Checking surface water flood risk on the long term service
Surface water risk never appears on the Flood Map for Planning. Here is how to read the bands, get the depth and velocity layers, and use the result in a householder drainage statement.
A surface water flood risk check takes about two minutes on the government's long term flood risk service, and the answer it gives is a risk band rather than a verdict on your house. Type in the postcode, and the service reports risk from rivers and the sea, from surface water, from reservoirs and from groundwater, each separately. Surface water is the one that matters for most extensions, garden rooms and driveways, and it is the one that never appears on the Flood Map for Planning.
The bands are probabilities, so read them as such before you decide whether to worry.
Finding the property and reading the risk bands
Search by postcode or address, then use the map to look at the area around the building rather than accepting the summary alone. The service reports four bands, and each one is an annual chance of flooding:
| Band | Chance in any given year |
|---|---|
| High | 3.3% or greater, about 1 in 30 |
| Medium | Between 1% and 3.3% |
| Low | Between 0.1% and 1% |
| Very low | Less than 0.1% |
A medium band means that in any single year the chance of flooding is somewhere between one in a hundred and one in thirty. Over a 30-year mortgage that is not a remote possibility, which is why lenders and insurers ask.
Take a screen capture with the date on it, and record which layer you were looking at. The service is updated as the national modelling improves, and an answer from three years ago may not match today's.
How surface water risk differs from rivers and the sea
Surface water flooding, sometimes called pluvial flooding, happens when rainfall lands faster than the ground can absorb it or the drains can carry it away. Water runs off roofs, roads and driveways, collects in the lowest place it can find, and stays there until the drainage catches up.
River and coastal flooding is different in three ways that change what you do about it. It usually comes with warning, it follows a mapped channel or shoreline, and it is what the Flood Zones on the Flood Map for Planning describe. Surface water arrives in minutes, can happen anywhere including on a hillside, and is not in the Flood Zone system at all.
For a householder that has a practical consequence. A site in Flood Zone 1, described as low risk for rivers and sea, can sit in a high surface water band, and the council's drainage officer will be interested even though no flood risk assessment is triggered by the zone.
Depth, velocity and extent behind the bands
The public service shows you a band. The national surface water mapping behind it holds more: the modelled extent for the 1 in 30, 1 in 100 and 1 in 1,000 rainfall events, and in the detailed layers the depth and the velocity of the water in each.
Depth and velocity matter because they decide whether a flood is a nuisance or a hazard. Fifty millimetres of water ponding in a driveway is an inconvenience. Three hundred millimetres moving at speed down a sloping side passage is dangerous and will get through a door.
Those detailed layers are not on the public site. The lead local flood authority holds them for its area, and the Environment Agency can supply the mapping on request. Ask for them if the public service shows medium or high risk and you are proposing habitable accommodation at ground level or below.
Why a low risk postcode can hide a high risk garden
The national model is built on a terrain grid at a scale designed for planning at the catchment level. It does not know about your kerb, your gullies, the 300 mm step up to your back door or the culverted ditch under the garden.
So two common situations get reported wrongly.
The first is a garden that sits lower than the road and the neighbouring plots. Water running off a long tarmac drive or a road with a broken gully collects there in a way the national model may smooth out. If your neighbours tell you the corner of the garden floods every couple of winters, believe them over the map.
The second is the reverse. A property shown in a high band because the modelled flow path runs along the street may be perfectly safe, because the flow stays in the carriageway and the house sits 400 mm above it. A topographical survey with spot levels answers this, and it is the single most useful document you can add to an application on a site with a surface water flag.
Walk the site in heavy rain if you can. It sounds obvious and almost nobody does it.
Using the result in a drainage statement
For most householder work the council wants a drainage statement rather than a full assessment: a page or two saying where surface water goes now, where it will go after the works, and why the position does not get worse.
Four things belong in it.
- The result from the long term flood risk service, dated, with the extract attached.
- The existing drainage arrangement, including whether surface water goes to a soakaway, a surface water sewer or a combined sewer.
- The proposed arrangement, with the calculation for any soakaway and the infiltration test it is based on.
- Finished floor levels relative to external levels, and any threshold detail where water could enter.
Two design points recur on householder schemes. Extending over an existing soakaway means replacing it, and a soakaway needs to sit a stated distance from the building, so check the space before the drawings are finished. And paving a front garden is caught by the permitted development rules: a hard surface between the principal elevation and a highway exceeding 5 m² needs permission unless it is made of porous material or run-off is directed to a permeable area within the curtilage.
For anything larger, a proper drainage strategy with attenuation volumes and discharge rates takes over.
Cross-checking with the lead local flood authority
The lead local flood authority is your county council or unitary authority, and it holds the local knowledge the national model lacks: records of historic flooding, surface water management plans, critical drainage area designations, and the surface water mapping used in the council's strategic flood risk assessment.
Email them with the address and ask three questions. Is the property in a critical drainage area or an area of known surface water flooding. Are there records of past flooding in the street. Is there a local requirement on discharge rates or on the use of infiltration. A designation changes what the council expects even where the Flood Zone is 1, as set out under critical drainage areas and what they add to an application and under the local surface water flag.
The water company is worth a separate email where the street drains to a combined sewer, because sewer flooding records sit with the undertaker rather than with either the council or the Environment Agency.
Reservoir and groundwater layers on the same service
Two other layers appear in the same search and cause unnecessary alarm.
The reservoir layer shows the area that could be flooded if a large reservoir upstream failed completely. It is not a probability. Reservoirs in England are inspected under a statutory regime and failure is treated as highly unlikely, so the layer describes a worst case rather than a likelihood. Councils rarely ask for anything about it on a householder application.
The groundwater layer is coarse, showing broad areas where groundwater may emerge in wet periods. It is most relevant on chalk and on some limestone, and it matters mainly if you are digging a basement or relying on infiltration drainage, because a soakaway in ground with a high water table will not work.
If both of those come back clear and surface water comes back low or very low, the drainage section of your application is a short one. If surface water comes back medium or high, get the levels surveyed before the design is fixed.
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Frequently asked questions
What do the risk bands on the long term flood risk service mean?
They are annual chances of flooding. High means 3.3 per cent or greater in any year, about one in thirty. Medium is between 1 and 3.3 per cent. Low is between 0.1 and 1 per cent. Very low is less than 0.1 per cent. Read them over the lifetime of the building rather than as a single year.
Why does the Flood Map for Planning not show surface water?
Because Flood Zones describe the probability of flooding from rivers and the sea only. Surface water flooding comes from rainfall that cannot soak in or drain away, can happen almost anywhere including on slopes, and sits outside the Flood Zone system. It has its own national mapping on the long term flood risk service.
How do I get depth and velocity information?
The public service reports bands only. The national surface water mapping behind it holds extent, depth and velocity for the 1 in 30, 1 in 100 and 1 in 1,000 rainfall events. The lead local flood authority holds those layers for its area and the Environment Agency can supply the mapping on request.
Can the map be wrong about my garden?
It can be misleading at plot scale, in both directions. The terrain grid does not know about kerbs, gullies, door thresholds or a culverted ditch. A topographical survey with spot levels resolves it, and neighbours' accounts of past flooding are usually better evidence than the national model.
Do I need permission to pave my front garden?
A hard surface between the principal elevation and a highway exceeding 5 m² needs planning permission unless it is made of porous material, or run-off is directed to a permeable or porous area within the curtilage. That rule exists precisely because paved front gardens increase surface water run-off into the street.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.