Section diagram of an extension in Datchet showing the design flood level at 18.93 mAOD, the ground floor at 19.45 mAOD and the 600 mm protection level at 19.53 mAOD

Flood Risk Standing Advice for Extensions: Is Your Floor Level High Enough?

Nick

Nov 2

9 min read

The flood risk standing advice for extensions is the Environment Agency's rule set for a flood risk assessment (FRA) on a minor extension in Flood Zone 2 or 3. Its main test is the floor level: at least 600 mm above the estimated flood level, or 300 mm where that level is known with a high level of certainty. The checker below applies that test to your levels and tells you by how many millimetres the floor passes or falls short. If the floor cannot be raised, it gives the height that flood resistance and resilience must reach. It also covers changes of use and new homes in Flood Zone 2, which follow a different version of the rule.

Need the FRA itself? Send us the plans and the council's letter through our fee proposal form. What a householder FRA contains is on our flood risk assessment for extensions page.

How to use the checker

You need two levels, both in metres above Ordnance Datum (mAOD): the estimated flood level and the proposed ground floor level.

The estimated flood level is the level of the design flood at the site: the 1% annual probability river flood (0.5% for the sea), with an allowance for climate change. The standing advice says: "You may be able to request the estimated flood level for river and sea flooding" through the Environment Agency's Flood Map for Planning. The data request returns modelled levels for many sites, often as a "Product 4" package. Where no model covers the site, the advice adds: "If not, you will need a flood risk specialist to calculate this for you."

The proposed floor level is the finished level of the lowest floor of the extension. It must be in mAOD, from a land survey or measured from survey points. A height above the garden, or above the existing floor, cannot be compared with a flood level. Officers reject an FRA that gives none, as the worked example below shows.

Where the flood level came from decides whether 300 mm can be enough. Choose "EA modelled data" only when the level comes from the Environment Agency's model for the site, including the right climate change allowance.

For a change of use or a new home, the checker also asks for the average ground level of the site and the level of the road next to the building, because the advice for vulnerable developments measures from the highest of the three.

What the result means

Meets the standing advice floor level. The floor is at least 600 mm above the estimated flood level (or above the highest of ground, road and flood level for vulnerable development). For an extension the advice still asks for more: "You will also need to use flood resistant materials up to at least 600mm above the estimated flood level."
Meets it only if the flood level is certain. The floor is between 300 mm and 600 mm above the flood level. The advice says: "You may be able to reduce this to 300mm if there is a high level of certainty about your estimated flood level." With EA modelled data the FRA can make that case, naming the model and the data reference. With an estimate, raise the floor or obtain the modelled level.
Below the standing advice. The floor is less than 300 mm above the flood level, or below it. If it can be raised, the checker gives the level to raise it to. If it cannot, the FRA has to explain why and specify resistance and resilience measures: "These measures should protect the property to at least 600mm above the estimated flood level." The design should "prioritise excluding flood water where possible" and "speed recovery in case water gets in".
You need the estimated flood level first. No floor level can be judged without it.
Flood Zone 3b: check PPG table 2 first. The site is in the functional floodplain, and the advice sends you to table 2 of the planning practice guidance before anything else.
Not covered by the standing advice. The scheme falls outside the advice, and the FRA has to follow the full planning practice guidance on flood risk.

Where water would stand more than 600 mm deep against the walls, the advice is direct: "If your design aims to keep out a depth of more than 600mm of water, you should get advice from a structural engineer." In our practice that depth is measured from the lowest ground outside the extension, not from the floor. Resistance and resilience measures should follow the CIRIA Code of Practice for Property Flood Resilience, which the standing advice names.

Our guide to finished floor levels in Flood Zones 2 and 3 covers the wider question, including floors that cannot be raised at all.

Worked example: a refusal in Lawn Close, Datchet

Application 26/01517/FULL was for a part single and part two-storey side and rear extension to a semi-detached house. The site is in Lawn Close, Datchet, in Flood Zone 2. The Royal Borough of Windsor and Maidenhead refused it on 15 September 2026. The FRA submitted with it was AFC's, issued in July 2026.

The second reason for refusal reads: "The proposal, by virtue of insufficient flood risk modelling and lack of compliance with EA householder standing advice, would fail to appropriately demonstrate that it would be sufficiently resilient to flooding in planning terms." The first reason concerned the design of the rear roofscape.

The officer's report explains the flood reason. The July FRA measured the 300 mm freeboard from the first-floor bedrooms and gave no design flood level and no floor levels in mAOD. The officer wrote: "This is not how EA standing advice defines freeboard or FFL, and it is not accepted that the proposal complies by defining FFL as starting where the bedrooms are located. The report does not define any of these levels". That FRA was issued a week before the Environment Agency's modelled data for the site arrived. The Environment Agency itself returned the consultation as outside its remit, so the council judged the FRA against the standing advice alone.

After the refusal AFC wrote a new FRA in October 2026 for a revised scheme, using the Environment Agency's Thames (Bourne End to Datchet) 2019 model. Its figures:

| Level | mAOD | |---|---| | Design flood level (1% AEP + 35%, central 2080s) | 18.93 | | Design flood level + 300 mm | 19.23 | | Existing and proposed ground floor | 19.45 | | Design flood level + 600 mm (flood resistance and resilience to here) | 19.53 |

The ground floor stands 0.52 m above the design flood level: 0.22 m above the 300 mm level and 80 mm below the 600 mm level. The FRA relies on the certainty of the modelled level for the 300 mm figure, and specifies flood resistance and resilience to 19.53 mAOD. The floor level is taken from LiDAR and the drawings, to be confirmed by measured survey. Entered in the checker, these numbers give "Meets it only if the flood level is certain".

Lawn Close section: design flood level 18.93 mAOD, ground floor 19.45, 300 mm level 19.23 and 600 mm level 19.53

Figure 1: Levels from AFC's October 2026 FRA for Lawn Close, Datchet. Vertical scale exaggerated.

The revised scheme had not been submitted at the time of writing, so the council has not yet decided whether it meets the advice. The lesson from the first application: give the flood level and the ground floor level in mAOD, and measure the freeboard from the ground floor. More on what this council asks for is in our guide to flood risk assessment in Windsor and Maidenhead. The job is written up as a case study.

When the floor cannot be raised: an extension in Datchet

A second householder extension in Datchet, in Flood Zone 3a, shows the other branch of the advice. The first application was withdrawn in October 2026. Its FRA was not AFC's: it relied on a neighbouring property's 2024 FRA and gave no design flood level and no levels in mAOD. The council asked for a full site-specific FRA to the current standing advice, because the advice had changed since the neighbour's approval.

AFC's FRA for the resubmission takes the design flood level from the Environment Agency's Product 4 data for the site. It is 19.40 mAOD: the 1% AEP flood with a 35% climate change allowance (central, 2080s). The ground floor stays at the existing 19.22 mAOD, 0.18 m below that level. The FRA sets out that departure from the advice and the measures that go with it:

flood resistance to 19.65 mAOD: a waterproofed new slab, the existing solid slab sealed, and air bricks covered or self-closing;
resilient materials and raised services to 20.20 mAOD, 800 mm above the design flood level;
bedrooms and a refuge on the first floor, at about 21.7 mAOD.

Resistance is set against the undefended 0.1% AEP level of 19.59 mAOD, because the integrity of the modelled defences is not shown. Entered in the checker as an extension that cannot be raised, these levels give "Below the standing advice" and a protection height of 20.00 mAOD. The resubmission had not been made at the time of writing.

Datchet extension section: ground floor 19.22 mAOD below the 19.40 flood level, resistance to 19.65, resilience to 20.20

Figure 2: Levels from AFC's October 2026 FRA for a householder extension in Datchet. Vertical scale exaggerated.

When the flood risk standing advice for extensions does not apply

The advice for minor extensions covers "a householder or non-domestic extension with a floor space of no more than 250 square metres" in Flood Zone 2 or 3.

Flood Zone 3b, the functional floodplain, is shown in the council's Strategic Flood Risk Assessment. The advice for minor extensions deals with it in one step: "Check table 2 of the planning practice guidance if your proposed minor extension is in Flood Zone 3b (functional floodplain)." Table 2 of the planning practice guidance (paragraph 079, Reference ID 7-079-20220825) says development should not be permitted in Flood Zone 3b if it is highly vulnerable, more vulnerable or less vulnerable. Water compatible uses can go there, and essential infrastructure only if it passes the Exception Test. A new home is 'more vulnerable', so the table rules it out in Flood Zone 3b, as it does a change of use to a 'more vulnerable' use. Speak to the council before going further.

The advice does not cover:

Extensions over 250 m², which need an FRA to the full planning practice guidance.
New homes in Flood Zone 3, and most changes of use there. In Flood Zone 3 the advice for vulnerable developments covers only a "change of use to 'water compatible'" and a "change of use where the vulnerability classification will reduce or remain unchanged as 'less vulnerable' or 'water compatible'". A change of use that ends as 'more vulnerable' or 'highly vulnerable', such as a house converted to flats, is outside the advice in Flood Zone 3.
Council requirements. The council can ask for more than the advice: a groundwater section, an evacuation plan or floodplain compensation. Our floodplain compensation calculator checks the storage a Flood Zone 3 extension displaces.

What to send for a quick FRA quote

the existing and proposed plans, elevations and a section through the ground floor;
a topographic survey with levels in mAOD, if there is one;
the council's letter or validation request, if the application is already in;
the flood level data, if you have requested it from the Environment Agency.

With those we can give a fixed fee and say whether the floor level is likely to meet the advice before the FRA is written.

What is the Environment Agency standing advice for extensions?

It is the Environment Agency's guidance on what an FRA for a minor extension in Flood Zone 2 or 3 must contain. It is published on GOV.UK as flood risk standing advice. It covers floor levels, flood resistance and resilience, and other consents. The council applies it when it decides the application.

How high above the flood level should my extension floor be?

At least 600 mm above the estimated flood level. The advice says the margin may be reduced to 300 mm where there is a high level of certainty about the flood level. Where the floor cannot reach that height, the FRA must say why and add flood resistance and resilience measures up to 600 mm above the flood level.

When can the freeboard be 300 mm instead of 600 mm?

When there is a high level of certainty about the estimated flood level. In practice that means a level from the Environment Agency's modelled data for the site, with the right climate change allowance. The FRA should name the model and the data reference. An estimate or old data does not support 300 mm.

What if I can't raise the floor of my extension?

Explain in the FRA why, for example level access to the existing house. Then specify flood resistance and resilience measures to at least 600 mm above the estimated flood level, following the CIRIA Code of Practice for Property Flood Resilience. If the design keeps out more than 600 mm of water, a structural engineer should check it. Our guide to finished floor levels sets out the measures.

Where do I get the estimated flood level for my site?

Request flood risk data for the site through the Environment Agency's Flood Map for Planning. If no modelled level is available, a flood risk specialist has to calculate one.

Does standing advice apply in Flood Zone 3?

Yes for a minor extension of 250 m² or less, and for a change of use in Flood Zone 3 where the use will be 'less vulnerable' or 'water compatible' afterwards (the classification reducing or staying the same). Not for a new home, and not for a change of use that ends as 'more vulnerable' or 'highly vulnerable'. For a minor extension in Flood Zone 3b, the advice says to check table 2 of the planning practice guidance. Table 2 says development should not be permitted in Flood Zone 3b if it is highly, more or less vulnerable. Speak to the council before going further.

Does the Environment Agency comment on householder extensions?

Usually not. On the Lawn Close application in Datchet it returned the consultation as outside its remit, and the council judged the FRA against the standing advice itself.

Ask us about your extension

If the checker shows a shortfall, or you have no flood level yet, send us your plans. Our fee proposal form takes the drawings and any flood level you have, and we reply with a fixed fee.