The flood risk in Horndean is real, just not the kind brochures mention. Almost no addresses sit in a river or coastal flood zone. About one in five sits in a high surface-water risk band. Another one in five sits in medium, and roughly three in ten in low. The water that worries you here falls from the sky, not from a riverbank.
The river and sea picture is the easy half. Effectively no addresses sit in Flood Zone 3, the high-risk river band. About one in a thousand sits in Flood Zone 2, the next band down, which rounds to nothing for practical purposes. No properties in the village carry a historic flood event on the record. The nearest coast is about four-and-a-half kilometres south, which is far enough that tidal flooding isn't on the village's worry list. The Portsmouth tide gauge is rising at about two millimetres a year, which matters a lot for places sitting on the harbour and not very much for an inland village on the chalk.
The surface-water picture is where Horndean earns its flood-risk rating. About one in five addresses sits in a high surface-water risk band. Roughly the same share again sits in medium. About three in ten sit in low. Add those together and well over half the village carries some kind of rainwater-flood flag against it. The cause is unspectacular. Horndean is a chalk-and-clay village on the southern slope of the Downs, and rainwater has to go somewhere. When the somewhere is a post-war estate built across what used to be drainage land, the somewhere is sometimes a back garden, and sometimes the road outside it.
The climate allowance, which adds projected sea-level rise and heavier-rainfall scenarios to the existing flood zones, brings the affected share to roughly half a percent. That stays tiny in river-and-sea terms. The surface-water story is the one that gets worse, not the river one, because pavements, concrete drives and mid-century drainage networks weren't built for what the climate models now project. The rivers stay safe. The drains keep being asked to do more than they were designed for.
What this means for insurance is a mixed picture. The river and coastal risk is low enough that mainstream cover is straightforward to find at reasonable rates. The surface-water risk is the awkward one. Premiums on properties flagged as high surface-water risk can come in heavier than the postcode average, and excesses on flood claims often sit higher than the standard. Anyone buying here would do well to get a quote on the actual address before committing, because the spread between best and worst within Horndean is wider than the village-wide number suggests.
Horndean's flood story is real, and it's a surface-water one. The river maps are clean. The drains are the problem nobody markets on.