Yes, on paper, and on paper is how mortgage lenders and insurers will look at it. Two-thirds of Chiswick sits in the highest river flood zone. The Thames forms its whole southern edge. The defences change the picture a great deal, but the map does not show them. A buyer here needs to know which of those two pictures their particular house is in.
On the Environment Agency's river-and-sea mapping, 67% of Chiswick's postcodes fall in Flood Zone 3, the high-probability band, and about 4% in Flood Zone 2, the medium-probability fringe beyond it. The two bands do not overlap, so the small Zone 2 figure is the strip around the edge of the large one. Those zones show the land as if the river walls were not there. The Boat Race is rowed past Duke's Meadows on a flood tide, and in Chiswick that is about the only time anyone is pleased to hear the phrase.
Surface water is a separate route. It is rain that falls faster than the drains can take it away, and it has nothing to do with the river. In a built-up district it collects in dips in the road and in basements, so it can reach streets nowhere near the Thames.
On the Environment Agency's climate-adjusted map, about 73% of Chiswick's postcodes fall in the flood zones. That map applies the agency's allowance for future change to the same land. It is a modelled scenario, and nobody can tell from it what the river will actually do.
For insurance, a postcode in Flood Zone 3 gets a closer look before a quote is offered, even where the house has never been wet. Premiums and excesses depend on the individual property, so ask an insurer to quote for the exact address while the solicitor is still doing the searches. The quote will tell you more than a map of the whole district can.
The postcode-level measures put surface water alongside rivers and sea and count a postcode if it rates on any one of them. On that basis about 23% of Chiswick's postcodes rate high on at least one measure, about 24% medium, and about 51% low. Because the measure counts several kinds of flooding, these figures cannot be compared directly with the flood zones.
The more useful number is the count of homes. The Environment Agency's rivers-and-sea assessment lists 139 Chiswick postcodes that carry some risk, covering 3,172 properties. Of those, 12 properties are rated high risk, one medium, 19 low and 1,588 very low. Postcodes with no assessed risk are left out entirely. A district mapped mostly into the highest flood zone has a dozen properties rated high once the defences are counted, and the question for any buyer is whether the house is one of them.
The defences are what make the difference. With no defences in the model, about 1% of Chiswick's postcodes fall inside the high-probability flood extent. With them, that falls to under 1%. "Defended" means only that a defence appears in the Environment Agency's model. It says nothing about how a particular wall will hold up in a particular storm. A listing that calls a riverside flat "defended" is using one word of a technical map to make a sale.
The nearest tide gauge in the national record is at Portsmouth, where the sea rose by about 2 millimetres a year between 1962 and 2023. That is a rate measured on the south coast over those years. It is the closest measured figure available for the tidal Thames, and it is a long way downstream.
Chiswick's flood risk is poor on the official mapping. The river defences reduce the number of homes actually rated high risk to a handful, but lenders and insurers start from the map, so a buyer has to start there too.