Theydon Bois has more flood risk than its river maps suggest. Very little of it is in a river flood zone. More than a quarter of its postcodes come up high on at least one flood measure. Rain is the bigger problem. A buyer who checks only for a nearby river has not checked enough.
On the Environment Agency's river-and-sea mapping, about 2% of postcodes in the Theydon Bois area are in flood zone 3, the highest-probability band, and a further 5% are in zone 2, the medium-probability band outside it. The two bands don't overlap, so together they cover a small share of the area. The Environment Agency has five recorded flood events, the most recent in December 1993, when a main river overflowed its channel where there were no raised defences.
Surface water does not get its own figure here. It shows up in the broader postcode measure further down, and that measure tells a very different story from the river zones. The village's name comes from Thecdene, the valley where thatching materials were cut, and a valley is where the rain ends up.
Allowing for climate change, the Environment Agency's modelled flood zone grows to about 7% of postcodes in the area. That figure comes from a model of future conditions, so it shows where the mapped zone could spread, and no single house's future can be read from it.
For insurance, the river zones are only part of what an underwriter looks at. Surface-water risk at the exact address counts too, and a home that sounds safe because it is outside the river zones can still be quoted a higher premium. Getting a quote for the specific property while the conveyancing is still early costs nothing, and it shows quickly whether that address is one the insurers worry about.
The broader postcode picture counts any of several measures, surface water included. On that basis about 28% of postcodes in the Theydon Bois area are high risk on at least one of them, about 26% medium and about 41% low. Particulars that only quote the river zone are giving you the smallest of these figures.
Counting homes instead of postcodes makes the risk easier to picture. The Environment Agency's rivers-and-sea register lists 320 properties in the postcodes carrying any river or sea risk at all, and postcodes with no such risk are not included. Of those, 20 properties are high risk, 7 medium and 45 low, and six of the high-risk ones are homes. From rivers alone, the number of homes at high risk is small enough to count individually.
On whether defences stand in the way, the Environment Agency's model puts about 3% of postcodes inside the high-risk river extent without defences, and the same 3% with defences included. The defences that exist in the model do not shrink that high-risk area. A defence being in the model does not make any house behind it safe, and here it makes no difference to the high-risk footprint either.
Flood risk is one of the weaker points about Theydon Bois. River flooding affects only a handful of homes, but the wider flood measures, surface water included, catch more than a quarter of the area's postcodes.