Some of Horley is, and the risk sits in specific streets rather than across the town. Only a sliver lies in the river flood zones. Surface water flags far more postcodes than the rivers do. The climate-adjusted map is a good deal less comfortable than today's. Check the address, not the town.
On the Environment Agency's river-and-sea mapping, about 1% of Horley's postcodes fall in flood zone 3, the highest-probability band, and a further 8% sit in zone 2, the medium-probability fringe beyond it. The two bands do not overlap: zone 2 is the extra ground at lesser risk, not a bigger zone containing the first. The water here comes from rivers, not the sea. The town grew up partly around the River Mole on what was once waterlogged Wealden clay, and the Environment Agency holds 75 recorded flood events for the area, the latest on 25 November 1974, when a main river burst its channel with no raised defences in the way.
Surface water is where the wider exposure lies, and no river-zone map will show it. Heavy rain on clay has nowhere quick to go, so low-lying roads and gardens can take water well away from any river bank. That risk is folded into the postcode indicators below rather than the flood zones, and the two should never be read as one measure.
Looking forward, about 32% of postcodes fall inside the Environment Agency's climate-adjusted flood zones, which model how the river footprint could grow with wetter conditions. That is a projection of the modelled zones, not a promise, but it is the figure that tells you how much of today's safe-looking map is safe on borrowed time.
Insurance follows the address, not the town. A home in a flood zone or with a recorded history can expect questions, higher premiums or a larger excess, and a home outside every indicator usually sails through. The sensible move is an indicative quote on the exact property before a solicitor is instructed, since discovering the premium after exchange is the one surprise nobody enjoys.
Taking every measure together, including surface water, about 11% of postcodes rate high on at least one flood indicator, 17% medium and 46% low. These bands overlap with the river zones above, because they include them alongside surface water, so the numbers cannot be added to the zone figures or treated as the same thing. A postcode can be clear of the river zones and still carry a high surface-water flag.
Counting homes rather than postcodes gives a clearer sense of scale. The Environment Agency's rivers-and-sea assessment lists 54 postcodes carrying some risk, covering 1,245 properties. Of those, 25 properties are at high risk, 15 of them homes, 97 at medium risk and 236 at low risk. Postcodes with no river or sea risk do not appear in that list at all, so the count describes the exposed pockets, not the whole town.
Defences barely change the modelled picture. About 1% of postcodes fall inside the high-risk flood extent with defences left out of the model, and the same 1% with defences included. A defence being present in the model does not mean a property is safe; here it means the modelled high-risk footprint is the same either way, which is an honest answer to the question buyers rarely think to ask.
The nearest tide gauge, at Portsmouth, recorded sea level rising at 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 Horley, and a town this far inland worries about its rivers and its rainfall long before it worries about the Channel.
Horley's flood risk is middling: small in the river zones, wider on surface water, and concentrated enough that the right house is untroubled and the wrong one is not. The street decides it, so ask about the street.