Horsham's flood risk comes mostly from rain rather than the river. Hardly any of the town sits in the river flood zones. About one postcode in ten carries a high reading on at least one flood measure once surface water is counted. The last recorded flood was caused by drainage rather than the Arun bursting its banks. Worry less about the river and more about what the gutters do in a storm.
Start with the rivers. The Environment Agency's river-and-sea flood zones barely touch the town. About a tenth of 1% of the area's postcodes fall in Flood Zone 3, the highest-probability band, and about a fifth of 1% in Flood Zone 2, the medium-probability fringe beyond it. The two bands don't overlap, so Zone 2 being slightly larger doesn't mean it contains Zone 3. The Arun cuts through the south of town past Chesworth Farm and the cricket field, but the mapping gives it very little of the housing.
Surface water is the more realistic risk. The Environment Agency's records hold 20 recorded flood events for the area, and the most recent, in June 1981, is logged against the main river but caused by local drainage and surface water. That pattern fits a town built on clay in the Low Weald, where heavy rain has to find somewhere to go and a low garden or a dropped driveway is often where it ends up.
Looking ahead, the Environment Agency's climate-adjusted flood zone covers about 1% of postcodes. It is a modelled allowance for wetter future conditions, not a forecast, and it remains small.
For insurance, most Horsham homes should find cover straightforward, because the river-and-sea zones cover so little. Homes in a low spot, or on a street with a history of standing water, are where underwriters ask more questions. Premiums follow the exact address rather than the town, so a quote on a specific property tells you more than any town-wide figure.
The postcode measure gives a broader view. Counting rivers, sea and surface water together, about 10% of postcodes rate high on at least one flood measure, about 14% medium and about 32% low. These bands overlap with the flood zones above and can't be added to them. They are wider mainly because surface water is included, and surface water is Horsham's real exposure.
Counting homes instead of postcodes makes the river risk look smaller still. Across the 22 postcodes the Environment Agency's rivers-and-sea mapping lists with some risk, holding 550 properties, three are rated high risk, one medium and 88 low, and none of the high-risk properties are homes. Only postcodes with some risk appear in that list, so the rest of the town isn't counted at all, which on the river measure counts in its favour.
On defences, about a tenth of 1% of postcodes sit in the high-probability flood extent with no defence modelled, and about the same share where a defence is present in the model. A modelled defence means something stands in the water's path in the model. It doesn't mean the house behind it is safe, and a buyer shouldn't read it that way.
Sea level is a distant concern for an inland town. The nearest tide gauge, at Portsmouth, recorded a rise of about 2 millimetres a year across its record from 1962 to 2023. That is a measured historic rate on the coast, not a forecast for the Arun's upper reaches.
Before buying, do these four things:
- Put the exact address into the Environment Agency's long-term flood-risk service and check the surface-water result as well as the river one.
- Ask the seller directly whether water has ever stood in the garden, driveway or road outside.
- Get an indicative insurance quote on the specific property before instructing a solicitor.
- Look at where the property sits relative to the street; a house below the road collects what the road sheds.
Horsham's flood risk is middling. The river poses little threat to homes, surface water is a real enough risk to check address by address, and the buyers who check are the ones who don't get caught out.