Amersham has a small flood risk from rivers and a larger one from rain. Very few homes are in the river flood zones. A larger share of postcodes is flagged once surface water is counted. The river through the old town is a chalk stream that sometimes dries up completely. The risk is real for a few streets and low for the rest of the town.
On the Environment Agency's river-and-sea flood zones, about 3% of Amersham's postcodes are in zone 2 and under 1% are in zone 3. The zones don't overlap. Zone 3 is the higher-probability band, and zone 2 is the medium-probability fringe beyond it, so the larger figure belongs to the lower-risk band. The River Misbourne runs along the valley floor through Old Amersham, in a valley far bigger than a stream its size could have cut. The river is small, and the valley it runs through is much older and larger.
Surface water is the bigger concern. The postcode measures elsewhere on this site, which count heavy rain running off the land as well as rivers, flag noticeably more of the town than the river zones do. Old Amersham is on the valley floor and Amersham-on-the-Hill is on the plateau above it, so the slopes between them decide where rainwater ends up.
The Environment Agency's climate-adjusted zone, which models river flooding with an allowance for future climate change, covers about 4% of the area. It is a modelled scenario, a little wider than today's zones.
Insurers price on the risk at each address, so most homes on the plateau should find cover routine, while the lower streets near the Misbourne or on a surface-water path may face more questions and higher premiums. The useful check is to put the exact address into the Environment Agency's long-term flood risk service, which shows surface water and rivers separately, and get an insurance quote on the strength of it.
At postcode level, about 8% of Amersham's postcodes rate high on at least one flood measure, about 11% medium and about 25% low. Each of those counts a postcode if any one of several measures flags it, surface water included, so the shares overlap and a postcode can appear in more than one.
Counting individual properties gives a sharper picture. The Environment Agency's rivers-and-sea mapping covers 526 properties in the 28 Amersham postcodes that carry any risk at all. Of those, 7 are at high risk, 7 at medium and 288 at low, and only 3 of the high-risk properties are homes. Postcodes with no river or sea risk are left out of the list entirely.
The share of postcodes in the high-risk modelled flood extent is under 1% whether or not defences are included in the model. The two figures are the same, so defences make little difference here. "Defended" means only that a defence appears in the Environment Agency's model, and it says nothing about whether a particular house will stay dry.
The nearest tide gauge is Portsmouth, which recorded sea level rising by about 2 millimetres a year between 1962 and 2023. That is the measured rate at a gauge on the south coast, and it has very little to do with a town in the Chilterns.
There is no recorded historic flooding anywhere in Amersham's area. That is reassuring, though the record only covers what was written down.
Amersham's flood risk is middling. River risk is small and confined to the valley floor, and surface water accounts for most of the exposure. Buyers looking at the lower streets should check the individual address. For homes on the plateau, flooding is unlikely to be a serious concern.