Checking surface water and reservoir flood risk online
Two national flood services answer different questions. How to read the surface water bands, what the reservoir layer is really for, and where groundwater risk hides.
If you want to check the long term flood risk for your house, you need the right service, and there are two of them. The Flood Map for Planning gives you the Flood Zone that a planning application is judged against, and it covers rivers and the sea only. The separate long term flood risk service covers surface water, reservoirs and coastal erosion, and it is the one that answers the question most homeowners are actually asking: could water come across the garden in a heavy summer storm.
Both are free, both are run by the Environment Agency, and neither is a substitute for a drainage design. What they give you is a starting picture accurate enough to decide whether you need to spend money on anything more.
The two national services do different jobs
| Service | What it shows | What it is used for |
|---|---|---|
| Flood Map for Planning | Flood Zones 1, 2 and 3 from rivers and the sea, ignoring defences | The sequential test and whether a Flood Risk Assessment is needed |
| Long term flood risk service | Likelihood of flooding from rivers and sea, surface water and reservoirs, plus coastal erosion | Understanding risk to the property and insurance conversations |
The zone shown on the planning map deliberately leaves flood defences out of the picture, because the zone describes the natural floodplain rather than the defended condition. The long term service takes defences into account, which is why the same address can look worse on one and better on the other. That is not an error in either map.
Reading the surface water layers
Surface water flooding, sometimes called pluvial flooding, is rainfall running over the ground before it reaches a drain or a watercourse. It is the source that catches householders out, because it happens away from rivers and often in places with no flooding history.
The service reports risk in four bands based on the annual chance of flooding: high at more than 3.3 per cent in any given year, medium between 1 and 3.3 per cent, low between 0.1 and 1 per cent, and very low below 0.1 per cent. Those percentages translate roughly into the old return period language of a 1 in 30, 1 in 100 and 1 in 1000 year event, which you will still see quoted in older reports.
Two things to hold in mind. The mapping is modelled on a national terrain dataset, so it does not know about your garden wall, the raised patio or the gully at the bottom of the drive. And it shows the flow route, which is often more useful than the depth: if the map shows water tracking down the side of the house, that route exists whatever the modelled depth says.
What the reservoir map is for
The reservoir layer shows the maximum extent of flooding if a large raised reservoir regulated under the Reservoirs Act 1975 were to fail completely. It is not a likelihood at all. Reservoir failure in this country is extremely rare, and the mapping exists so that emergency planners know which areas to evacuate rather than so that planners can refuse applications.
Being inside a reservoir inundation extent does not normally stop a householder extension. It may appear in a search report and alarm a buyer, so it is worth understanding before you are asked about it at the point of sale.
Groundwater is the gap in the national picture
There is no national groundwater flood map of the same quality. Groundwater flooding happens where the water table rises to the surface, typically in chalk valleys after a long wet winter, and it can persist for weeks rather than hours.
Where you can look instead:
- The council's Strategic Flood Risk Assessment, which usually maps known groundwater problem areas from historic records.
- The lead local flood authority, normally the county or unitary council, which keeps records of past flooding incidents under the Flood and Water Management Act 2010.
- The British Geological Survey, which publishes susceptibility datasets based on geology and groundwater levels.
- Neighbours who have been there thirty years. Not a dataset, but frequently the best evidence available.
Putting the layers together for your site
Work through it in this order, and write down what you find as you go:
- Check the Flood Map for Planning first and record the Flood Zone. Zone 1 is low probability from rivers and sea, Zone 2 medium, Zone 3 high.
- Check the long term flood risk service for surface water at the property and for a hundred metres around it, because the flow route upstream matters more than the point value.
- Note the reservoir extent if there is one, and whether the property is within it.
- Look at levels. A site that falls towards the house from the road collects water regardless of what any map says.
- Ask the lead local flood authority whether the site sits in a critical drainage area, which is a local designation the national maps do not show.
- Search the council's planning register for nearby applications, because a previous Flood Risk Assessment on an adjoining site is free evidence.
A constraints exercise like this pays for itself before a single drawing is done, which is the argument made in the note on building a constraints plan before the design starts. Levels are the half of it that people skip, covered separately in establishing site levels from survey and elevation data.
An illustrative example
A 1930s semi on a gentle slope sits in Flood Zone 1, so no Flood Risk Assessment is triggered by zone alone. The long term service shows a medium surface water band running along the rear boundary where an old field ditch was culverted. Illustrative only, but the practical consequence is real: the council's drainage officer will want the rear extension raised slightly, the surface water routed away from the culvert line and a soakaway tested rather than assumed. None of that is hard if it is known at sketch stage. All of it is painful at validation.
What the council will ask for
For a householder extension in Flood Zone 1 with no other flood source flagged, usually nothing beyond a note on the drainage layout. Planning Practice Guidance on flood risk and coastal change sets the national triggers for a Flood Risk Assessment: sites in Flood Zone 2 or 3, sites of one hectare or more in Zone 1, land in Zone 1 with critical drainage problems, and sites where other sources of flooding are known. Councils add their own validation requirements on top, so check the local list.
Where surface water is the issue rather than the river, the document that answers it is a Drainage Strategy showing discharge rates, storage and the exceedance route. PlanScope can draft both from your postcode and the constraint data it picks up automatically, and then you send them to your agent or engineer to check the numbers.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why does my address look different on the two flood maps?
The Flood Map for Planning shows the undefended floodplain from rivers and the sea, because Flood Zones describe natural risk before defences are counted. The long term flood risk service takes existing defences into account and adds surface water and reservoirs. A defended property can therefore sit in Flood Zone 3 while showing a lower long term risk.
Does a high surface water rating mean I cannot extend my house?
No. Surface water risk is a material consideration rather than a bar, and most householder schemes are resolved by raising finished floor levels slightly, keeping the flow route clear and designing drainage that does not discharge into the problem. The council's drainage officer will want to see how water moves across the site, so include levels on the drawings.
When is a Flood Risk Assessment actually required?
National guidance triggers one for sites in Flood Zone 2 or 3, sites of one hectare or more in Flood Zone 1, land in Flood Zone 1 with critical drainage problems, and sites where flooding from other sources is known. Councils publish local validation lists that can go further. Check both before assuming a householder application escapes the requirement.
Is groundwater flooding shown on the national maps?
Not in the same detail. The national services concentrate on rivers, sea, surface water and reservoirs. For groundwater you need the council's Strategic Flood Risk Assessment, records held by the lead local flood authority, and British Geological Survey susceptibility data. Local knowledge of past events is often the most useful evidence you will find.
What is a critical drainage area?
It is a locally identified area where surface water drainage problems are known to be acute, usually defined by the lead local flood authority or in the council's Strategic Flood Risk Assessment. Being inside one can trigger a Flood Risk Assessment even in Flood Zone 1. The national maps do not show these boundaries, so you have to ask the authority directly.
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.