Sydenham has a modest river risk and a bigger rain risk, and most of the danger comes from the sky. Only a sliver of the district sits in a river flood zone. About one postcode in seven rates high on at least one flood measure. The River Pool is the local river. The water most homes have to worry about comes down the hill.
The Environment Agency's river-and-sea flood zones cover about 2% of Sydenham's postcodes in Zone 3, the highest-probability band, and less than 1% in Zone 2, the medium-probability fringe that lies beyond it. Those zones follow the River Pool, which runs along the east of Lower Sydenham beside the Hayes railway line. The agency has 17 recorded flood events in the area, the most recent in 1968, when a main river overflowed its channel with no raised defences in the way.
Surface water is a separate problem, and the river zones leave it out entirely. Sydenham Hill is one of the highest points in London, and the rest of the district runs downhill from it, so heavy rain has a long way to travel before it reaches a drain. That risk shows up in the postcode-level figures further down, and it is why the river zones on their own make Sydenham look drier than it is.
The Environment Agency's climate-adjusted mapping puts about 3% of Sydenham's postcodes inside the flood zone once a changing climate is allowed for. That is a modelled outline of how the zone could grow, and it adds very little to today's map.
Insurance follows the address. A home in the River Pool corridor, or at the bottom of a slope where surface water collects, can cost noticeably more to cover than a flat on the ridge, and some insurers will ask questions the others skip. Put the exact address into the Environment Agency's long-term flood risk service and get a quote early, because a district average will not tell you about your own drive.
At postcode level, counting a postcode if it rates on any of several flood measures, surface water included, about 14% of Sydenham's postcodes are high on at least one, 18% medium and 36% low. A postcode only needs one measure to count, which is why these shares run well above the river zones.
Counted by property instead of postcode, the Environment Agency's rivers-and-sea mapping lists 501 properties in the postcodes that carry any risk, and postcodes with none are left off entirely. Of those, 13 are at high risk, 336 at medium and 33 at low. None of the 13 high-risk properties is a home, so the river's worst case lands on sheds, garages and commercial premises.
The defended and undefended outlines tell the same story. About 2% of Sydenham's postcodes fall inside the high-risk extent with no defences modelled, and about 2% still fall inside it with defences included. Whatever defences the model knows about along the River Pool do not shrink the area at risk, and "defended" on a flood map only ever meant that something is there.
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 the rate measured on the south coast, and Sydenham's flooding comes from a small river and heavy rain.
Flood risk in Sydenham is middling: the river problem is small and confined to one corridor, and the rain problem is wider, spread across the slopes, and the one most buyers will need to check.