Flood risk: what to check before you buy, and what the maps do not say

Flood risk is one of the few things about a property that can be catastrophic, largely invisible on a viewing, and entirely checkable in advance. It is also one of the most commonly misread.

Three different kinds of flooding

River and sea flooding is what the maps show best. The Environment Agency models it in detail, and the risk categories describe the chance of flooding in any given year.

Surface water flooding is rain that cannot drain away fast enough. It is mapped less precisely, it can happen a long way from any river, and in many years it affects more properties than rivers do. A house on a hill can flood if it sits at the bottom of a slope with a blocked drain at the end of it.

Groundwater flooding happens where the water table rises, typically after a long wet period. It is the least well mapped and can affect cellars and foundations in areas that look entirely dry.

What the risk categories mean

The categories describe an annual chance. "High" does not mean it floods every year, and "low" does not mean it never will — a one-in-a-hundred-year event has a one per cent chance every single year, and can happen twice in a decade. The phrase describes probability, not a schedule.

Crucially, these maps describe areas, not buildings. A property raised above the surrounding ground, or with flood defences, may be considerably safer than its postcode suggests. One in a basement flat on the same street may be at far greater risk than the map implies.

What to actually do

Check the official maps first. They are free, and they cover river, sea and surface water separately. Look at all three; a property can be clear on one and not another.

Ask the seller directly. The standard property information form asks whether the property has flooded. A vague answer is itself informative.

Get a quote for buildings insurance before you exchange. This is the single most useful test available to you. Insurers price risk for a living, and a refusal or a very large excess tells you something no map will.

Ask about Flood Re. It is a reinsurance scheme that helps make cover available for homes at high flood risk. It does not cover everything — properties built after 2009 are excluded, as are most leasehold flats in certain circumstances and buildings used commercially. If a property depends on it, find out whether it qualifies.

Walk the street when it is raining. Unglamorous and genuinely useful. You will see where water collects, whether drains cope, and how the ground slopes in a way no map conveys.

Defences and their limits

A defended area is safer than an undefended one, and defences are designed to a standard rather than to be infallible. Overtopping happens. Maintenance matters. And defences can change the risk downstream, which is why a scheme upstream is worth knowing about even when it does not touch the property you are buying.

What we show

MoveCompass shows live flood warnings and alerts from the Environment Agency for English postcodes, so you can see whether anything is in force right now. That is a different thing from long-term risk, and we label it as such.

Live warnings tell you what is happening today. For the long-term picture, use the official flood risk service for the specific address, and read the insurance quote as carefully as you read the map.

Scotland, Wales and Northern Ireland each have their own flood authority publishing their own maps, and their data is not in the Environment Agency's feed.