Flood Map for Planning or the long term risk service
The planning flood map is undefended and river and sea only. The long term service includes defences, surface water and reservoirs. Use each for what it is built for.
Two government services will tell you about flood risk at the same address and give you different answers. The Flood Map for Planning shows flood zones for rivers and the sea ignoring defences, and it is the map the planning system uses. The long term flood risk service shows risk from rivers and the sea taking defences into account, plus surface water, reservoirs and groundwater, and it is written for householders and insurers. Neither is wrong. They are measuring different things, and a self-builder who quotes the friendlier one in a planning submission will be corrected by the council at consultation.
Use the planning map for the sequential test and for deciding whether a flood risk assessment is needed. Use the long term service to understand what could actually happen on site.
What each service is built for
The Flood Map for Planning exists to support planning decisions. It shows Flood Zones 2 and 3 from rivers and the sea, the location of flood defences, main rivers and areas benefiting from defences, and flood storage areas. It is deliberately drawn on an undefended basis, because a defence can be overtopped, can fail, and may not be maintained for the lifetime of the development.
The long term flood risk service exists to tell people what risk they face where they live. It takes defences into account, covers surface water and reservoirs as well as rivers and the sea, and presents the answer in bands rather than zones.
The distinction is deliberate policy. Planning asks where development should go over the next hundred years. The householder service asks what might happen to this house.
Flood zones and the undefended basis
The zones are probability bands, defined by annual chance of flooding ignoring defences.
| Zone | Annual probability, rivers | Annual probability, sea |
|---|---|---|
| Zone 1 | Less than 0.1 per cent | Less than 0.1 per cent |
| Zone 2 | Between 0.1 and 1 per cent | Between 0.1 and 0.5 per cent |
| Zone 3 | 1 per cent or greater | 0.5 per cent or greater |
Zone 3 is subdivided into 3a, high probability, and 3b, the functional floodplain where water has to flow or be stored in times of flood. Zone 3b is not shown on the national map. It is defined locally in the council's strategic flood risk assessment, so a site shown as Zone 3 nationally may be 3b in the council's own evidence, which changes what can be built there.
Zone 1 is not zero risk. It is simply below the threshold for river and sea flooding, and it says nothing at all about water running off a hillside behind the site.
Surface water, reservoirs and groundwater
Surface water flooding is the source that catches self-builders out, because it does not appear on the planning map and it is not the Environment Agency's to manage. Responsibility for local flood risk sits with the lead local flood authority, which is the county or unitary council.
The long term service shows surface water risk in bands, along with reservoir inundation extents and information on groundwater. Councils increasingly ask for this to be addressed even in Zone 1, particularly where the site is in an area the strategic flood risk assessment identifies as having critical drainage problems.
The underlying national flood risk assessment has been updated in recent years and the bands are presented differently from earlier versions of the service, so read the categories as the service states them on the day you check rather than from memory.
Why the two disagree at one address
Four reasons account for nearly all of it.
Defences. A site behind a maintained embankment can sit in Zone 3 on the planning map and show as low risk on the long term service, because one ignores the embankment and the other does not.
Source. A site can be Zone 1 for rivers and high risk for surface water, which looks like a contradiction and is not.
Resolution. Flood zones are produced from national scale modelling and their edges are approximate. A boundary that appears to clip the corner of a plot may be an artefact of the model rather than a real line on the ground.
Date. The two datasets are updated on different cycles, so a defence completed last year may appear in one before the other.
When a council's consultee queries your submission, it is usually one of these four. Saying which one, with the evidence, resolves it faster than resubmitting.
Drawing the red line into the product search
Both services let you look up an address, but for a planning submission you want the site rather than the postcode. The Flood Map for Planning includes a facility to draw or upload the site boundary and generate a report for that area, which is what a case officer expects to see rather than a screenshot of a pin.
Use the same red line as the application. If the access strip is inside the red line, it goes into the flood search too, because a house on dry ground reached by a road that floods twice a winter still has a safe access problem.
That report also tells you whether detailed modelled data exists for the reach, which determines how much of the flood risk assessment can be written from public data and how much needs an enquiry.
Recording the date and version of a check
Flood datasets change. An assessment that cites an undated screenshot is the easiest thing in the world for a consultee to reject.
Record, for every check: the service used, the date, the reference number of any report generated, the extent searched, and the dataset version where the service states one. Put that in a short table at the front of the flood risk assessment. It takes ten minutes and it removes an entire category of consultation comment.
Redo the checks if the application sits undetermined for months. A determination made against superseded mapping is a problem for everyone.
When to ask for detailed data
Public mapping is enough for a Zone 1 site of under a hectare with no surface water risk. It is rarely enough for anything else.
Ask the Environment Agency for detailed flood risk data where the site is in Zone 2 or 3 and you need modelled levels, where you need the 1 in 100 year level plus a climate change allowance to set finished floor levels, or where the mapped zone boundary crosses the site and you need to know whether that reflects real topography. Allow time. These requests are not instant, and building the programme around an assumption that they are is how self-build projects lose a season.
For surface water, the lead local flood authority holds the local data and will usually say what it expects a drainage strategy to demonstrate before you design one. Its consultee response is also where a poorly documented model gets unpicked, which we cover in the note on what a consultee checks in model output.
What to submit with the application
A site-specific flood risk assessment is expected where the site is in Zone 2 or 3, where it is a hectare or more in Zone 1, and where the site is at risk from other sources or in an area with critical drainage problems. Where the sequential test applies, it is a separate exercise from the assessment and has to be evidenced in its own right, as our note on the flood sequential test sets out.
Change of use applications follow different rules from new build, and the vulnerability classification can shift even where nothing is built, a point covered in the piece on flood risk and change of use.
Start with the boundary, not the postcode. Then check both services, date the evidence, and write down which one you relied on for what.
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Frequently asked questions
Which flood map does a planning application use?
The Flood Map for Planning. It shows Flood Zones 2 and 3 from rivers and the sea on an undefended basis, which is what the sequential test and the requirement for a flood risk assessment are built on. The long term flood risk service is useful background but is not the planning dataset.
Why does my site show as Zone 3 but low risk on the other service?
Almost always because of flood defences. The planning map deliberately ignores them, since defences can be overtopped, can fail and may not be maintained for the lifetime of the development. The long term service takes them into account. Both statements can be correct at the same address.
Does the Flood Map for Planning show surface water flooding?
No. It covers rivers and the sea only. Surface water risk sits with the lead local flood authority, which is the county or unitary council, and appears on the long term flood risk service. A Zone 1 site can still carry a high surface water risk and need drainage evidence.
What is Flood Zone 3b and why can I not see it?
Zone 3b is the functional floodplain, where water has to flow or be stored in times of flood. It is not shown on the national map. Councils define it in their strategic flood risk assessment, so a site shown nationally as Zone 3 may be 3b locally, which sharply limits what can be built.
When do I need detailed data from the Environment Agency?
Where you need modelled flood levels to set finished floor levels, where the mapped zone boundary crosses the site and you need to know whether it reflects real ground levels, or where a climate change allowance has to be applied. Allow several weeks, because these requests are not returned immediately.
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.