It depends entirely on which type of flooding. Surface water is the live concern, not rivers or sea. Roughly one in seven postcodes carries the highest surface-water risk band. River and sea flood zones cover essentially none of the town. The map you need to look at is not the one estate agents usually show you.
River and sea flooding is, on the published flood-zone maps, essentially absent from the town. The higher-probability river and sea flood zone covers none of the postcodes here. The lower-probability one catches around one in a thousand. That reflects the simple geography. Hailsham sits inland, with the nearest coast around seven kilometres away across the Pevensey Levels. The historic flood record carries no event for the town itself. The rivers and the coast are not the threat. The threat is the rain that lands here.
Surface water flooding is where the picture changes, and where buyers should pay attention. About one in seven postcodes is in the highest band for surface-water or groundwater risk. Roughly one in ten is in the medium band. About one in six is in the low band. Add those together and the question stops being whether some streets are exposed and becomes which streets you happened to be looking at. The town centre is largely fine. Specific roads close to drainage ditches, the watercourses around the Levels, and the lower-lying edges of the urban area are not. The risk concentrates where water concentrates, which is in the places drainage engineers have been quietly worrying about for years.
Climate projections do not change the river and sea picture much. The climate-adjusted flood zone, which models a higher-rainfall future, brings the share of postcodes affected up to under 1% across the town. That is not the kind of number that should drive a buying decision on its own, but it is also not zero. Anyone buying a property at the lower edge of the town should ask whether their specific plot sits in that climate-adjusted band, not whether the town as a whole does.
Insurance is where surface-water risk usually shows up first. A property in a high surface-water postcode is more likely to be quoted with a flood-cover exclusion, or loaded with a higher premium and a higher excess on the flood peril specifically. Most mainstream insurers will quote on most properties in Hailsham without flagging anything. The ones that get flagged tend to get flagged consistently across insurers. Before exchanging on a property in this town, get an actual quote for buildings cover on the specific address, not a generic one based on the postcode area. The difference between the two can be substantial, and the gap will not narrow.
The honest verdict is mixed. The river and sea risk is essentially absent. The surface-water risk is real in specific places and irrelevant in others. Read the address-level data, not the headline. Flooding in Hailsham is a street question, not a town question.