Yes, Mortlake has a real flood risk, and it is concentrated on the river side of the district. A fifth of its postcodes are in the highest river-and-sea flood zone. Allowing for climate change, that rises to about a third. When the risk is counted house by house, only one property is rated high. Both figures are accurate, and they measure different things.
Mortlake sits on the south bank of the Thames between Kew and Barnes. On the Environment Agency's river-and-sea mapping, about 20% of the area's postcodes are in Flood Zone 3, the highest-probability band. Another 11% are in Flood Zone 2, a separate medium-probability band beyond it. The two zones do not overlap, so Zone 2 being the smaller figure is not an error. The name "Mortlake" may come from an old word for a long, narrow marshy meadow, so the people who named the place may well have noticed the water.
The Environment Agency gives no separate surface-water share for Mortlake. Surface water, meaning rain that cannot drain away fast enough, is included in the postcode-level measure below, and it is the kind of flooding that can affect streets well away from the river.
The Environment Agency also maps how the flood zones would grow under climate change, and on that version about 35% of Mortlake's postcodes fall inside. That is a modelled future, but it covers a larger part of the district than today's Zone 3 does.
In practice, insurers price flood risk house by house and pay little attention to which district a home is in. A house inside a mapped flood zone can still get cover, but it may cost more or come with a higher flood excess, and a buyer finds this out only by getting a quote. The useful check is to enter the exact address into the Environment Agency's long-term flood risk service and get an insurance quote for that same house while the purchase is still at an early stage.
A wider postcode measure records whether a postcode rates high, medium or low on any one of several flood indicators, surface water included. On that measure, about 21% of Mortlake's postcodes rate high on at least one indicator, 29% medium and 66% low. One postcode can appear in more than one band, so these shares are not meant to add up to a whole.
The more useful figure is a count of homes. The Environment Agency's rivers-and-sea mapping lists 2,055 properties in Mortlake postcodes that carry some risk. Of these, 1 is rated high, none medium, 964 low and 269 very low. Postcodes with no risk at all are left out of the count entirely. Many postcodes are inside the flood zones while very few individual homes are rated high, and that gap is what the next figure helps explain.
Flood defences are part of the reason. In the Environment Agency's model, about 8% of Mortlake's postcodes fall inside the high-risk flood area with no defence, and about 7% fall inside it with a defence present. A defence present in the model means a wall or embankment is assumed to be there and working. It does not mean the water cannot get past it. "Defended" describes the model's assumptions, not a promise about any house.
Sea level is rising slowly. The nearest tide gauge in the dataset is at Portsmouth, which recorded an average rise of about 2 millimetres a year between 1962 and 2023. That is the rate measured at that gauge over those years, and it is a long way from Mortlake.
No historic flooding is recorded in the area. That is reassuring, though it only means no flood has been recorded here, not that the area has never flooded.
Flood risk is a genuine weak point for Mortlake, even though very few individual homes are rated high. For buyers near the river, flood risk should be checked as carefully as the price.