Harrow has a real flood risk, and it comes out of the drains, not out of a river. Almost nowhere is in a river flood zone. About one postcode in five is high risk on at least one flood measure once rainwater is counted. Nothing in the area has a recorded flood. The danger here is a downpour with nowhere to go.
On the Environment Agency's river-and-sea mapping, under 1% of Harrow's postcodes fall in Flood Zone 2, the medium-probability band, and none falls in Flood Zone 3, the high-probability band. The two zones are separate bands, so the tiny Zone 2 share is the whole of the river and sea exposure. A town that grew up around a hill and a railway was never going to be a riverside town.
Surface water is a different story. Rain that falls on roofs, roads and the paved forecourts of a busy suburb has to go somewhere, and when the drains are full it runs along the lowest street it can find. That is the risk that puts Harrow in the flood conversation at all, and it is decided by the dip in the road and the slope of the garden far more than by any river.
The Environment Agency's climate-adjusted flood zone, which allows for heavier rivers in future, covers under 1% of postcodes, the same small share as today. For river flooding, the projected future looks much like the present.
For insurance, the line that counts is the surface-water risk on the specific address. A home in a high-risk pocket can find cover dearer or carrying a bigger excess even with no river in sight, and a home a street away may see no difference at all. Get a quote on the exact address early, while there is still time to walk away.
Counting every measure together, including surface water as well as rivers and sea, about 21% of Harrow's postcodes rate high risk on at least one of them, about 19% medium and about 36% low. A postcode counts as high if any single measure puts it there, so these shares are much bigger than the river zones alone. Most of that weight is rainwater.
In terms of actual buildings, the Environment Agency's rivers-and-sea mapping lists only 4 Harrow postcodes with any risk at all, covering 96 properties. Of those, 12 properties are at high risk, 14 at medium and 45 at low, and 8 of the high-risk ones are homes. Postcodes carrying no river or sea risk do not appear in that count at all.
On defences, the modelled share of postcodes in the high-risk river extent is zero whether defences are included or not. There is nothing for a wall to protect here, because the river model barely touches the place. That says nothing about rainwater, which no flood wall was ever built to stop.
The nearest tide gauge to Harrow is at Portsmouth, where sea level rose by about 2 millimetres a year between 1962 and 2023. That is a measured rate on the south coast, and a long way from a town on a hill in north-west London.
There is no recorded flooding in the area's historic flood records. That is good news, and it is also a statement about what was written down, which tends to favour rivers bursting their banks over a flooded side road after a summer storm.
Harrow's flood risk is below average, because it lives in the gutters and low corners of a hard, built-up town instead of in any river, and a buyer has to look for it street by street.