Yes, Westminster sits on the Thames floodplain, and the official maps say so plainly. Most of the area falls in the highest river-and-sea flood zone. Take the defences away and more than half of it would sit in the modelled high-risk extent. With them in place, the number of homes actually rated at high risk is tiny. The whole district depends on walls it rarely thinks about.
On the Environment Agency's river-and-sea flood zones, about 79% of the area's postcodes are in Flood Zone 3, the highest-probability band. Flood Zone 2, the medium-probability fringe beyond it, takes in under 1% more. That small second figure tells you how the ground is laid out: the land is either down on the old floodplain or clear of it, with very little in between. Westminster Abbey was founded on Thorney Island, between the arms of the River Tyburn, and the zone map has never forgotten the island.
Surface water is a separate problem, and the river maps leave it out. The Tyburn still runs under the streets in its buried channel, and the slum once called Devil's Acre stood in its flood ravine until clearance and new drainage removed both. In a district of offices, stations and paving, heavy rain has few places to soak in, so the risk from a cloudburst is decided by the lie of each street.
The Environment Agency's climate-adjusted zone, which allows for higher river and sea levels, takes in about 80% of postcodes. Since nearly everything low-lying is already inside Flood Zone 3, the forward look adds very little to the map.
For insurance, a Flood Zone 3 postcode is the first thing an insurer's system flags, whatever the floor you live on. Upper-floor flats tend to fare better than basements and ground floors, and the building's own insurance, arranged by the freeholder, is where a flood claim often lands. Ask the managing agent for the building's insurance schedule and any flood claims history, and get an indicative quote for the exact flat early on, because a basement flat with a garden in SW1 can be a very different proposition from the one three floors above it.
Postcode by postcode, about 19% are rated high on at least one of several flood measures, with surface water counted alongside rivers and sea. About 7% rate medium and 15% low on that same any-measure basis. Those shares overlap with the flood zones and measure a different thing, so they sit alongside them without adding up.
The Environment Agency's property-level assessment for rivers and sea counts the defences in, and it rates 8 properties at high risk, none at medium and none at low, out of about 11,260 assessed, with about 10,710 at very low risk. Only 3 of the high-risk properties are homes. Those counts only include postcodes carrying some risk in the first place. The figure in the zone map and the figure for real buildings are two very different conversations, and it is the defences that bridge the gap.
The defended-or-not picture makes the point sharply. In the Environment Agency's modelled flood extents, about 57% of postcodes fall inside the high-probability extent when no defences are modelled, and under 1% when they are. "Defended" only means a defence is present in the model, and the Environment Agency's own records show what happens when one is beaten: the most recent recorded flood here, in 1928, came from the river overtopping its defences.
The nearest tide gauge in the national record, at Portsmouth, measured sea level rising by about 2 millimetres a year between 1962 and 2023. That is the rate at that gauge over those years, and it is the slow pressure every river wall in London is built to hold back.
Westminster's flood risk is poor on any map that ignores the defences, and very low for nearly every individual home that counts them, so a buyer here is placing a long bet on the Thames walls holding.