How this check works, and what it cannot tell you

From postcode to coordinate

Your postcode goes to postcodes.io, which is built on the ONS Postcode Directory, and comes back with a latitude and longitude, the local authority district and the country. That coordinate is the centre of the postcode unit, which typically contains about fifteen addresses.

That is the biggest limit of this site and we would rather say it than bury it. A river or sea flood zone is usually wide enough that the centre of a postcode is a fair guide. Surface water flooding follows the fall of the land street by street and even plot by plot, so the postcode centre can sit in a different band from your front door. That is why the map lets you drag the pin onto your own plot: the check then runs on that exact point, the result says so, and the report records which point was used.

A pin more than two kilometres from the postcode is ignored and we fall back to the centre of the postcode. Otherwise a stray click could produce a report about a different village.

From coordinate to answer

Five Environment Agency map layers are asked about your point, all at the same time, so the whole check takes a few seconds:

• Flood Map for Planning. Flood Zone 2 or 3 from rivers and the sea. If your point is in neither, it is in Flood Zone 1. This map ignores flood defences.
• Risk of Flooding from Rivers and Sea. Four bands, High, Medium, Low and Very low, this time taking account of defences such as walls and embankments.
• Risk of Flooding from Surface Water. The same four bands for water that runs off the ground and collects in low spots when drains and watercourses cannot cope.
• Historic Flood Map. Land the Environment Agency knows has flooded at some time.
• Recorded Flood Outlines. Individual recorded floods, with a name, a date, a source and a cause where the Environment Agency has them.

A sixth source is not a map: the Environment Agency flood-monitoring service lists the flood warning and alert areas near the point and any warnings that are active right now.

A map that returns nothing at your point is an answer, not an error. For the zone map it means Flood Zone 1; for the two risk maps it means Very low. A source that does not respond at all is a different thing. In that case the site says that the check is incomplete and does not offer a report.

The Environment Agency describes the bands by the chance of flooding in any given year. High is more than 3.3% (1 in 30), Medium is between 1% and 3.3% (1 in 100 to 1 in 30), Low is between 0.1% and 1% (1 in 1,000 to 1 in 100) and Very low is less than 0.1% (1 in 1,000).

The ring of eight points

A single point can hide the fact that Flood Zone 2 or 3 starts a short walk away. So the check also asks the zone map about eight further points around yours, one in each compass direction. The free check does this at 250 metres. The Flood Risk Report does it at 100, 250 and 500 metres. It is a way of seeing how close the zones come, not a measurement of a boundary.

Pages for a postcode district or a council

The pages for a postcode district or a council area cannot check every property in them. They show a sample of 5 points spread across the area, and they say so. A figure such as "2 of 5 sample points are in Flood Zone 2 or 3" describes the sample, not a count of homes. Use it as an indication of the area and check your own postcode for the answer that matters.

Caching, and why the report never uses it

A free check is cached for seven days on a grid of about sixty metres, so that neighbours share an answer and the source is not queried needlessly. A report is never served from that cache: when you order one, every layer is queried again, live, and the answer is frozen into the order. Flood maps are updated from time to time, and a cached statement about flood risk is a complaint waiting to happen.

Coverage

England only. The Environment Agency maps used here cover England. Wales is covered by Natural Resources Wales, Scotland by SEPA and Northern Ireland by the Department for Infrastructure, each with its own maps. Enter a postcode in one of those countries and the site says so and offers nothing.

Within England we hold pages for 197 postcode districts and 11 councils.

What the maps do not show

• Groundwater flooding, where water rises up through the ground after prolonged rain.
• Reservoirs and dam failure. These have their own reservoir flood maps from the Environment Agency.
• Sewer flooding and blocked or burst drains, which are the responsibility of the water company and are not in the Environment Agency maps.
• Small local watercourses that are too small to appear in the national river models. The surface water map picks up some of this, but not all.
• Future climate. The maps describe present-day risk. A wetter, higher-sea-level future is not in these bands.
• The height of the ground floor, the condition of the building, or what the owner has already done to protect it.

What this is not

• It is not an official flood search. Those are ordered by a conveyancer, usually as part of the legal work on a purchase.
• It is not a flood risk assessment of a building and it is not advice from an engineer or surveyor.
• It does not tell an insurer or a lender anything they will accept in place of their own assessment.
• It cannot say that a property will or will not flood. A low risk is a chance, and low-probability floods happen.