Stevenage has a small river problem and a larger rain problem. Fewer than one postcode in a hundred is in the highest-risk river zone. About one in ten scores high on at least one flood measure once surface water is counted. The sea is so far away that its nearest official measuring point is on the south coast. Most of the flooding a buyer here needs to think about comes straight down out of the sky.
On the Environment Agency's river-and-sea flood maps, under 1% of Stevenage's postcodes are in Flood Zone 3, the highest-probability band. Another 5% are in Flood Zone 2, the medium-probability fringe outside it. The Environment Agency has six recorded flood events for the area, the latest in July 2015, when a main river overflowed its channel at a point with no raised defences. For a town this size, river flooding is confined to a narrow strip.
Surface water is a different story. It comes from rain that cannot drain away fast enough, and it does not respect the river zones, which is why the postcode-level picture below looks so different from the zone figures above. A house can be a long way from any watercourse and still be on the route the water takes downhill.
The Environment Agency's climate-adjusted map, which redraws the river-and-sea zones for heavier future rainfall, puts about 5% of postcodes inside them. That is barely more than today's Zone 2, so the extra allowance moves the boundary line on the river map only slightly.
For insurance, the effect follows the property, not the town. A home outside every flood band should be insured on ordinary terms, while one in a river zone or a surface-water hotspot can expect more questions and a higher premium. The Environment Agency's long-term flood risk check runs on an exact address, and running it alongside an insurance quote takes ten minutes before a solicitor is instructed.
Count every measure the Environment Agency publishes, surface water included, and about 10% of Stevenage's postcodes are rated high on at least one of them, 14% medium and 42% low. A postcode only needs to trip one measure to be counted, so these shares run well ahead of the river zones and should not be added to them. The rain gets more postcodes into these figures than the rivers do.
Defences hardly come into it. In the Environment Agency's high-probability model, about 0.3% of postcodes are in the undefended extent and 0.2% in the defended one. "Defended" means only that a defence appears in the model at that spot, and says nothing on whether it would hold on the night. Here the defended share is too small to change any buyer's decision either way, and the July 2015 event happened where there were no raised defences at all.
The nearest tide gauge is Portsmouth, which recorded sea-level rise of about 2 millimetres a year between 1962 and 2023. That is the rate measured on the south coast over those years, and its main relevance to Stevenage is to show how little the sea has to do with this town's flood risk.
Stevenage's flood risk is middling. The river picture is mostly reassuring, but rain gives about one postcode in ten a high reading, so every address here needs checking on its own.