BREEAM — alongside sequential and exception tests and surface-water drainage strategies (SuDS). If you want a quote or need to talk a Horsham District site through, contact one of our experienced flood risk consultants on 01293 214444.

Horsham District's emerging Local Plan is being rewritten in front of us: the Inspector's suggested main modifications, published 26 March 2026, rename the council's water neutrality policy to "Water Efficiency" and renumber its flooding policy — so an application submitted today has to satisfy the adopted 2015 policy and read the direction the incoming one is taking.

Flood risk and drainage for Horsham District Council planning, at a glance

• Two-tier planning authority. Horsham District Council decides the application; West Sussex County Council is the Lead Local Flood Authority and statutory consultee for surface water drainage on every major application; the Environment Agency covers the main rivers Arun and Adur.
• The adopted flood and drainage policy is Policy 38 (Flooding), read with Policy 24 (Environmental Protection) and Policy 35 (Climate Change) in the Horsham District Planning Framework, adopted November 2015.
• An emerging Local Plan is mid-examination. The Horsham District Local Plan 2023–2040 restarted its examination in April 2026 under a newly appointed Inspector; its flooding policy and its renamed water neutrality policy carry real, if still limited, weight.
• Water neutrality is no longer required in Horsham District. The Sussex North constraint that stalled roughly 21,000 homes for four years has been withdrawn, and the council's own emerging policy is being renamed "Water Efficiency" to match.
• A Surface Water Drainage Statement is required on every application that adds buildings or hard surfacing — not just major schemes — signed by a competent drainage engineer and reviewed by West Sussex County Council as the LLFA.

Who decides flood risk and drainage in Horsham District

Horsham District Council is the local planning authority: it registers, assesses and determines the application, and its officers apply the Horsham District Planning Framework to reach a decision. It is not, however, the body that reviews your drainage strategy in detail. That role belongs to West Sussex County Council, acting as the Lead Local Flood Authority and a statutory consultee on surface water drainage for every major planning application in the district. The Environment Agency sits alongside both, as the authority for the district's main rivers — the Arun and the Adur — and for any tidal influence on them. A single ordinary watercourse quirk is worth knowing early: consenting works to an ordinary watercourse in Horsham District is Horsham District Council's own responsibility under a local designation agreement, not West Sussex County Council's, even though the county council is the LLFA for everything else surface-water related.

Understanding the role an LLFA plays is the single most useful thing to grasp before submitting anything to Horsham District Council, because a technically strong flood risk assessment can still stall behind an unresolved drainage objection from the county council. Our guide to how Local Planning Authorities handle flood risk and SuDS sets out the wider process this sits inside. West Sussex County Council sets out its own role and contact details on its sustainable drainage systems page.

Does a Horsham District Council application need a flood risk assessment or drainage strategy?

Horsham District Council's own validation requirements set out exactly when each document becomes mandatory, and they go further than the flood zone map alone. A flood risk assessment is required for any part of a site in Flood Zone 2 or 3, for Flood Zone 1 sites of a hectare or more, for land the Environment Agency has identified as having critical drainage problems, for land the Strategic Flood Risk Assessment flags as being at increased risk in future, and for other flood sources introducing a more vulnerable use. A Surface Water Drainage Statement, by contrast, is asked for on every application that adds buildings, hard surfacing or an extension — major or minor.

The Sequential Test and, where needed, the Exception Test follow paragraph 175 of the NPPF (December 2024) and paragraph 027 of the Planning Practice Guidance on flood risk and coastal change, with the usual exemptions for householder applications, small non-residential schemes under 250m² and most changes of use. Our explainer on what the Sequential and Exception Tests actually require and our note on why planning applications get invalidated for flood risk and drainage are worth reading before you submit, because Horsham District Council will not register an incomplete application at all — its own Local Information Requirements for Validation sets out exactly what must be in place before that happens.

Horsham District Council's Local Plan flood and drainage policies

The adopted Horsham District Planning Framework, adopted in November 2015, still governs decisions today. Its flood and drainage requirements sit across three named policies — Policy 38 (Flooding), Policy 24 (Environmental Protection) and Policy 35 (Climate Change) — cited together as the policy basis for every Surface Water Drainage Statement Horsham District Council asks for. Behind it, the emerging Horsham District Local Plan 2023–2040 is being examined right now, and its own flooding and water policies are already shifting.

Policy 38 — Flooding

Policy 38 of the Horsham District Planning Framework requires development to follow a sequential approach to flood risk, to be safe from flooding for its lifetime, and to demonstrate that it will not increase flood risk elsewhere — the adopted basis Horsham District Council applies to every flood risk assessment submitted today. It is cited, alongside Policy 24 and Policy 35, as the policy driver behind the council's Surface Water Drainage Statement requirement, which means a site's drainage strategy and its flood risk assessment are read together, not as separate submissions.

Strategic Policy 10 — Flooding (emerging)

Strategic Policy 10 in the emerging Horsham District Local Plan 2023–2040 will, once adopted, sharpen several of Policy 38's requirements: it makes explicit that the sequential approach applies to flooding from all sources, ties compliance to the Horsham District and Upper Mole Valley Strategic Flood Risk Assessments by name, prohibits surface water discharge to the foul sewer, and asks any SuDS scheme to demonstrate amenity value, green and blue infrastructure and natural flood management — not hydraulic performance alone. The Council submitted these suggested modifications to the Planning Inspector on 26 March 2026, after the examination restarted under a newly appointed Inspector; they carry real, though not yet decisive, weight while the Plan remains unadopted.

Strategic Policy 9 — Water Efficiency (formerly Water Neutrality)

Strategic Policy 9 was submitted for examination as a water neutrality policy, reflecting the Sussex North Water Supply Zone constraint that has affected Horsham District since 2021. The Inspector's March 2026 modifications delete the water neutrality wording entirely, rename the policy Water Efficiency, and replace it with a straightforward efficiency standard: no more than 110 litres of mains-supplied water per person per day for new homes, and BREEAM ‘Excellent’ in the Water category for non-domestic buildings. Horsham District's own application form still asks for a water neutrality statement "where necessary", so it is worth confirming your scheme's current position before you submit. The full suggested wording sits in the council's Local Plan Examination documents.

West Sussex County Council's drainage and SuDS requirements

West Sussex County Council reviews every major application's surface water drainage as the Lead Local Flood Authority, and its expectations are exacting. A Surface Water Drainage Statement, completed on the council's own pro forma and signed by a drainage engineer recognised by the Engineering Council or the Institution of Civil Engineers, must accompany every major application — and Horsham District Council asks for a proportionate version on minor schemes too.

Applicants must work through the discharge hierarchy in sequence, under paragraph 080 of the Planning Practice Guidance: infiltration and storage for reuse come first, and each subsequent discharge destination has to be justified in turn before falling back to a sewer. Where infiltration is proposed, the design must be based on actual percolation testing to BRE Digest 365, with a minimum factor of safety of 2 applied to the observed rate under CIRIA Report 156 and the SuDS Manual (C753) — house or rubble soakaways are not acceptable, and any infiltration unit must half-empty the water it stores within 24 hours for a 1-in-10-year storm. West Sussex County Council, working with the South East Seven authorities, has also produced the ‘Water People Places’ guidance document to help developers plan SuDS properly from the outset, and strongly recommends pre-application discussion before a scheme is finalised.

Getting this wrong is one of the most common reasons a Horsham District application stalls: our note on why an LLFA objects to a planning application and on what drainage information you actually need at application stage explain how to pre-empt it. Horsham District Council's own Surface Water Drainage Statement guidance sets out the pro forma and every technical expectation in full, and applicants are strongly advised to use West Sussex County Council's pre-application advice service before finalising a drainage scheme. A dedicated SuDS proforma now accompanies drainage submissions to a growing number of councils, and while Horsham District's pro forma is its own Surface Water Drainage Statement rather than the London-style form, the underlying discipline — showing your figures, not just asserting them — is identical. The four pillars of SuDS — water quantity, water quality, amenity and biodiversity — and the 2025 National Standards for SuDS both sit behind what West Sussex County Council will accept.

Our free geology and ground conditions checker gives an early, indicative read on whether a Horsham District site's ground is likely to support infiltration drainage before you commission a trial pit, our BRE 365 infiltration rate calculator turns soakage test timings into the design rate West Sussex County Council expects to see, and our drainage calculators help size the wider scheme.

Is water neutrality still required for development in Horsham District?

Not in the way it was between 2021 and 2025. Natural England's September 2021 position statement over the Sussex North Water Supply Zone required new development in Horsham, Crawley, Chichester and the South Downs National Park to prove that its water use would not increase abstraction that risked harming the Arun Valley's protected wetlands, and the constraint stalled around 21,000 homes over four years. That requirement was withdrawn in late 2025, and Horsham District's own emerging Local Plan is catching up: the Inspector's March 2026 modifications strip the water neutrality wording from Strategic Policy 9 entirely and rename it Water Efficiency.

For a district where roughly 21,000 homes were held up for four years by a single hydrological constraint, a policy quietly being renamed from "Water Neutrality" to "Water Efficiency" in March 2026 is the clearest sign yet of how much has changed — and how easy it still is to submit an application carrying assumptions that are now out of date.

None of this removes water efficiency requirements altogether. The emerging Strategic Policy 9 sets a 110-litres-per-person-per-day standard for new homes and a BREEAM ‘Excellent’ Water category expectation for non-domestic buildings, reflecting Horsham District's continued designation as an area of serious water stress by the Environment Agency. Our full explainer on water neutrality in planning covers the Habitats Regulations background, the Hardham abstraction issue and what the 2025 withdrawal means in more detail.

Horsham's Strategic Flood Risk Assessment and what it means for your site

Horsham District Council's evidence base includes a District-wide Level 1 Strategic Flood Risk Assessment, dated January 2020, covering the district outside the South Downs National Park, alongside the separate Upper Mole Valley / Gatwick sub-region Level 1 SFRA (December 2023) for the northern part of the district around Crawley and Gatwick. Both feed directly into Strategic Policy 10 of the emerging Local Plan, which ties compliance to "the Horsham District Strategic Flood Risk Assessments... or Upper Mole Valley SFRA... or any future updates" by name.

• Two catchments, two SFRAs. The main Horsham District SFRA covers the Arun and Adur catchments; the separate Upper Mole Valley SFRA covers the district's northern edge, which drains toward the Thames via the Mole rather than south to the coast.
• A live Sequential Test. Horsham District Council carried out a Sequential and Exception Test in April 2014 for three strategic mixed-use and residential sites partly affected by flood risk — a reminder that the council applies this test at both plan-making and application stage.
• The SFRA defines the functional floodplain locally. Flood Zone 3b is not simply read off the national map in Horsham District; the SFRA sets out how it is defined for the district's watercourses, and an FRA has to work from that basis.

Our guide to what a Strategic Flood Risk Assessment actually is explains how Level 1 and Level 2 SFRAs differ and why they matter for site allocation as well as individual applications. Horsham District Council publishes its evidence base, including the SFRA, on its Planning Policy evidence base pages.

How national policy applies to flood risk and drainage in Horsham District

Behind Horsham's own policies sits the same national framework every English authority applies: the NPPF's risk-based approach, the September 2025 update to the Planning Practice Guidance confirming that all sources of flooding are assessed, not only rivers and the sea, and the 2025 National Standards for SuDS.

• The NPPF steers development to the lowest-risk land first, via the Sequential Test, with the Exception Test as the second gate where higher-risk development is justified — see our guide to planning for flood risk under the NPPF.
• Vulnerability classification matters as much as the flood zone. A basement flat is treated as highly vulnerable regardless of mitigation; our NPPF Annex 3 guide explains the five categories.
• A Critical Drainage Area designation can require an FRA even in Flood Zone 1 — the Environment Agency's May 2026 surface water update has brought some previously unaffected Flood Zone 1 land in the South East into a mapped surface water risk area for the first time. See what a Critical Drainage Area means for planning.
• From April 2026, planning appeals leave far less room to fix flood or drainage evidence after submission. Our note on the new 2026 planning appeals rules explains why getting Horsham's requirements right first time now matters more than ever.

Building control and planning drainage requirements are not the same thing either, which trips up more Horsham District schemes than it should: a building regulations completion certificate does not discharge a planning drainage condition, and our explainer on whether building control checks planning permission sets out why the two can pull in different directions on the same site.

Flood risk in Horsham District's area (in brief)

Horsham District drains through two river systems: the River Arun, which rises on the High Weald and runs south through Horsham town before reaching the coast at Littlehampton, and the River Adur to the east. Both carry tidal influence some distance inland — the Arun's tidal limit sits at Pallingham Locks, where defences currently provide roughly a 1-in-30-year standard of protection. The River Arun also carries England's one surviving West Sussex Internal Drainage District, administered directly by the Environment Agency rather than a separate board, covering low-lying land within the wider Arun catchment. Around Horsham town itself, the land beside the Arun's banks sits in Flood Zone 3b, the functional floodplain, in places such as Arunside, Tanbridge Park and Ridgehurst Drive.

This is a compact summary only — our flood risk assessment in Horsham page covers the town's flood history, its key watercourses and the wider physical picture in full, and our flood risk assessment in West Sussex page covers the county's chalk groundwater, coast and river systems beyond the district boundary.

How Unda helps with flood risk and drainage for Horsham District Council planning

We are experts on Horsham District's planning system — its adopted and emerging policies, its Strategic Flood Risk Assessments and West Sussex County Council's drainage expectations — not a generic report mill working from a national template.

A flood risk assessment or drainage strategy for a Horsham District site from Unda is built against the council's actual requirements: Policy 38, 24 and 35 of the adopted Framework, the direction the emerging Strategic Policies 9 and 10 are taking, the district's SFRAs, and West Sussex County Council's discharge hierarchy and infiltration standards. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report. Where a drainage strategy is needed alongside the flood risk assessment, we prepare it as a coordinated but separate report, so the two cases stay consistent and an LLFA objection has nowhere to land.

Flood risk and drainage services for every situation in Horsham District'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 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 all three for Horsham District sites, alongside the right form of assessment for buyers, lenders and BREEAM.

Flood risk assessments

• Flood risk assessment for planning. The core report, prepared to satisfy the NPPF, the Environment Agency and Horsham District Council. Groundwater and, where needed, flood modelling are assessed within it.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged Arun or Adur valley 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. A defensible design water table for chalk and valley sites — 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 location at all. Horsham District Council has applied both before, including in its April 2014 Sequential and Exception Test for three strategic sites, and a clean site-specific assessment showing the development is not actually within an area at risk can remove the need at application stage.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering — calculates existing versus proposed runoff and proposes SuDS to hold discharge within West Sussex County Council's standards.
• 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.
• Flood modelling. 1D and 2D hydraulic modelling — a technique that feeds the assessment, Sequential Test or drainage design where national mapping is not enough.

Every report is overseen by a chartered flood risk consultant who stays involved past submission to answer objections, and you can screen a Horsham District postcode first on our free flood risk map or check current conditions on the live river levels map. For live warnings and river levels on the Arun and Adur, the Environment Agency's Check for flooding service and Floodline on 0345 988 1188 are the most reliable sources; for an immediate risk to life in a flood, always call 999.

Horsham District flood risk and drainage: frequently asked questions