Herne Hill is at risk of flooding from rain, and almost none from rivers or the sea. No river-and-sea flood zone touches it. Nearly a fifth of its postcodes still carry a high surface-water or similar rating. Another fifth or so carry a medium one. The danger here comes from above and from under the road, and it comes street by street.
On the Environment Agency's river-and-sea mapping, none of Herne Hill's postcodes fall in Flood Zone 3, the highest-probability band, and none fall in Flood Zone 2, the medium-probability fringe beyond it. The River Effra used to flow down through Herne Hill on its way to Brixton, and it no longer shows up on a flood zone map at all.
That leaves surface water, the rain that arrives faster than the drains can take it and runs to the lowest ground it can find. It does not need a river nearby, and on a hill with a valley through the middle it has plenty of low ground to choose from. The worst flood in Herne Hill's recent memory had nothing to do with weather either. In 2013 a burst water main under Half Moon Lane flooded Herne Hill, Dulwich Road and Norwood Road, which is a hazard no flood zone map was ever drawn to catch.
Looking ahead, the Environment Agency's climate-adjusted mapping puts about 2% of Herne Hill's postcodes in its flood zones, a small share where today there is none.
For insurance, the river-and-sea picture helps and the surface-water picture complicates it. Insurers price on the exact property, and a house on a postcode with a high surface-water rating can be quoted very differently from one a street away. An indicative quote on the specific address, early in the process, tells a buyer more than any district figure.
The wider postcode indicators fold surface water in alongside rivers and the sea, and they count a postcode if it rates on any one of those measures. On that basis, about 18% of Herne Hill's postcodes rate high on at least one measure, about 23% medium and about 42% low. Those are different shares from the river-and-sea zones, because they ask a different question. The Environment Agency's long-term flood risk service lets anyone check a single address for surface water, and that one search is worth doing for any house here.
The Environment Agency's modelled flood extents put none of Herne Hill's postcodes inside a high-probability area, whether a flood defence is present in the model or not. So the question of whether anything stands between the river and the house never really arises here. Where a defence does appear in that model, it only means a defence is present, and says nothing on whether a given house stays dry.
The nearest tide gauge is at Portsmouth, where sea level rose by about 2 millimetres a year across its record from 1962 to 2023. That is a measured rate at a gauge a long way from south London, and it has little bearing on a district with no coastal or tidal flood zone.
There is no recorded history of flooding across the area in the Environment Agency's historic flood records. That counts in Herne Hill's favour, though a missing record means nobody wrote a flood down, which is a slightly different thing from it never happening. The 2013 water main shows how an unrecorded kind of flood can still put water through the front door.
Flood risk is one of Herne Hill's weaker points, with almost no exposure to rivers or the sea and a surface-water problem on enough postcodes that the exact address matters more than the district's name.