In Chesham the flood risk is in the valley bottom, and the hills above it stay dry. About one postcode in twenty is in the highest river flood zone. Count rainwater as well and nearly one in five rates high on at least one measure. No flooding is on record. The valley decides which half of the town you're in.
The Environment Agency's river-and-sea mapping puts about 5% of Chesham's postcodes in flood zone 3, the highest-probability band. Another 2% are in flood zone 2, the medium-probability fringe beyond zone 3. The river behind those figures is the Chess, a chalk stream that rises from springs above the town and runs through culverts under the streets. It comes back into daylight at Waterside. The Chess is a gentle stream for most of the year, but it has nowhere to spread except the bottom of the valley.
Surface water is the other half of the story. Chesham grew where dry valleys from the surrounding hills meet, and when heavy rain falls on the slopes it runs downhill into the town. A house can be well away from the river and still be in the path of that run-off. Defence walls do nothing about water coming down a valley side.
The Environment Agency also models what climate change could do. With its allowance added, the share of Chesham's postcodes in the flood zone rises to about 9%.
For insurance, the deciding factor is which part of the valley the house is on. Up the slopes, the river barely registers with an insurer. Down by the river, insurers will ask more questions, and the premium can move with the answers. Get a quote for the exact address before you instruct a solicitor, because that is when a surprise is still cheap.
At postcode level, the Environment Agency rates each postcode on several separate measures, including surface water as well as rivers and sea. About 18% of Chesham's postcodes are rated high on at least one of them, 16% medium on at least one and 28% low on at least one. Each figure counts a postcode if any one measure reaches that level, so a postcode can appear in more than one band.
Counting homes is more useful than counting postcodes. In the Environment Agency's rivers-and-sea mapping, 306 properties in Chesham are at high risk, 173 of them homes, 124 at medium risk and 209 at low risk. They come from 1,519 properties across the 65 postcodes that carry some risk, and postcodes with no risk are left out of the list entirely.
The Environment Agency also models whether anything stands between the water and the house. About 5% of the town's postcodes fall inside the high-probability extent without defences in the model, and about 5% with them. Defended here only means a flood defence appears in the model. The model makes no claim about how well that defence would perform, or that the house behind it would stay dry.
The nearest tide gauge is at Portsmouth, where sea level rose by about 2 millimetres a year between 1962 and 2023. That is the rate the gauge measured over those years, at the coast. For a Chilterns town this far inland, the sea is somebody else's problem.
Nobody has recorded a flood in the town on the historic flood map. That counts in Chesham's favour, though it only tells you no flood was mapped. It doesn't prove the water never came.
Flood risk is the weak spot in Chesham, concentrated where the river and the run-off meet in the valley bottom. If you're buying high on the valley sides you can mostly put it out of your mind. If you're buying down by the river, take it seriously.