From the rain, more than its height above the valley suggests. No postcode here is in a river or sea flood zone. The Misbourne keeps to its own valley. Still, nearly one postcode in five is rated high risk on at least one flood measure. Gerrards Cross is on the slopes of the Chilterns, and the water reaches it anyway.
In the Environment Agency's river-and-sea flood zones, 0% of Gerrards Cross postcodes are in zone 3, the highest-probability band, and 0% are in zone 2, the medium-probability fringe beyond it. The Environment Agency's property-level risk list includes just one postcode in the area, covering three properties, and only one of those is rated at all, at low risk. The River Misbourne runs through the parish north-east of the town, and the main rivers barely touch the houses.
Surface water is a different story. When heavy rain lands on built-up slopes faster than the drains can take it, it runs downhill and gathers in dips, roads and gardens, however far away the nearest river is. Ponds like Latchmoor Pond on the common have always shown where water collects. On this measure Gerrards Cross is exposed in a way the river map never shows.
The Environment Agency's climate-adjusted flood zones also put 0% of postcodes at risk, so the river picture holds up even with the allowance for a wetter future added in.
For insurance, the river and sea maps work in a buyer's favour, but insurers also look at surface-water risk, and a postcode rated high on that can show up in the quote. Get an insurance quote on the specific house early in the process, alongside the survey, so the premium is known before the money is committed.
Taking every flood measure together, rivers, sea and surface water, about 18% of Gerrards Cross postcodes are high risk on at least one measure, about 23% are medium and about 33% are low. These bands overlap with the flood zones and can't be added to them. With the river and sea zones empty, nearly all of that high-risk share comes from the other sources, chiefly the rain. Particulars that mention "flood zone 1" and stop there have answered the easiest question.
On the Environment Agency's modelled flood extents, 0% of postcodes are in the high-risk extent, with or without defences. In the model, defences don't come into it at all, because the modelled river water never reaches the town. That tells you nothing about the surface-water risk, because no wall is built against rain coming down a road.
The nearest tide gauge is at Portsmouth, where sea level rose at about 2 millimetres a year between 1962 and 2023. That gauge is a long way from a town on the Chiltern slopes, and the sea is not the risk here.
The Environment Agency has no recorded historic flooding anywhere in the area. That's reassuring, but it covers only floods somebody recorded, and surface-water flooding in a back garden or a dip in the road often goes unrecorded. Before you offer, check the Environment Agency's long-term flood risk service for the exact address, and look at the surface-water layer rather than the river one.
Gerrards Cross is below average for flood risk, and the reason is the rain. The rivers aren't the problem, and a buyer who only checks the river map will miss the risk that matters.