On the official maps, Hammersmith is about as exposed to flooding as anywhere gets, and in practice it relies almost entirely on the Thames defences. Nine in ten postcodes are in the highest river flood zone. Only a few dozen homes are rated high risk once the defences are counted. The last recorded flood was nearly a century ago. Lenders and insurers start from the maps, though, and every buyer here has to deal with that.
On the Environment Agency's river-and-sea mapping, about 91% of Hammersmith's postcodes fall in Flood Zone 3, the highest-probability band, and another 4% fall in Flood Zone 2, the medium-probability fringe beyond it. The two bands don't overlap, so Flood Zone 2 is the smaller number because almost everything is already in Flood Zone 3. These zones take no account of defences. They show where the river would reach if nothing stood in its way.
Surface water, the rain that falls faster than the drains can take it away, is a separate risk. It has no figure of its own here and only shows up in the postcode-level indicators below.
The Environment Agency's climate-change version of the river-and-sea zones covers about 100% of Hammersmith's postcodes once rounded, against the 95% in the two current zones combined. That is a modelled picture of how the zone spreads under a warmer climate, and it puts no date on any flood.
For insurance, the mapping matters more than the history. Every postcode in Flood Zone 3 gets flagged when insurers and lenders run their checks, whatever the defences do in practice, so expect the flood questions to come up on every quote and every mortgage application. Get an indicative buildings insurance quote on the exact address while the conveyancing is still early, because the price of cover varies from one building to the next along the river.
The broader postcode measure counts a postcode if it rates on any one of several flood indicators, surface water included. On that basis, about 16% of Hammersmith's postcodes rate high on at least one, 27% medium and 50% low. Those shares overlap with the flood zone figures and measure something different, so they shouldn't be added together.
Counting homes is more useful. The Environment Agency's rivers-and-sea assessment lists 7,259 properties in the 215 Hammersmith postcodes that carry any risk at all. Postcodes with no risk don't appear. Of those 7,259, 44 are rated high risk, none medium and none low, and 5,646 are very low. All 44 high-risk properties are homes. "Thames views" goes in the particulars, and Flood Zone 3 goes in the search report.
Counted with defences, about 0.5% of Hammersmith's postcodes fall in the Environment Agency's high-risk undefended flood extent, and none in the high-risk defended extent. "Defended" only means the model includes a defence; it says nothing about whether that defence will hold. The Agency has 53 flood events on record for the area. The most recent, in January 1928, was the Thames overtopping its defences. That is the kind of flood a wall cannot fully rule out, which is why the wall, and not the river, now decides how most of these homes are rated.
The nearest tide gauge with a long record is at Portsmouth, where sea level rose about 2 millimetres a year between 1962 and 2023. That is a rate measured on the south coast over sixty years. It says nothing about the tidal Thames at Upper Mall next year.
Hammersmith's flood risk is poor on paper, and lenders, insurers and searches all go by the paper. The defences and nearly a century without a recorded flood are what keep it from being worse than that.