Brentwood has a flooding problem, though rivers have very little to do with it. Hardly any of the town is in a river flood zone. About a quarter of postcodes still score high on at least one flood measure. The water that causes trouble here falls as rain and can't drain away fast enough. Buyers who only check the river map are checking the wrong thing.
On the Environment Agency's river-and-sea mapping, under 1% of Brentwood's postcodes are in flood zone 3, the highest-probability band. Zone 2, the medium-probability fringe outside it, takes in an even smaller share, also well under 1%. For a town on the London road this far inland, the river-and-sea picture is about as clean as it gets.
Surface water is a different matter. That is rain landing faster than drains, gardens and gutters can take it, and it pools on roads and runs downhill wherever the ground slopes. It needs no river at all, and it accounts for most of Brentwood's flood rating. A river defence does nothing about a blocked drain at the end of a cul-de-sac.
Allowing for climate change, the Environment Agency's adjusted zone covers about 1% of postcodes. That is a slight widening of a zone that was small to start with, and it reinforces the point that rain, not the rivers, is the risk to watch here.
For insurance, a home outside the river zones but with a surface-water rating can still draw questions from insurers, and a high rating on one measure can affect the quote. Get an insurance quote on the exact address early in the process, and ask the seller directly whether water has ever come into the house or garden. Insurers are not reassured by a postcode average, and neither should a buyer be.
Taking the postcode indicators together, which count a postcode if it rates on any one of several measures including surface water, 24% of Brentwood's postcodes come out high on at least one, 22% medium and 44% low. One measure is enough to put a postcode in a band, so these shares run well above the river zones and should not be added to them.
The Environment Agency's rivers-and-sea mapping counts actual buildings, and only 20 postcodes in the area appear in it at all. Across them it lists 387 properties, of which 97 are at high risk, 25 medium and 39 low. Of the high-risk ones, 86 are homes. For a town this size that is a short list, though it is a very long list for the people on it.
On whether anything stands in the way, the Environment Agency's modelled extents put about the same share of postcodes, under 1% each way, in the high-risk extent with defences and without them. "Defended" here means a defence appears in the model. A defence in the model has never kept water out of a real hallway.
No historic flooding is recorded in the area. That is reassuring as far as it goes, though an empty record only covers what someone wrote down, and surface water that came up a driveway and went away again rarely makes the official books.
Flood risk is one of Brentwood's weaker points. Almost none of it comes from rivers, and too much of it comes from rain that has nowhere to go.