Yes, and most of that risk comes from rain, not rivers. The river flood zones touch only a small corner of the place. Nearly a quarter of postcodes rate high risk on at least one flood measure. The gap between those two figures is surface water. A listing that says "not in a flood zone" is telling you about the smaller of the two problems.
On the Environment Agency's river-and-sea mapping, about 3% of South Merstham's postcodes sit in flood zone 3, the highest-probability band. Another 3% sit in flood zone 2, the medium-probability fringe beyond zone 3. The two zones don't overlap, so a postcode is in one or the other, never both. On rivers and the sea alone, this looks like a place where most buyers can stop worrying.
Surface water changes that. There's no separate river-free figure for surface water here, but the combined postcode measures below include it, and they're much higher than the river zones. The recorded history is modest: there are seven recorded flood events, and the most recent was in September 1968, when an ordinary watercourse overflowed its channel where there were no raised defences. About 2% of postcodes fall within the outline of a recorded historic flood. One quiet half-century is not a guarantee. Surface-water flooding tends to happen suddenly, on one street, after one bad storm.
Looking ahead, the Environment Agency's climate-adjusted flood zone covers about 6% of postcodes, more than the present-day zones. That's the Agency's projection of how the river-and-sea picture shifts as the climate changes, not a promise about any one house.
For insurance, the postcode matters, and the exact address matters more. Insurers price flood cover on their own mapping, which includes surface water, so a home outside the river zones can still get a higher quote or an exclusion. Get an indicative quote on the specific address before you instruct a solicitor, not after the mortgage offer arrives.
Across all measures combined, rivers, sea and surface water, about 23% of postcodes rate high risk on at least one measure, about 20% medium, and about 34% low. "At least one measure" is doing important work: these figures overlap with the zone figures above and can't be added to them. The two sets of numbers answer different questions, and this set is the one that includes the rain.
Counted by property rather than by postcode, the Environment Agency's rivers-and-sea mapping lists 16 postcodes carrying some risk, covering 521 properties. Of those, 110 are rated high risk, 107 of them homes, 20 are medium and 35 are low. Only postcodes carrying some risk appear in this list, so every other postcode in South Merstham isn't counted at all, and that absence is the good news.
On defences, about 3% of postcodes fall within the high-risk extent with no defences, and about 3% with defences included in the model. Modelled defences don't shrink the high-risk area here. "Defended" only means a defence exists in the model. It doesn't make a property safe, and a seller who presents it as reassurance is reading more into the map than it says.
The nearest tide gauge is at Portsmouth, which shows how far the sea is from any immediate concern here. Its record from 1962 to 2023 shows sea level rising at about 2 millimetres a year. That's a measured past rate at a distant coast, not a forecast for South Merstham.
Flood risk is South Merstham's weak spot, and the problem is surface water, not the river. Check the exact address before you fall for the house.