Flood Risk Assessment in Horsham

The River Arun rises on the southern slopes of the High Weald and cuts through Horsham town centre before heading south towards the coast. Where the river narrows between the town's older streets and the Arunside, Tanbridge Park and Ridgehurst Drive neighbourhoods, the land beside its banks sits in Flood Zone 3b — the functional floodplain. For anyone developing, buying, insuring or managing property in Horsham District, that geography matters.

We prepare flood risk assessments in Horsham for planning, property purchase, mortgage and insurance, and BREEAM — alongside sequential and exception tests and surface water drainage strategies. A flood risk assessment, a sequential test and a drainage strategy are three distinct documents. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be in this location, and where does the surface water go?

Parts of Horsham town centre have sat in Flood Zone 3b — the functional floodplain of the River Arun — for decades. When Storm Ciarán struck in November 2023, the Environment Agency issued a severe flood warning for the Arun through the town, placing properties from Amiesmill Bridge to Hills Farm Lane at immediate risk.

To speak to one of our experienced flood risk consultants about a site in Horsham, call 01293 214444 or email enquiries@unda.co.uk. Our flood risk consultants have extensive experience of flood risk and planning policy across West Sussex.

Flood risk along the River Arun and across Horsham District

The River Arun is the defining flood risk source for Horsham town. It enters the district near Slinfold to the west, picks up the Ewhurst Stream, the Hammer Stream and the Boldings Brook, then narrows as it passes through the town's historic core. The Environment Agency's Flood Zone 3 — where flooding is most probable — follows the river closely through Arunside, Millbay Lane, Tanbridge Park and Ridgehurst Drive. Flood Zone 3b, the functional floodplain where water must be allowed to flow and be stored, covers the river's immediate margins through these neighbourhoods.

Surface water is a major secondary risk across the district, particularly on the Lower Weald clay soils to the south and west of the town where infiltration rates are poor and runoff accumulates quickly in heavy or sustained rainfall. Groundwater flooding can affect basements and lower-lying gardens in parts of Horsham built on sands and silts overlying the Weald Clay — a risk that is seasonal and less well-mapped than fluvial risk, but one the Environment Agency's Areas Susceptible to Groundwater Flooding dataset flags as a moderate concern in parts of the district.

Key watercourses in Horsham District

• River Arun (main river, EA responsibility). Rises on the High Weald and flows south through Horsham town centre. The EA's active flood warning area “Horsham on the River Arun” (reference 065FWF5001) runs from Amiesmill Bridge to Hills Farm Lane, covering Millbay Lane, Tanbridge Park, Arunside and Ridgehurst Drive.
• Boldings Brook. Flows through residential neighbourhoods to the north of the town centre. Localised Flood Zone 2 and 3 floodplain exists along its lower reaches, adding risk to sites that sit outside the main Arun corridor.
• Chennells Brook. To the east of the town; a source of surface water and fluvial flood risk after sustained rainfall events, and relevant to development on the eastern fringe of Horsham.
• Billingshurst watercourses. The Limeburner Stream and smaller ordinary watercourses cross the A29 corridor — directly relevant to Horsham District’s strategic growth allocations in and around Billingshurst.
• Ordinary watercourses (OWCs). Managed by West Sussex County Council as the Lead Local Flood Authority and by riparian owners. OWCs across the district can deliver localised flooding that does not always appear on the EA’s national flood maps, and must be considered in any site-specific flood risk assessment.

The Horsham Level 1 Strategic Flood Risk Assessment (January 2020), prepared for Horsham District Council, is the definitive local evidence base for flood zone designations and flood risk policy across the district. Our consultants work with it routinely. Use the Unda Flood Risk Map and the interactive map below to check the flood risk at any Horsham postcode.

A history of flooding in Horsham

The River Arun has flooded Horsham repeatedly. Understanding the pattern matters for flood risk assessments, insurance reports and planning decisions alike, because the planning system, the Environment Agency and insurers increasingly expect a site-specific flood risk assessment in Horsham to account for the area’s actual flood history alongside the modelled risk data.

• Storm Ciarán — November 2023. One of the most significant rainfall events in decades struck southern England. The EA issued a severe flood warning (“Horsham on the River Arun”) as the Arun rose rapidly through the town. Properties in Arunside, Tanbridge Park and Millbay Lane were at immediate risk of flooding. The event placed Horsham’s flood management infrastructure under sustained pressure and reinforced the case for site-specific flood risk work on any development near the river corridor.
• December 2023 — River Arun on high and rising. Within weeks of Storm Ciarán, a second sustained rainfall period sent river levels back up. A flood warning remained in place for Horsham on the River Arun for several days, affecting the same reach from Amiesmill Bridge to Hills Farm Lane. Two severe events in rapid succession in a single season highlighted the increasing frequency of high-water events on this watercourse.
• January 2020 — widespread bank overtopping. Sustained winter rainfall following storms Ciara and Dennis caused the Arun to overtop its banks at multiple points between Horsham and Billingshurst. Surface water flooded roads and access routes across the western fringe of the district, affecting land that sits nominally outside the EA’s Flood Zone 3 maps but is clearly at risk in practice.
• February 2014 — prolonged fluvial flooding. The winter of 2013–14 saw the longest run of storm events in recorded UK history. The Arun within Horsham District was at or above flood alert level for weeks, and parts of the town’s riverside were inundated for extended periods. The event is referenced in the Horsham SFRA 2020 as part of the district’s historic flood record.

Sign up for Environment Agency flood warnings for the Horsham area at gov.uk/get-flood-warnings, or track live river levels on our monitoring map. In an emergency, always call 999.

When does development in Horsham need a flood risk assessment?

Planning Policy Guidance requires a site-specific flood risk assessment for any planning application in Flood Zone 2 or 3, and for major development — 10 or more dwellings, or 1,000 m² or more of non-residential floorspace — in any flood zone including Flood Zone 1. In Horsham District, Horsham DC’s Local Plan and the Horsham SFRA 2020 are the evidence base the local planning authority uses alongside national policy. Change of use to a more flood-vulnerable building class, or development near an ordinary watercourse, will often trigger an FRA even where the standard thresholds are not met on their own.

If you are unsure whether your site requires a flood risk assessment in Horsham, contact one of our experienced flood risk consultants. We work with Horsham DC and WSCC LLFA requirements routinely and can give you a clear answer quickly. Read our guide to when you need a flood risk assessment or explore sequential and exception tests for sites with more complex location choices.

Horsham’s development landscape and the water neutrality change

Horsham is one of the fastest-growing districts in the South East. The emerging Horsham Local Plan 2023–2040 proposes approximately 13,212 new homes and 114,000 m² of employment floorspace across the district over its plan period. Strategic allocations include a major new neighbourhood at Ifield (up to 3,000 homes), urban extensions at Southwater (around 1,000 homes) and Billingshurst (around 650 homes), and a mixed-use scheme at Mowbray in the town centre (around 500 homes). Every major site coming forward in Horsham will require a flood risk assessment and, on any significant area of greenfield or brownfield land, a drainage strategy.

Until recently, development across the Arun Valley faced an additional constraint. Natural England’s water neutrality position statement required new development to demonstrate that it would not increase net water consumption in the Arun & Rother Levels Special Area of Conservation. This effectively stalled around 2,600 planning permissions across several catchment districts, including Horsham, for several years. In November 2025, Natural England withdrew the position statement, confirming that the water neutrality constraint no longer applies to new applications. The change has unlocked a substantial pipeline of schemes, and the volume of planning applications requiring flood risk assessment and drainage work in Horsham District has risen accordingly. Applicants with schemes that were stalled, or that carry historic water neutrality conditions, should confirm their current position with specialist planning and environment consultants.

• Ifield Garden Village (up to 3,000 homes). Large strategic allocation in the west of the district, partly within the Arun catchment. Requires a comprehensive flood risk assessment and drainage strategy at masterplan stage; ordinary watercourses and surface water management are central design issues.
• Southwater strategic sites (c. 1,000 homes). Greenfield land south of the town; Weald Clay substrate means surface water management is a primary WSCC LLFA concern. Discharge rates and SuDS solutions must be agreed with WSCC before determination.
• Billingshurst strategic extension (c. 650 homes). Located close to the River Arun and the Limeburner Stream. Parts of the allocation boundary fall within Flood Zone 2 or 3, so FRA and sequential test work will be required alongside a drainage strategy.
• Mowbray mixed-use site (c. 500 homes). Urban regeneration site within the town centre; surface water management and groundwater are the primary drainage considerations given the site’s position in the town.
• Water neutrality position withdrawn November 2025. Arun Valley catchment constraint removed. Stalled permissions can now progress; applicants with historic conditioned obligations should seek specialist advice to confirm their current requirements.

Our consultants are experienced in flood risk assessment across West Sussex and understand Horsham DC’s development landscape, the SFRA evidence base and WSCC LLFA’s surface water drainage requirements. For an overview of how flood risk fits into the planning process, see our guide to the Environment Agency and flood risk planning.

Flood risk assessments for every situation in Horsham

We prepare flood risk assessments in Horsham for a wide range of purposes. Each is a distinct piece of work addressing a specific question; none is a secondary or bolt-on service.

• Planning flood risk assessments. For applications in Flood Zone 2, 3 or on major sites in Flood Zone 1. Prepared to satisfy Horsham DC’s planning policy requirements, the Horsham SFRA 2020, and the Environment Agency’s standing advice and pre-application consultation requirements. We prepare FRAs for residential, commercial, industrial and mixed-use sites across the district.
• Property purchase flood risk surveys. For buyers, solicitors and mortgage lenders concerned about flood risk at a property in Horsham. A Flood Risk Survey and Report gives a proper site-level picture of the flood risk, insurance implications and any relevant mitigation — going well beyond the EA’s national flood map.
• Insurance flood risk reports. Where a policy has been refused, restricted or priced at a level that requires justification, a flood risk report can provide insurers and loss adjusters with site-specific evidence. We prepare insurance flood risk reports for residential and commercial properties across Horsham District.
• BREEAM flood risk assessments. The BREEAM Man 05 credit requires a formal site flood risk assessment as part of the scheme’s sustainability appraisal. We prepare BREEAM-compliant reports for commercial and residential schemes in Horsham seeking BREEAM ratings from Pass to Outstanding.
• Sequential and exception tests. Where a site is in Flood Zone 2 or 3 and the LPA requires a sequential test to demonstrate no reasonably available lower-risk alternative exists, we prepare the sequential test report. Where the exception test is also required, both documents travel together. Read our analysis of the Yatton judgment on exception test planning balance.
• Groundwater flood risk assessments. For sites where seasonal groundwater is a concern — basements, undercroft parking or lower-lying garden and amenity areas in parts of Horsham built on sands and silts overlying the Weald Clay. A groundwater FRA considers the source and seasonality of the risk alongside the surface-water and fluvial picture.
• Surface water drainage strategies and SuDS. A planning FRA is often accompanied by a drainage strategy. These are separate commissioned documents answering a different question: where does the surface water go, and how does it meet the requirements of WSCC LLFA and Horsham DC? See the next section for more detail on SuDS in Horsham.
• Preliminary drainage design. For applications where a full drainage strategy is required at outline or detailed planning stage, we prepare preliminary drainage designs that set out the SuDS approach, the attenuation volumes and the indicative layout for WSCC LLFA and Horsham DC consideration.

We also prepare Flood Consequence Assessments (FCAs) for sites where a detailed assessment of flood consequences — including residual risk behind defences — is required. Read more about residual flood risk in our technical blog. Our full range of services for Horsham sits within our wider coverage of flood risk assessment across Sussex and all areas we cover.

Surface water drainage strategies and SuDS in Horsham

West Sussex County Council is the Lead Local Flood Authority (LLFA) for Horsham District. For major planning applications — broadly those of 10 or more dwellings or 1,000 m² of non-residential floorspace — WSCC is a statutory consultee for surface water drainage. The LLFA expects surface water to be managed sustainably on site using Sustainable Drainage Systems (SuDS), with post-development discharge rates limited to the pre-development greenfield rate or, where that is not achievable, the 1-in-1 year rate for brownfield sites. The national SuDS standards (in force from 2024) require all new major development to incorporate SuDS meeting the four pillars: water quantity, water quality, amenity and biodiversity.

Horsham DC also requires a Surface Water Drainage Statement for most residential development, even below the major-application threshold. On the Weald Clay soils that dominate much of Horsham District, infiltration rates are typically very low — class 5 or F under the BRE 365 infiltration rate assessment, meaning soakaway drainage is rarely viable. Above-ground attenuation features — detention basins, swales, ponds and permeable surfaces — are the primary SuDS solution for most sites in the district. WSCC LLFA provides pre-application drainage advice for proposals that raise complex surface water questions, and we recommend engaging WSCC early on any large or complex site.

• Water quantity. Attenuation of runoff to greenfield rates for the 1-in-1, 1-in-30 and 1-in-100 year events (plus 40% climate change allowance). On Weald Clay sites across Horsham District, this typically means above-ground storage. Use our drainage calculators for initial sizing estimates.
• Water quality. SuDS treatment trains that remove pollutants before any discharge to watercourses or public sewer. Important given the Arun Valley’s SAC designations and the sensitive receiving water environment. Vegetated filter strips, swales and pond systems are the preferred approach for most Horsham District sites.
• Amenity. SuDS features designed as attractive, usable landscape elements, not purely as engineering infrastructure. Horsham DC and WSCC LLFA expect drainage that enhances the quality of spaces in new development. Green swales and naturalistic detention features score well on both drainage performance and amenity criteria.
• Biodiversity. SuDS contributing to biodiversity net gain (BNG), which is mandatory on most planning applications in Horsham District under the Environment Act 2021. Ponds, wetland margins and planted swales count positively toward the BNG metric. Read more about nature-based drainage and SuDS and the four pillars of SuDS in our technical blog.

Our drainage strategy and preliminary drainage design services cover the full range of drainage work required for planning applications in Horsham, from a short Surface Water Drainage Statement for a single dwelling to a full SuDS design strategy for a strategic allocation of hundreds of homes. We know WSCC LLFA’s technical expectations and design to meet them from the outset. Our ground conditions and soakaway tool gives indicative guidance on infiltration feasibility for Horsham postcodes.

The authorities behind Horsham’s flood risk system

A flood risk assessment in Horsham must be addressed to the right audience. Understanding the roles of each body — and what each will scrutinise — is essential for preparing an assessment that gets approved first time.

• Environment Agency (EA). Statutory consultee for development in Flood Zone 2 or 3, and for development within 8 metres of a main river (including the Arun). Responsible for the Flood Map for Planning, Flood Zone designations, and issuing flood warnings on the River Arun. The EA maintains the Horsham flood warning area 065FWF5001 covering the reach from Amiesmill Bridge to Hills Farm Lane. Sign up for warnings at gov.uk/get-flood-warnings. Read our guide to the EA’s role in flood risk planning.
• West Sussex County Council — Lead Local Flood Authority (LLFA). Statutory consultee for surface water drainage on major planning applications. WSCC manages risk from ordinary watercourses, surface water and groundwater across Horsham District. WSCC publishes a Local Flood Risk Management Strategy and provides pre-application drainage advice. Drainage strategies must meet WSCC’s technical requirements. See our blog on the role of LLFAs in planning.
• Horsham District Council (HDC) — local planning authority. Applies the Horsham Local Plan, the Horsham SFRA 2020 and national planning policy to all planning applications with a flood risk or drainage element. HDC is the decision-maker; the EA and WSCC LLFA are consultees whose objections HDC will normally follow unless the applicant’s technical evidence addresses their concerns.
• Southern Water. Responsible for foul and surface water sewers across most of Horsham District. Development must not increase surface water discharge to the combined sewer network; applicants on larger schemes may need a Southern Water capacity assessment alongside the drainage strategy.
• Regional Flood & Coastal Committee (RFCC). The Solent & South Downs RFCC approves funding for EA-managed flood risk assets in the Horsham area, including any works on the River Arun flood defences. Works on main rivers require EA consent under the Environmental Permitting Regulations.

In a flood emergency: call 999. Report flooding to the EA Floodline on 0345 988 1188 (24 hours). Power cut: 105. Check for live warnings at check-for-flooding.service.gov.uk.

Frequently asked questions about flood risk assessment in Horsham