Some of it is, and the risk is concentrated on the low ground. About one postcode in five is in a river flood zone. Roughly one in eight scores high on at least one flood measure. A few hundred properties are at high risk from rivers. Most of the town stays dry, and the parts that don't tend to be the same streets each time.
In the Environment Agency's river-and-sea flood zones, 8% of Addlestone's postcodes are in zone 3, the highest-probability band, and another 12% are in zone 2, the medium-probability ring that lies beyond it. Each postcode is counted in only one of the two. The town's lowest land follows the Bourne and runs out to the Thames meadows at Addlestonemoor. The highest point is Row Hill recreation ground in Row Town, which is about as high as north Surrey ground gets.
Rivers are only part of it. The Environment Agency has 87 recorded flood events in and around the town. The most recent, in January 2003, came from a sewer, after a channel without raised banks couldn't carry the water. About 13% of the area is within outlines of past floods. Most of the soil is described as loam with naturally high groundwater, which explains a lot of soggy lawns.
The Environment Agency's climate-adjusted mapping widens the flood zone to cover 28% of postcodes. That's a modelled picture of how far the zone could spread as the climate changes. It doesn't predict any particular flood, but it does show which way the line on the map is expected to move over the life of a mortgage.
Insurers price from the same kind of mapping, so a home inside zone 3 or close to the Bourne can cost more to insure, and some policies will ask questions others don't. Get an indicative quote for the exact address, and ask the seller for any claims history, while you can still walk away.
Taking each postcode at its worst rating on any measure, including surface water, 13% of Addlestone's postcodes are rated high, 21% medium and 41% low. These bands overlap with the river-zone figures above, because a postcode in zone 3 usually also scores high here, so the two sets shouldn't be added together.
Counting properties, the Environment Agency's rivers-and-sea assessment lists 2,252 across the 96 Addlestone postcodes that carry some risk. Of those, 340 are rated high, 264 medium and 1,085 low, and 236 of the high-risk properties are homes. Postcodes with no river or sea risk don't appear in the count at all, so this is a tally of the exposed corner of the town, set against nothing else.
The Environment Agency also models where defences change the picture. The high-risk extent covers 8% of postcodes with defences included and 8% without them, so in the model the defences don't reduce the high-risk area at all. Where a defence is in place, it lowers the odds without removing the water, and here the model hardly credits them with even that.
Portsmouth is the nearest tide gauge, and it recorded sea level rising by about 2 millimetres a year between 1962 and 2023. That figure is for the coast. Addlestone's water arrives by river and drain, and the Bourne matters more to anyone living beside it than the Solent does.
Addlestone's flood risk is middling: most homes are clear, but a few hundred near the low ground carry real exposure.