Using the long term flood risk service for a site

A long term flood risk check for a site tells you what the Environment Agency's national models say about present day risk at an address, from rivers and the sea, surface water, reservoirs and, in some areas, groundwater. It is free, it takes a minute, and it is the right first move before you commission anything. What it is not is a planning document. The planning system runs on Flood Zones, which come from a different map built for a different purpose, and the two services can give you answers that look contradictory until you understand why.

What the service is built to do

The check long term flood risk service answers a question a homeowner, a buyer or an insurer asks: how likely is this property to flood in a typical year, as things stand.

It reports risk in four bands for each source. High means a chance of flooding greater than 3.3 per cent in any year. Medium is between 1 and 3.3 per cent. Low is between 0.1 and 1 per cent. Very low is below 0.1 per cent.

Crucially, for rivers and the sea the service takes existing flood defences into account. A property behind a defended embankment can therefore show as low risk here while sitting squarely in Flood Zone 3 on the planning map.

Where it differs from the flood map for planning

The Flood Map for Planning shows Flood Zones 2 and 3. Those zones describe the probability of flooding from rivers and the sea ignoring the presence of defences, because planning policy wants to know what would happen if a defence failed or was removed.

Flood Zone 1 is everything outside Zones 2 and 3 and is not drawn on the map at all. Flood Zone 3b, the functional floodplain, is defined locally in the authority's strategic flood risk assessment rather than nationally, which is why an officer sometimes tells you that you are in 3b when the national map shows 3.

Taking the correct extract from the planning map, with a date and a site boundary on it, is a job in itself, and the article on reading a flood map for planning extract correctly goes through it.

Reading the surface water layer honestly

Surface water is where most householders get a surprise, and it is the layer that deserves the most care.

The mapping is modelled rather than observed. It represents water running overland during intense rainfall, routed across a national terrain model, and it does not know about your drainage. It does not know that the gully at the bottom of the garden was cleared last year, or that the neighbour raised their patio by 300 millimetres.

What it is good at is showing flow paths. If the mapping draws a blue ribbon down the line of an old field ditch, through your side passage, that ribbon is almost certainly real even if the depths are approximate. Take flow paths seriously and treat depths and extents as indicative.

What it is weak at is small features. Kerbs, walls, buildings and culverts are either absent from the terrain model or crudely represented, so a property can show as at risk because the model routes water through a building that in reality diverts it.

Groundwater and reservoirs

The reservoir layer shows the area that could be affected in the unlikely event of a reservoir failing. It is shown for awareness. It almost never changes a planning decision and it should not change a purchase decision on its own.

Groundwater information is less complete. Where it is reported, it reflects areas susceptible to groundwater emergence rather than a modelled depth, and it matters most for basements, deep foundations and infiltration drainage. If you are planning a soakaway, groundwater level is the number that decides whether it works, and that comes from a trial pit and a percolation test rather than from a national map.

Where the resolution misleads on a small site

Zoom in far enough and the service will give you a boundary running through the middle of a semi-detached house. Do not believe it to that precision.

The national models work on a grid. On a typical back garden, the difference between a cell shown as medium risk and the one next to it shown as low is often within the model's own uncertainty. For a small extension this rarely matters. For a decision about finished floor levels, it matters a great deal, and the answer is a site survey rather than a screenshot.

Three practical habits help. Print the map at a sensible scale with the address marked. Record the date, because the underlying data is updated periodically. And look at the surrounding area rather than the single property, because the useful information is where the water comes from and where it goes.

What to tell a buyer, a lender or an insurer

Be factual and be complete. Give the risk band for each source, the date of the check, and anything you know that the model does not: a history of flooding, a watercourse that was culverted, a pump in the basement.

Flood Re exists to support household insurance for homes at flood risk, subject to its own eligibility rules including the property's council tax band and build date. Properties built after 1 January 2009 are outside the scheme, which is a point worth checking before a new home is marketed on the assumption that affordable cover will be available.

Do not summarise a medium risk property as low risk because the defences are good. The defences are already in the figure.

When a modelled assessment is needed instead

The national service never satisfies a planning requirement on its own. A flood risk assessment is needed for any development in Flood Zone 2 or 3, for sites of one hectare or more in Flood Zone 1, and for sites in Flood Zone 1 identified as having critical drainage problems.

A site-specific assessment adds what the national map cannot: a topographic survey, finished floor levels referenced to Ordnance Datum, climate change allowances applied to the right river basin district, safe access and egress, and residual risk after mitigation. Where surface water is the issue rather than fluvial flooding, the work usually moves into a drainage strategy with run-off rates and storage volumes.

One honest caveat. Flood mapping is revised as models improve and as new survey data arrives, so a check carried out two years ago may not match one carried out today. Date every extract you rely on, and re-check before you submit.

Enter your postcode on PlanScope to see whether your site needs a flood risk assessment and what else goes with it.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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