Streatham's flood risk falls out of the sky, not out of a river. Only a sliver of the district sits in the river-and-sea flood zones. Nearly one postcode in five is flagged high on at least one flood measure. No flood is on record. The worry here is drainage, and drainage is easy to miss on a river map.
On the Environment Agency's river-and-sea mapping, about 2% of Streatham's postcodes fall in Flood Zone 3, the highest-probability band, and a further 3% in Zone 2, the medium-probability fringe beyond it. The two bands do not overlap, so Zone 2 is the extra ring and Zone 3 the core.
Surface water is the bigger story. The postcode-level flood indicators, which count rain that pools and runs off alongside rivers and sea, flag far more of the district than the river zones do. Streatham first made its name selling water that came up out of the ground at Streatham Wells, and the modern concern is water that will not go back down into it.
Under the Environment Agency's climate-change allowance, the share of postcodes inside the flood zones rises to about 5%. That is a modelled scenario for a wetter future, drawn by the agency, and it moves the lines on its map without telling you what any given winter will do.
For insurance, the effect is uneven. Insurers look at the individual property, so a home in one of the flagged postcodes may face higher premiums or extra questions, while the house two streets up the slope may notice nothing. The Environment Agency's long-term flood risk service lets you check the exact address, surface water included, and that is the check worth doing on any home here.
Taking every measure together, about 18% of Streatham's postcodes rate high on at least one of them, about 19% medium and about 37% low. Each postcode is counted under whichever measure flags it, river, sea or surface water, so these shares run much higher than the zone figures and describe something broader.
Counting homes instead of postcodes gives the clearer picture. The Environment Agency's rivers-and-sea mapping lists 4,051 properties in Streatham postcodes that carry some risk: 576 at high risk, 427 at medium, 1,250 at low and 2 at very low. Of the high-risk properties, 488 are homes. Postcodes with no modelled risk do not appear in that count at all.
In the modelled flood extents, about 2% of postcodes fall in the high-risk area with defences left out, and about 2% with them included. The defences make no visible difference to the modelled outline, so a listing that calls a Streatham home "defended" is quoting a model that drew the same line either way. Even where a defence is present in the model, that says a structure exists, and nothing about whether the house stays dry.
The nearest tide gauge in the national record is at Portsmouth, where sea level rose by about 2 millimetres a year between 1962 and 2023. That is the rate measured at Portsmouth over those years and has little bearing on an inland district this far from the coast.
No historic flood outlines are recorded anywhere in Streatham. That is reassuring as far as it goes, since it means nothing big enough to be mapped has been written down here, though it is not proof that a street has never had water across it.
Flood risk is one of Streatham's weaker points. The river zones are small, but the surface-water flags cover enough of the district that every address deserves its own look.