Not much, and most of what risk there is comes from rain. Almost none of Stanwell is in a river flood zone. About one postcode in fifty is high risk on some measure. No floods are on record. In Stanwell, the water to watch for comes down from the sky.
On the Environment Agency's river-and-sea flood maps, under 1% of Stanwell's postcodes are in Flood Zone 2, the medium-probability band, and none are in Zone 3, the high-probability band. Each zone is counted separately, and Zone 2 covers only the land just outside Zone 3. In Stanwell, Zone 3 is empty, and Zone 2 barely reaches the edges. The River Colne once marked the parish's western boundary, and it now gives the floodplain modellers very little to draw on this side of the reservoirs.
Surface water is the bigger risk. It is what happens when a downpour arrives faster than the drains can take it. The Environment Agency's modelled flood extents put about 1% of Stanwell's postcodes inside the low-probability outline. A large share of the ground is covered by roofs, roads and depot yards, so the rain that does fall has fewer places to soak away.
The Environment Agency has also redrawn its flood zones with an allowance for climate change. On that version, about 1% of Stanwell's postcodes fall inside a zone. That is barely more than today, and it describes a change in the climate, not a change in the river.
At these levels, most homes in Stanwell should be insured on ordinary terms. The few properties on the edge of a zone, or where surface water collects, are the ones where an insurer may ask more questions. Put the exact address into the Environment Agency's long-term flood risk service and get an insurance quote early. Insurers price each address separately, and the postcode average does not help much if your street is the exception.
A broader postcode measure tracks river, sea and surface water risk together, and records the highest rating a postcode gets on at least one of them. About 2% of Stanwell's postcodes are high on at least one measure, 9% are medium and 41% are low. These bands overlap with the flood-zone figures above and should not be added to them. On this measure, low risk covers a lot of Stanwell, and high risk covers very little.
The Environment Agency's models can also show whether a flood defence stands between a postcode and the water. In Stanwell, no postcodes fall in the high-probability extent, with or without defences, so there is no defended area to weigh up. A defence in a model only means one exists, and it should never be read as a promise the house will stay dry.
The nearest tide gauge is at Portsmouth, a long way south, where sea level rose about 2 millimetres a year between 1962 and 2023. That is a measured past rate for the south coast, and it says almost nothing about an inland parish next to an airport.
No historic flooding is recorded for Stanwell. That is reassuring, though it only means no flood reached the record, which is a slightly smaller claim than no flood ever happened.
Flood risk is one of Stanwell's real strengths. Rivers barely come into it, and the remaining risk is a cloudburst on a few low spots.