Surface water flood risk: what it means, and why a house far from any river can flood

Most people think of flooding as a river bursting its banks. In England more homes are at risk from rain that simply has nowhere to go. A heavy summer storm overwhelms the drains, water runs down the road, and it collects in the lowest place it can find. That place can be a cellar, a garden at the bottom of a slope, or a ground floor flat on a hill nowhere near a river.

Why buyers miss it

The Flood Zone that most people hear about covers rivers and the sea only. A house can be in Flood Zone 1, the lowest, and still sit in a High surface water band. The two are separate maps, made by the Environment Agency from separate models, and a check that only reads the zone says nothing about the rain.

What the bands mean

• High: 3.3 percent or more chance of surface water flooding each year, 1 in 30.
• Medium: between 1 percent and 3.3 percent each year, 1 in 100 to 1 in 30.
• Low: between 0.1 percent and 1 percent each year, 1 in 1000 to 1 in 100.
• Very Low: less than 0.1 percent chance each year.

A point outside every mapped extent is the lowest answer the dataset can give. It is not a promise. The map is built from a model of the ground and the drains, not from a visit to the street.

What makes it worse, street by street

• A house lower than the road, or at the bottom of a slope.
• A basement, a cellar or a sunken patio.
• A front garden paved over for parking, on this house or its neighbours.
• Old combined drains that also carry sewage, which back up in a storm.
• Air bricks close to the ground, which let water straight under the floor.

What to ask before you buy

Ask the seller whether water has ever come into the house or the garden in heavy rain. Look at where the drive and the garden fall. Look for a tide mark on a garden wall. Ask the insurer whether the surface water band changes the quote, because some insurers weigh it as heavily as the river band.

The free check on this site reads the surface water band for any English postcode in seconds, with the source and the date. The £12.99 report puts it beside the river and sea band, the warning areas and the nearest gauges, as one dated PDF.