Great Dunmow has very little river flood risk, and its main water risk comes from rain with nowhere to drain. Almost no postcodes sit in the river flood zones. None are in the highest one. Surface water is what catches a minority of streets. The risk here comes from above more often than from the riverbank.
On the Environment Agency's river-and-sea mapping, about 1% of Great Dunmow's postcodes fall in flood zone 2, the medium-probability band, and none fall in zone 3, the high-probability band. The two zones don't overlap, so the town has a thin edge of medium risk and no high-risk centre at all. The only flood the Environment Agency has recorded here was in February 1947, when a main river ran over its banks with no raised defences in the way. That was a long time ago, and it is the only entry.
Surface water is a different story. It is heavy rain that can't drain away fast enough, pooling in dips, gardens and low roads well away from any river, and it can reach houses that a river map would mark as safe. On the river measure alone, Great Dunmow looks almost dry. Once surface water is counted, it looks more like an average town.
The Environment Agency's climate-change version of the river-and-sea zones puts about 1% of postcodes at risk, barely more than today's mapping. On that forward look, the Chelmer is not expected to cause much more trouble than it does now.
Insurers price each address on its own mapping, and surface water counts. A home in a dry river zone can still get a higher quote because of a hollow at the end of the garden. The Environment Agency's long-term flood risk service gives a separate surface-water result for every address, and looking it up for a specific house takes about two minutes.
On the wider postcode measure, which flags a postcode if any one of several indicators puts it at risk, surface water included, about 5% of Great Dunmow's postcodes rate high, about 12% medium and about 31% low. Because that measure picks up any flood source, its numbers are naturally bigger than the river-zone shares.
Counted by building, the Environment Agency's rivers-and-sea mapping lists 541 properties across the 18 postcodes that carry some risk, and postcodes with none are left out entirely. Of those, 28 properties are rated high, 21 medium and 109 low. Twenty-two of the high-risk properties are homes. A house that appears on that list is a matter of public record, whatever the sales brochure says about the view of the river.
On the question of defences, the Environment Agency's modelled high-probability flood extent includes none of Great Dunmow's postcodes, defended or undefended, and about 1% sit in the low-probability extent with no defence in the model. Where a defence does appear in that modelling, it means a wall or bank exists, and says nothing about whether it would hold.
Great Dunmow's flood risk is middling overall. The rivers are mostly harmless and the rain does most of the damage, so a buyer should check the surface-water map before the river map.