Yes, a real slice of Lewisham is at risk of flooding, and the risk comes from the rivers more than the sea. About one postcode in eight is in the highest river-flood zone. About a quarter rate high once rainwater is counted too. Thousands of properties carry a high rating. The town was founded where two rivers meet, and the rivers have kept the appointment.
On the Environment Agency's river-and-sea flood maps, about 12% of the area's postcodes are in Flood Zone 3, the highest-probability band, and a further 9% are in Flood Zone 2, the medium-probability fringe beyond it. The old village grew up at the meeting of the Quaggy and the Ravensbourne, and the town later spread north onto the wetter ground once drainage improved. About 15% of postcodes lie inside the outlines of recorded historic flooding, and the agency holds 114 recorded flood events here, the latest in 1968, when a main river exceeded the capacity of a channel with no raised defences. Two rivers and a long memory make an awkward combination for a mortgage lender.
The river zones leave out water that never reaches a river. Heavy rain on a town of flats, roads and paved yards has to go somewhere, and where the drains cannot take it, it collects in the low streets. That risk turns up in the combined postcode measure further down, and nowhere on the zone maps.
Under the Environment Agency's climate-change allowance, the share of postcodes inside the modelled flood zone rises to about 28%. That is the agency's picture of how far the zone could reach as the climate changes, and it names no year in which the water arrives.
Insurers price a home in or near a flood zone on the risk at that address, so a property inside the higher bands can mean a higher premium, a larger excess, or fewer companies willing to quote. Put the exact address into the Environment Agency's long-term flood risk service and get a buildings insurance quote at the same time as the survey, when a bad answer can still change the offer.
Counting every measure the Environment Agency uses, surface water included alongside rivers and sea, about 26% of the area's postcodes rate high on at least one of them, about 27% medium on at least one, and about 39% low on at least one. A postcode can appear in more than one of those bands, because each line asks only whether any single measure reaches that level.
The agency's rivers-and-sea mapping counts the homes and buildings directly. In the postcodes that carry any risk at all, it lists 7,831 properties, and 3,723 of them are rated high risk, 1,008 medium, 585 low and 568 very low. Postcodes with no river or sea risk are left off the list entirely, so these counts describe the exposed part of town only. A high rating covering more than three and a half thousand properties is a lot of front doors for one town centre.
About 14% of postcodes fall within the high-probability flood extent with no defence in the model, and about 13% within it with a defence present. A defence on the agency's map means a wall or channel exists in the calculation; it says nothing about whether that wall holds on the night it is tested, or whether the house behind it stays dry. A flood defence is a piece of engineering, and engineering has a design limit.
The nearest tide gauge with a long record is at Portsmouth, where sea level rose by about 2 millimetres a year between 1962 and 2023. That is a measured past rate on the south coast, and for Lewisham the more pressing water has always been the stuff coming down the Quaggy and the Ravensbourne.
Flood risk is one of Lewisham's weak spots. Large parts of the town are clear, but enough homes sit on the wrong side of the rivers that every buyer should check the flood map early on.