Chislehurst has almost no river risk, but heavy rain is another matter. Hardly any of it is in the river flood zones. About one postcode in ten rates high on at least one flood measure. The last river flood on record is older than the moon landings. If water reaches a house here, it most likely fell from the sky first.
On the Environment Agency's river-and-sea flood map, less than 1% of Chislehurst's postcodes are in flood zone 3, the highest-probability band. About 4% are in flood zone 2, the medium-probability band just outside it. Those are separate bands, so the second figure doesn't include the first. The Environment Agency holds 26 recorded flood events for the area, and the latest, in 1968, was a main river overtopping a channel with no raised defences. Chislehurst is a gravel hill, which the Saxons knew when they named it, and the rivers have mostly left it alone since.
Surface water is a different story. Heavy rain on clay and paved ground has nowhere to go, and it collects in dips and low roads whatever the river map says. "Outside the flood zone" in a set of particulars refers to rivers and sea and says nothing about a cloudburst. The Environment Agency's long-term flood risk service shows surface-water risk for the exact address, and that is the check worth making on any house at the bottom of a slope or near the ponds in Chislehurst West.
The Environment Agency's climate-adjusted map puts about 5% of postcodes inside the flood zone once its modelled allowance for future rainfall is added. That allowance comes from a model, and the extra area is small, but it's in the direction you'd expect.
For insurance, the low river exposure is good news for most homes. Insurers price on their own models of the specific address, though, and a house flagged for surface water can draw a higher premium or an excess on flood claims. Get a quote on the actual house early in the process.
Counting every flood measure the Environment Agency publishes, surface water included, about 11% of Chislehurst's postcodes rate high on at least one, 12% medium and 30% low. These figures measure something different from the flood-zone shares above, so the two sets shouldn't be added or swapped. A "low" rating means a small chance in any given year, and that still isn't zero.
Counting homes, the Environment Agency's rivers-and-sea assessment lists 67 properties across five postcodes. Only postcodes with some risk appear at all. Twelve of those properties are rated high risk, nine of them homes, none medium and three low. Nine homes is a specific number, and a buyer only needs to know whether the house being viewed is one of them.
On the modelled high-risk extent, about 0.4% of postcodes fall inside it with defences included and the same 0.4% without them. That means flood defences make no measurable difference to the modelled picture here. "Defended" on these maps means the model includes a defence. It doesn't mean the house stays dry.
The nearest tide gauge is at Dover, and over its record from 1961 to 2010 it measured sea level rising by about 2.2 millimetres a year. Dover is a long way from Chislehurst, and the sea is the least of its concerns.
Chislehurst's flood risk is middling. The rivers are hardly a factor, and the surface water decides which streets carry most of the risk.