Balham's flood risk is real, and it arrives through the drains. No river or sea reaches it. About one postcode in five still carries a high-risk flag. Heavy rain is the whole story here.
The Environment Agency's river-and-sea flood zones leave Balham's postcodes untouched: none are in zone 3, the highest-probability band, and none in zone 2, the medium-probability fringe beyond it. For anyone house-hunting in south-west London, that clears the question most people arrive with. The nearest tide gauge to Balham is at Portsmouth, which is a fair indication of how far away the sea is.
Surface water is the risk that remains. The postcode indicators used across this site count the river-and-sea zones alongside surface water and other measures, and since the zones here are empty, every flag Balham carries comes from those other measures. It is rain landing on a dense spread of roofs, roads and paving faster than the drains can take it away. Look up the exact address on the Environment Agency's long-term flood risk service for surface water specifically, because the river result will come back clear and tell you nothing useful.
Looking ahead, the Environment Agency's climate-adjusted flood zone also covers none of Balham's postcodes. That allowance models how the river-and-sea zones could grow in a wetter future, so it speaks to rivers and the sea and leaves the rain question exactly where it was.
For insurance, a clean river-and-sea result helps, and a surface-water flag on the specific street is what an insurer will weigh. Expect premiums to follow the property's own record and its height above the road, and lower-ground and basement flats to get closer questions than the floors above them.
Counting a postcode as high, medium or low if it rates that way on any one of several measures, about 21% of Balham's postcodes rate high, 24% medium and 45% low. Those measures include surface water, so these shares and the zone figures describe different things. "Low" here still means a small chance in any year, and a high flag on one measure is enough to put a postcode in the top group.
The Environment Agency's modelled flood extents put none of Balham's postcodes inside a high-risk extent, with or without defences in the model. There is no defence question to answer, because the modelled river and sea flooding never arrives. Defences would be irrelevant to surface water anyway, which comes down from above.
The sea-level record at Portsmouth shows a rise of about 2 millimetres a year, measured there between 1962 and 2023. It describes that gauge on the south coast over those years and has little bearing on a street in Balham.
There is no recorded historic flooding in Balham. That is reassuring as far as it goes. The record captures floods someone mapped and kept, and a flooded basement after a summer storm rarely makes it onto a map.
Balham's flood risk is below average for a place with no river problem at all, because a fifth of its postcodes are flagged high for water that falls straight out of the sky. Buyers looking at lower floors should take the surface-water map as seriously as the asking price.