Flood Risk Assessment If You're Not in a Flood Zone: Which Map Counts
A council can ask for a flood risk assessment if you're not in a flood zone on the map you checked, and both sides can be right. The council goes by one map, the Environment Agency's flood map for planning, and reads it across the whole application site inside the red line, not at the house's address point. A garden, a garage plot or a driveway inside Flood Zone 2 or 3 is enough to trigger the assessment, even where the house itself sits in Flood Zone 1. This guide explains why the maps disagree, when an assessment is needed in Flood Zone 1 anyway, how a decided Air & Flood Consultants (AFC) job in Maidenhead was handled, and how higher ground is used properly: by comparing levels inside the assessment, not by arguing with the map.
Got an "FRA required" letter for a garage, outbuilding or extension? Send us the letter and the red line plan through our fee proposal form for a fixed fee proposal. For an example of a proportionate householder assessment, see our Partridge Mead case study.
Why the flood maps disagree
Several public and commercial maps describe flood risk, and they answer different questions. A homeowner usually meets three of them before a council letter arrives: a free online flood map, the flood section of a purchase search, and the government's own property-owner service. None of them is the map the council uses to decide that a planning application needs a flood risk assessment.
The flood map for planning is the Environment Agency's service for planning applications. It shows Flood Zones 1, 2 and 3 for river and sea flooding, and it now carries further layers, including flood zones plus climate change, surface water and defences. The Planning Practice Guidance defines the zones in its Table 1 (Reference ID 7-078-20220825): Flood Zone 1 is "Land having a less than 0.1% annual probability of river or sea flooding", Flood Zone 2 is land between a 1% and 0.1% annual probability of river flooding (0.5% to 0.1% for the sea). Flood Zone 3a is land at a 1% or greater annual probability of river flooding (0.5% or greater for the sea). The table adds that the flood zones themselves "do not take account of the possible impacts of climate change".
Check your long term flood risk is the GOV.UK service for people looking at an area, such as owners and buyers. It covers rivers and the sea, surface water, reservoirs and, where data is available, groundwater. In its own words: "This service will tell you the flood risk of an area, not a specific property." It does not give the planning flood zone.
Surface water maps show where rainwater may pond or flow, with banded depths. Surface water is now also a layer of the flood map for planning, and in Flood Zone 1 it is a separate trigger for an assessment (see below).
Purchase searches and free online maps are commercial products for buyers, lenders and insurers. They apply the provider's own scoring and wording. A search can rate river risk "high" for a property that sits in Flood Zone 2 on the planning map, or "not in a flood zone" for a plot that touches Flood Zone 3. Neither result binds the council.
Figure 1: Four kinds of flood map and what each is for. Only the flood map for planning sets the flood zone for a planning application.
The council reads the map at the red line, not the address point
GOV.UK's guidance Flood risk assessments if you're applying for planning permission (last updated 28 May 2026) tells applicants to "use the Environment Agency flood map for planning to find if the development is in Flood Zone 1, 2 or 3". It continues: "Find the correct area by entering the postcode or location of the development site. Then you can draw the boundary of your site to get relevant flood risk information."
The boundary is the red line on the location plan submitted with the application. Validation officers check the flood zone across that line. If any part of it falls in Flood Zone 2 or 3, they expect an assessment, whatever the address search says.
This is where most disagreements start. A postcode search, a purchase search and most free maps place a single point on the address, usually on the house. A house on higher ground can sit in Flood Zone 1 while the garden behind it, the plot for a new garage or the end of a long drive runs down into Flood Zone 2 or 3. The opposite also happens: an address point can land on the road or on a neighbouring plot and report a zone that has nothing to do with the site. The reliable check is the one GOV.UK describes: draw the site boundary on the flood map for planning and download the report for that area.
Figure 2: The address point and the red line can sit in different flood zones. The council goes by the red line. Schematic, not a real site.
The printed report from the flood map for planning says the same thing in its notes. Its first page states the zone for the selected location and, underneath: "This information relates to the selected location and is not specific to any property within it." The selected location is the area that was drawn or searched. A report for a drawn red line describes that red line, not the house.
Figure 3: The first page of the official report for the Partridge Mead site (4 June 2026). It describes the selected location, not any one property.
When you need a flood risk assessment if you're not in a flood zone
Flood Zone 1 does not end the question. The same GOV.UK page lists the cases where a flood risk assessment is needed for "all development (including minor development and changes of use)". Beyond Flood Zones 2 and 3 and Flood Zone 3b, the list includes development:
"within Flood Zone 1 with a site area of 1 hectare or more";
"within 'Flood Zones plus Climate Change', showing it is at increased risk of flooding from rivers or sea in future";
"within Flood Zone 1 and the flood map for planning shows it is at risk of flooding from surface water, now or in future";
"in areas with critical drainage problems";
"within Flood Zone 1 where the LPA's strategic flood risk assessment (SFRA) shows it will be at increased risk of flooding during its lifetime";
"that increases the vulnerability classification and may be subject to sources of flooding other than rivers or sea".
The Environment Agency's standing advice covers Flood Zone 1 sites shown at increased risk from rivers or the sea in future. For these, the applicant should complete an assessment and "apply this guidance as if the site were in Flood Zone 2".
Councils also publish their own validation checklists, and many repeat this list in their own words or add to it. Large rural plots pass the one-hectare test more often than owners expect, because the red line of a farmhouse with paddocks can easily exceed it.
One of AFC's own case studies shows the Flood Zone 1 triggers at work. The barn conversion at Hoxne is in Flood Zone 1 but at high risk from surface water in the courtyard between the barns, and the change to dwellings raised the vulnerability of the use. Our page on Flood Zones 1, 2 and 3 explains the zones themselves.
Worked example: a householder extension in Partridge Mead, Maidenhead (26/01427/FULL)
The application, 26/01427/FULL to the Royal Borough of Windsor and Maidenhead, was for a single storey rear extension to a link-detached 1970s house, replacing a conservatory. The officer's report describes it as about 4 m deep and 3.5 m high, with a gable roof, rooflights and bi-fold doors to the garden. No bedrooms were added and there is no basement.
The flood map for planning put the front and rear gardens in Flood Zone 2. The house, and the footprint of the extension, sat outside it. The officer's report records this at paragraph 2.2: "The front and rear gardens of the site lie within Flood Zone 2, although the dwelling is not within Flood Zone 2." Because part of the red line was in Flood Zone 2, a site-specific flood risk assessment was required, and AFC prepared it (4 June 2026).
Figure 4: The flood map for planning at the Partridge Mead site (Environment Agency, 4 June 2026). Flood Zone 2 reaches into the selected area.
The assessment covered all sources. Its executive summary rated the risk from rivers and the sea as medium (Flood Zone 2), surface water low, groundwater high, sewers low and artificial sources low. On rivers, it found that "the site lies outside the design flood extent for the 1 in 100-year plus climate change event". It gave no modelled flood levels. Its mitigation, on page 15, included profiling external ground levels to direct overland flow away from entrances, solid concrete floors with resilient finishes, resilient skirtings, doorframes and sills, and sealed service entries, with waterproof concrete to below-ground structures for the groundwater risk.
The officer's report set the assessment against the standing advice at paragraph 8.9: "The EA's standing advice requires specific details with reference to estimated flood levels of the existing building(s) and the finished floor levels of the extensions, which would need to be at least 300/600mm above the flood level to minimise flood risk or demonstrate flood resistance and resilience measures up to 600mm above the estimated flood level." It then accepted the assessment: "However, as the dwelling and proposed extension would be located outside Flood zone 2, flood levels are not given. In this particular circumstance, and taking a proportionate approach, the FRA provides appropriate details on mitigation and resilience measures to be incorporated in the proposed development."
The council granted permission on 6 August 2026. Condition 3 ties the extension to the assessment: the development is to be carried out in accordance with the flood mitigation and protection measures in the AFC assessment of 4 June 2026, and "The flood mitigation and protection measures shall be maintained for the lifetime of the development."
The lesson is narrow. The officer accepted an assessment without flood levels only because the house and the extension were outside Flood Zone 2. Where the footprint itself is in Flood Zone 2 or 3, the standing advice the officer cited at paragraph 8.9 expects the estimated flood level and the floor level in mAOD. Our guide to flood risk assessment in Windsor and Maidenhead covers what the borough's officers check.
The assessment also shows why the zone is not the whole story. Surface water and groundwater are separate sources with separate maps, and here they gave different answers.
Figure 5: Surface water is a separate map and, in Flood Zone 1, a separate trigger for an assessment. At Partridge Mead the site was clear of it (AFC assessment, page 12).
Figure 6: The borough SFRA's groundwater map placed the site in an area of 75% or greater susceptibility to groundwater flooding, so groundwater was rated high although the river zone was only Zone 2 (AFC assessment, page 12).
The same question the other way round: a garage beside a house near Coventry
A current AFC job shows the opposite case. The owner of a house near Coventry applied for a new garage and an extension. The house is in Flood Zone 1, and the owner says the council accepts that the house itself is not in a flood zone. The council's validation email still said: "The site of the garage is within flood zones 2 and 3, therefore a flood risk assessment is a standard validation requirement."
Three sources had given the owner three answers. A free online flood map said the property was not in a river or sea flood zone. The purchase search had rated river flood risk high. A council record then showed a culverted watercourse running past the buildings. The council goes by the flood map for planning, read across the red line, and the garage plot is inside Flood Zones 2 and 3 on that map.
AFC did not argue the zone line. It asked for the red line plan and the position of each new building on it, and ordered the Environment Agency's modelled flood levels for the site (Product 4). It also asked the owner for spot levels and photographs, and logged the culvert record for the assessment. When the modelled levels arrive, the assessment compares the measured ground and slab levels with them and states the result. The report on this job has not yet concluded, so no levels are given here.
How a levels comparison works, and when higher ground helps
Higher ground is a real argument, but it is made with levels, not with a different map. The comparison needs two sets of numbers, both in metres above Ordnance Datum (mAOD).
The modelled flood level comes from the Environment Agency's model for the river, requested through the flood map for planning. The data package is known as Product 4. The standing advice says: "You may be able to request the estimated flood level for river and sea flooding and information about residual risks through the Environment Agency's flood map for planning. If not, you will need a flood risk specialist to calculate this for you." The Environment Agency gives a due date when it acknowledges the request. On a recent AFC request it was four weeks after the order. For surface water, the standing advice asks for the banded depths on the flood map for planning to be converted "to a maximum estimated flood level by comparing it against land level information for your site".
The site levels come from a topographic survey, or from Environment Agency LiDAR where there is no survey. The standing advice asks for all levels "in relation to Ordnance Datum (the height above average sea level)". Where Ordnance Survey data is not enough, it asks for a land survey by a qualified surveyor.
For a minor extension in Flood Zone 2 or 3, the standing advice (last updated 28 May 2026) then sets the test: "We recommend floor levels are set at least 600 millimetres (mm) above the estimated flood level." It continues: "You may be able to reduce this to 300mm if there is a high level of certainty about your estimated flood level." Where the floor cannot be raised, extra flood resistance and resilience measures "should protect the property to at least 600mm above the estimated flood level". Our guide to finished floor levels in flood zones works through the test with a decided example.
Figure 7: A levels comparison. Every level is in mAOD, and freeboard is measured from the modelled flood level, not from the ground.
What higher ground does, and does not do:
A site whose ground and floor levels sit above the modelled flood level still needs the assessment, because the red line is still in the zone. The assessment becomes shorter. It can show the building is above the design flood and set modest mitigation, as at Partridge Mead.
A building on a footprint inside the flood extent needs the floor level and the flood level in mAOD, and floodplain storage may need replacing. Our floodplain compensation calculator shows how the volumes are checked.
Changing the zone line itself is a separate challenge to the Environment Agency, with survey and modelling evidence. It is not a quick way past a validation request, and the application still needs a flood risk assessment while it runs.
What to send to speed up the flood risk assessment
Most of the time on a small assessment goes into fixing what is being assessed and where. These items, sent with the first enquiry, shorten it:
The council's letter or email asking for the assessment, and the planning reference if there is one.
The red line (location) plan submitted with the application.
The site plan showing where each new building, extension or area of hardstanding sits on that plan.
Floor plans and elevations, if drawn, including the proposed floor level.
Spot levels or a topographic survey, if one exists.
Photographs of the slab edge, the door thresholds and any ditch, drain, culvert or watercourse near the site.
A note of what the ground was like before any work started, if work has begun.
Our flood risk assessment for extensions page sets out what the assessment covers, and the flood risk assessment service page covers larger schemes.
The traps
Relying on the address point. A postcode search reads one point. The council reads the red line. Run the check on the drawn boundary before deciding whether an assessment is needed.
Using the wrong map. A purchase search, an insurer's rating or a free online map is not the flood map for planning. Quoting one in reply to a validation letter does not change what the council asks for.
Building the slab before the assessment. A slab that is already laid can still be assessed, but its level is then fixed. If it turns out below the modelled flood level, the options narrow to resistance and resilience measures, or to compensation for lost floodplain storage.
Forgetting other sources. Surface water, groundwater and culverted watercourses can govern a Flood Zone 1 site. A culvert on or next to the site may also need a separate consent: the standing advice says that if there is a watercourse "(including culverted watercourses) on or next to your site", the owner should read the guidance on owning a watercourse.
Letting the validation deadline pass. Validation letters set a date for the missing documents. If it has passed, ask the council in writing whether the application is still live before paying for an assessment against it.
Frequently asked questions
The council says I need a flood risk assessment, but my house is not in a flood zone. Why?
The council reads the Environment Agency's flood map for planning across the whole application site inside the red line, not at the house. If a garden, driveway or garage plot inside the red line is in Flood Zone 2 or 3, an assessment is expected. It can also be needed in Flood Zone 1 for surface water risk, sites of 1 hectare or more, or where the council's SFRA shows increased risk. See GOV.UK's list.
Which flood map does the council use for planning?
The Environment Agency's flood map for planning. GOV.UK tells applicants to use it to find the flood zone and to draw the boundary of the site to get the relevant flood risk information. Councils also use their own Strategic Flood Risk Assessment for local detail.
Is my garden in flood zone 2 if my house is in flood zone 1?
It can be. Flood zone boundaries follow ground levels, not property boundaries, so a plot can cross from Flood Zone 1 into Flood Zone 2 or 3. At Partridge Mead in Maidenhead the front and rear gardens were in Flood Zone 2 and the house was not. Draw the red line on the flood map for planning to see which zones it crosses.
Do I need a flood risk assessment for a garage or outbuilding?
If any part of the application site is in Flood Zone 2 or 3, or one of the Flood Zone 1 triggers applies, yes: GOV.UK's list covers "all development (including minor development and changes of use)". A garage is less vulnerable than a house, so the assessment is usually short, but it still has to deal with flood levels where the garage sits inside the flood extent. Our page on flood risk assessments for extensions explains the scope.
The flood map is wrong: my ground is higher. Can I challenge it?
Changing the zone line is a separate challenge to the Environment Agency, supported by survey and modelling evidence. For a planning application the practical route is a levels comparison inside the assessment: survey levels in mAOD compared with the modelled flood level. If the building sits above the modelled level, the assessment says so and the mitigation is modest.
What is the difference between the flood map for planning and "Check your long term flood risk"?
The flood map for planning gives the planning flood zones and is used for planning applications. Check your long term flood risk describes an area's risk from rivers and the sea, surface water, reservoirs and groundwater; it says it "will tell you the flood risk of an area, not a specific property". Only the first sets the flood zone for an application.
My purchase search said high flood risk, but the council map says Zone 2. Which is right?
For planning, the council map. A purchase search applies its provider's own scoring for buyers, lenders and insurers. The planning flood zone comes from the flood map for planning, and the assessment then looks at every source of flooding at the site.
What is EA Product 4 and do I need it for a householder extension?
Product 4 is the Environment Agency's package of modelled flood levels and extents for a site, requested through the flood map for planning. It is needed when the extension footprint is inside the flood extent and the floor level has to be compared with the estimated flood level. Where the footprint is outside the flood zone, as at Partridge Mead, a proportionate assessment without flood levels can be accepted.
What can I send to make the flood risk assessment quicker?
The council's letter, the red line plan, a site plan showing where the new building sits, any drawings with floor levels, spot levels or a survey, and photographs of thresholds and any nearby drain, ditch or culvert. The list is under "What to send" above.
Do I need a flood risk assessment in flood zone 1?
Sometimes. GOV.UK lists sites of 1 hectare or more, land shown at risk from surface water on the flood map for planning, land in flood zones plus climate change, critical drainage areas, land the council's SFRA shows at increased risk, and changes to a more vulnerable use that may be subject to other sources of flooding. Local validation lists can add more.
Send us the letter
Got an "FRA required" letter for a garage, outbuilding or extension? Send us the letter and the red line plan through our fee proposal form and we will reply with a fixed fee proposal and the information the assessment needs. The Partridge Mead case study shows a proportionate householder assessment that the council accepted and conditioned.
Comments