Flood Risk Assessment in Horsham

Every flood risk assessment in Horsham has to deal with two watercourses and with the town's own streets. The River Arun runs along the south side of the town past Tanbridge and Hills Farm Lane, and the Boldings Brook comes south from Warnham Mill to meet it, each with its own Environment Agency flood warning area; away from both, surface water is the commonest cause of recorded flooding in the district. We prepare flood risk assessments in Horsham for people applying for planning, buying a house, arranging a mortgage or renewing insurance, and write sequential and exception tests and surface water drainage strategies alongside them.

Horsham, West Sussex, is taken here as the ONS built-up area: the town centre around the Carfax and North Street, New Town and Tanbridge by the river, Roffey, Littlehaven and Oakhill to the east, and North Heath and Holbrook to the north, across the RH12 and RH13 postcode districts. Broadbridge Heath, Warnham and Southwater are separate built-up areas outside the outline, though the same district council, county council and flood evidence apply to them.

On Christmas Eve 2013 the Arun at the Tanbridge gauge reached 34.96 m, its highest level on record. The Environment Agency puts the point where property flooding becomes possible there at 35.20 m, so the worst flood the gauge has measured stopped 24 cm short of it.

To talk through a site in the town with one of our flood risk consultants, call 01293 214444 or email enquiries@unda.co.uk.

The Arun, the Boldings Brook and the streams that feed them

The Arun rises in St Leonard's Forest, north-east of the town, and is still a small river when it reaches Horsham. West Sussex County Council's 2025 flood strategy describes the upper catchment as Weald Clay that makes the river "very reactive to rainfall variations", so it rises fast after heavy rain. Within the town the flood risk sits in a few named corridors rather than across the whole place:

• River Arun. The Horsham on the River Arun warning area runs from Amiesmill Bridge to Hills Farm Lane and takes in Millbay Lane, Tanbridge Park, Arunside and Ridgehurst Drive. Below the town the river passes Broadbridge Heath and Slinfold on its way to Pallingham, which our River Arun page follows to the sea.
• Boldings Brook. It leaves Warnham Mill Pond and flows south to the Arun, and has a warning area of its own, Horsham on the Boldings Brook, covering Warnham Mill, Redford Avenue, Merryfield Drive, Irwin Drive, Hills Farm Lane and River Walk.
• Chennells Brook. On the eastern side of Horsham, it shares an Environment Agency flood alert area with the Boldings Brook. Both brooks, and Warnham Mill Pond, are main rivers, so the Environment Agency rather than the county council is the authority for them.
• Hornbrook Stream. A smaller tributary on the south-east edge of the town. Its flood zones cut across the Hornbrook Farm allocation, which is why that site has to pass the exception test.
• Ordinary watercourses and culverts. Ditches and piped streams through the estates are ordinary watercourses. West Sussex County Council's culvert policy opposes new culverting except where no reasonable alternative exists, and in Horsham district the consent for that work is issued by the district council. Our explainer on culverts and who owns them covers the detail.

Because two warning areas meet near Hills Farm Lane, a site in the south-west of the town can face both rivers at once. Our live river levels map shows the Tanbridge and Provender Mill gauges and any warning in force.

What the Tanbridge and Provender Mill gauges have recorded

Two Environment Agency stations on the Arun give a working scale for Horsham. The River Arun at Tanbridge gauge sits by the river south-west of the town centre and is read in metres above sea level:

1. 32.15 m to 32.60 m. The normal range at Tanbridge.
2. Above 34.20 m. Fields and other low ground beside the river may flood, and alerts can go out.
3. 34.96 m on 24 December 2013. The highest level the station has recorded.
4. Above 35.20 m. The level above which the agency says property flooding is possible.

Upstream, the Provender Mill gauge set its record in a different season: 37.40 m on 11 August 2007, in high summer rather than winter. A riverside assessment in Horsham therefore allows for summer downpours as well as long wet winters. It also works from modelled flood levels, not the gauge record alone, because the flood zones are drawn from modelled floods with a 1% and 0.1% annual chance, not from what a gauge happens to have seen.

Check a postcode from Roffey to Tanbridge

Type a postcode into the interactive flood map below to screen a Horsham address against Environment Agency data for rivers, surface water and reservoirs, or use our full flood risk map for anywhere in England. The coral line is the ONS built-up area, so Roffey, North Heath and the town centre fall inside it, while Broadbridge Heath and Warnham, each mapped by ONS as a settlement of its own, lie just beyond the western and north-western edges.

Floods on record in Horsham since 1968

Horsham's local flood history mixes river floods on the Arun with surface water in the streets, and the reservoir at Warnham Mill has needed its own safety work. These are the dated events the councils and the local press have set down:

1. September 1968. The district's 2010 flood study lists up to 50 damaged properties in Horsham in that month's Arun flood. The county's 2011 preliminary flood assessment logs flooding at various locations in the town on 20 September 1968, with a main road under water and a figure of 422 properties, though it does not say where the water came from.
2. 11 August 2007. The Provender Mill gauge reached 37.40 m, still its highest level.
3. 24 December 2013. The Arun at Tanbridge peaked at 34.96 m, still the station's record.
4. Autumn 2014. The Environment Agency began a £1.1 million safety scheme on the Warnham Mill Pond dam after its routine inspection, adding a steel sheet pile wall and strengthening the grass spillway, to better protect people living downstream from a breach or overtopping.
5. 7 January 2016. Water ran down North Street from Kings Road and stood on Station Road and the A24, and the county council said that much of the surface water drainage system was struggling with the capacity.
6. 20 December 2019. The Arun burst its banks in several places, the Memorial Gardens went under and staff at Afon House on Worthing Road were told to leave the building.

Surface water is the more common problem: the district's 2020 SFRA found it the most frequent source of recorded flooding. The 2010 flood study estimated that about 1,800 properties in Horsham were susceptible to surface water flooding, more than in any other settlement in the district, and the county's 2025 strategy still names Horsham, with Billingshurst, as a place where surface water and sewer incidents are concentrated. A reservoir adds a further source to check below Warnham Mill, which our note on reservoir flooding explains.

Sandstone, clay and soakaways under the town

Ground conditions change across Horsham, and they decide whether rain can soak away on a site or has to be stored and released slowly. British Geological Survey mapping shows the town sitting mostly on the Upper Tunbridge Wells Sand, a mix of sandstone and mudstone, with Weald Clay on the western side and river alluvium along the Arun:

Horsham District Council's Surface Water Drainage Statement guidance asks for soakaways to be sized from percolation tests to BRE Digest 365, with a factor of safety of at least 2, and for them to half-empty within 24 hours after a 1 in 10 year storm. It does not accept house or rubble soakaways. Our BRE 365 infiltration rate calculator works through the test results, and our guide to when infiltration testing is needed explains when the council will expect it.

On major schemes West Sussex County Council, as Lead Local Flood Authority, applies its August 2026 guidance on complying with the national standards for SuDS: discharge held to the greenfield rate, the first 5 mm of rainfall kept on site, winter groundwater monitoring before infiltration is relied on, and at least 1 m between the highest groundwater level and the base of a soakaway. Those standards are summarised in our article on the 2025 national standards for sustainable drainage.

Which flood policies apply to a Horsham planning application?

Applications in Horsham are decided by Horsham District Council under the Horsham District Planning Framework, adopted on 27 November 2015, whose Policy 38 on flooding takes a sequential approach and asks for a site-specific assessment on every proposal in Flood Zones 2 and 3 and on sites over a hectare in Flood Zone 1. A new Local Plan for 2023 to 2040 is still at examination, with hearings held in October 2026 and adoption expected in early 2027.

• The emerging flood policy. Main modification MM61 rewrites Strategic Policy 10 on flooding: no risk to occupants or to land elsewhere, an assessment for every proposal in Flood Zones 2, 3a and 3b, the sequential test, SuDS unless technically unfeasible and no surface water sent to the foul sewer. It gains weight as the examination moves on.
• The current SFRA. AECOM's Level 1 Strategic Flood Risk Assessment, revised in September 2024, replaces the January 2020 study. For the Arun through Horsham it uses the Environment Agency's 2017 Upper Arun model and, as a precaution, combines the 45% climate change extent with Flood Zone 2.
• Functional floodplain. Where the models do not separate Flood Zone 3b, the SFRA treats Flood Zone 3a as functional floodplain, which narrows what can be built beside the Arun and the brooks.
• Detailed modelling. Any site in Flood Zone 2 or 3, or inside the future flood extent, needs a detailed hydraulic modelling study using current climate change allowances.
• Climate allowances. For the Arun and Western Streams catchment the SFRA gives peak river flow uplifts of 13%, 19% and 36% for the 2050s on the central, higher central and upper end figures, and 20% to 45% on peak rainfall for the 1% storm. Our note on climate change allowances explains which one a scheme uses.

National policy sits in chapter 18 of the August 2026 NPPF: policy F4 says when an assessment is needed, F5 and F6 set the sequential and exception tests, and F8 expects every development to follow the 2025 national standards for sustainable drainage. The district-wide picture, including validation and the Surface Water Drainage Statement form, is set out on our Horsham District Council page.

Hornbrook Farm, Mercer Road and the other sites on the edge of town

The council's June 2026 flood tests for the modified Local Plan give each allocation around Horsham a flood zone split and a surface water figure. Only one of the town's sites needs the exception test, but several carry enough surface water risk to shape the layout:

Paragraph 5.7 of the paper expects a site-specific flood risk assessment on every allocation with flood risk, and the sequential approach to run through the site design. A windfall site that is not allocated starts with the sequential test of its own.

The water constraint that held back Horsham's housing has gone. Natural England withdrew its water neutrality position on 31 October 2025, and the district council says the Environment Agency formally agreed the reduced Hardham abstraction licence on 9 October 2026, so applications no longer need to deal with it. It was a water supply test, not a flood one, but its end is why the Inspector asked the council to plan for more homes, as the council reported in July 2026. Our article on water neutrality sets out what changed.

From Hills Farm Lane to Roffey: when an assessment is required

Policy 38 and chapter 18 of the NPPF decide when a Horsham application needs a flood risk assessment, and the Environment Agency's standing advice covers small extensions. The table reads those rules against the kinds of site that come forward in the town:

Our guide to when a flood risk assessment is needed goes through the national triggers in more detail, and our Environment Agency planning guide explains when the agency is consulted.

What we prepare for sites beside the Arun, in the town centre and on its edge

A buyer in Arunside, a householder in Roffey and the promoter of an allocation each need a different report, and most Horsham sites have to answer for both the river and the ground. Our consultants are chartered, and a senior consultant checks and signs off each report.

• Applications near the river and the brooks. Homes, conversions and commercial schemes in the Arun or Boldings Brook flood zones need a flood risk assessment for planning that answers Policy 38 and the 2024 SFRA.
• Every source on one site. A flood risk assessment in Horsham looks at the Arun and the brooks, surface water on the clay, sewers and the Warnham Mill Pond dam as one picture.
• Before exchange in the warning areas. Lenders and searches pick up Millbay Lane, Tanbridge Park and the Boldings Brook streets, and a flood risk survey for property purchase answers for one house.
• Housing on the river corridor. New homes in the Arun flood zones, and the Hornbrook Farm allocation with its Zone 3 strip, need the sequential and exception test.
• Drainage evidence on every scheme. The district council asks major applications for a drainage strategy or statement and will not take rubble soakaways, so most Horsham schemes need drainage strategy work of some kind.
• Greenfield rates on mixed ground. A surface water drainage strategy tests the sandstone for soakaways and falls back to storage and a greenfield-rate discharge where the mudstone and clay will not drain.
• Clearing the condition. Where a Horsham permission has a drainage condition attached, discharge of drainage conditions provides the details and the maintenance plan the council expects.
• Levels for a riverside plot. The 2024 SFRA asks for detailed hydraulic modelling on any site in Flood Zone 2 or 3 or the future extent, which is where flood modelling of the Arun or the Boldings Brook comes in.
• Warning and a safe route. Two Environment Agency warning areas meet near Hills Farm Lane, and a flood warning and evacuation plan ties a riverside scheme to them.
• Insurance near the river or on a flow path. With about 1,800 Horsham properties estimated to be susceptible to surface water, a flood risk assessment for insurance gives an insurer evidence for one address.
• Offices and works by the river. Staff at Afon House on Worthing Road were told to leave the building when the Arun rose in December 2019, and a business continuity flood risk assessment plans for that kind of day.
• Employment land at Hop Oast. A third of the EM1 allocation falls within the 1,000-year surface water extent, and a BREEAM flood risk assessment covers the Pol 03 credits for new commercial buildings there.
• Testing a site before an offer. With Hornbrook Farm, Mercer Road and the extra 500 homes north of the town in the modified Local Plan, a flood risk feasibility study shows early whether a layout works.

Warnings, drains and sewers: the Horsham contacts

If floodwater is putting anyone in Horsham in danger, ring 999; for a power cut, ring 105. For everything else, the organisations below each look after part of the town's flood risk:

The county as a whole is covered on our West Sussex page. East of Horsham, Crawley drains north to the Mole rather than south to the Arun, and across the Surrey border lie Cranleigh and Dorking. Other places are in the full list of areas.

Horsham flooding and planning: questions clients ask

Unda Consulting Limited prepares flood risk assessments, sequential and exception tests and drainage strategies for sites across Horsham, from riverside plots at Tanbridge to the allocations on the edge of town. To discuss one, call 01293 214444 or email enquiries@unda.co.uk.