Finished Floor Levels in Flood Zones 2 and 3: How High, and What If You Can't Raise the Floor

Finished floor levels in Flood Zone 3 or Flood Zone 2 are, for a minor extension, set from the estimated flood level for the site, not from the ground. The Environment Agency's standing advice recommends floors at least 600 mm above that level, allows 300 mm where the flood level is known with a high level of certainty, and asks for flood resistance and resilience to at least 600 mm above it where the floor cannot be raised. This guide sets out which flood level to use, when 300 mm is accepted, what an extension that matches the existing floor has to show, and what goes on the drawings, with a decided AFC job in Rushcliffe as the worked example.

Setting floor levels for a planning application? Send your drawings through our fee proposal form and we will tell you what level the council and the Environment Agency will expect. Our flood risk assessment service covers the levels, the Product 4 data request and the mitigation.

The short answer: 600 mm, 300 mm with certainty, or resistance to 600 mm

The Environment Agency's Preparing a flood risk assessment: standing advice applies where the Environment Agency is not consulted directly, which covers most minor extensions and many smaller schemes in Flood Zones 2 and 3. For minor extensions it states: "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." Flood resistant materials are also expected up to at least 600 mm above the estimated flood level.

The 600 mm recommendation dates from the update of 24 May 2024, which also removed the earlier option of setting an extension's floor no lower than the existing floor levels or 300 mm above the estimated flood level. The latest update, of 28 May 2026, added a design flood for surface water and asks for estimated flood levels for all sources of flooding, as set out under "Which flood level" below.

Where the floor levels cannot be raised, the same section asks for extra flood resistance and resilience measures that protect the property to at least 600 mm above the estimated flood level. The current text of the standing advice contains no option to set an extension's floor no lower than the existing floor and stop there. An extension that matches the house floor below the flood level needs the resistance and resilience case set out in full.

For more vulnerable development in Flood Zone 2, such as new dwellings, the standing advice sets the floor at a minimum of whichever is higher of 600 mm above the average ground level of the site, the adjacent road level or the estimated river or sea flood level, again with 300 mm possible where the flood level is known with a high level of certainty. In Flood Zone 3 the standing advice for vulnerable development covers only certain changes of use; the Environment Agency is consulted on new dwellings. Where the minimum cannot be met, the standing advice asks for floors raised as much as possible, says to "consider moving vulnerable uses to upper floors" (in a house, the bedrooms), and asks for extra resistance and resilience measures, with electrical equipment, wiring and sockets raised, to at least 600 mm above the estimated flood level. It adds: "Internal flooding of new vulnerable development like residential dwellings is unlikely to be considered appropriately flood resistant and resilient."

Figure 1: The order the standing advice works through. Drawn from the EA's Preparing a flood risk assessment: standing advice, last updated 28 May 2026.

Which flood level: the design flood level in mAOD

Most figures in the standing advice are measured from the estimated flood level, so the first job is to establish that level. The exception is more vulnerable development in Flood Zone 2, where the floor is set at least 600 mm above whichever is highest of the average ground level of the site, the adjacent road level or the estimated river or sea flood level. For a river site the estimated flood level is normally the 1% annual exceedance probability (AEP) flood level, the 1 in 100-year event, plus the climate change allowance for the site's management catchment and epoch, taken from the Environment Agency's climate change allowances guidance. For a tidal site it is the 0.5% AEP level with the sea level allowance. Our page on climate change allowances in a flood risk assessment explains which allowance applies.

The update of 28 May 2026 added a design flood for surface water: "an event with 1 in 100 annual probability plus an allowance for climate change". The flood map for planning shows surface water extents and banded depths, and the standing advice asks for that depth information to be converted "to a maximum estimated flood level by comparing it against land level information for your site". Where a site is at risk from surface water and from rivers or the sea, flood levels are estimated for all sources. If the surface water level is higher than the river or sea level, "you may need additional mitigation".

The river or sea level comes from the Environment Agency's modelled data, requested as a Product 4 for the site, or from the council's Strategic Flood Risk Assessment (SFRA) where it publishes modelled levels. It is stated in metres above Ordnance Datum (mAOD), the national height datum, not as a height above the ground. A floor "600 mm above ground" means nothing to the officer until the ground level and the flood level are both given in mAOD.

National policy and the Planning Practice Guidance (PPG) on flood risk and coastal change set the wider test that the levels serve: development in a flood risk area is to be safe for its lifetime without increasing flood risk elsewhere. The PPG defines the design flood as a river flood with a 1% annual probability, a tidal flood with a 0.5% annual probability or a surface water flood with a 1% annual probability, in each case "plus an appropriate allowance for climate change" (paragraph 002, Reference ID 7-002-20220825).

Flood risk assessments are to "identify sources of uncertainty and how these are accounted for in a mitigation strategy" (paragraph 003, Reference ID 7-003-20220825); the certainty of the flood level, which decides between 300 mm and 600 mm, is one of those sources. Paragraph 005 (Reference ID 7-005-20220825) lists what is considered in judging whether development is safe for its lifetime, including the depth, velocity and duration of a flood and the safety of people in the building. Paragraph 006 (Reference ID 7-006-20220825) states: "Residential development can be assumed to have a lifetime of at least 100 years".

Policy F7 of the National Planning Policy Framework (17 August 2026) states that development in a location at risk of flooding should be refused unless, among its other criteria, "The development will be safe throughout its lifetime taking account of the vulnerability of its users" (F7(2)(b)) and "The development is appropriately flood resistant and resilient such that, in the event of a flood, it could be quickly brought back into use without significant refurbishment" (F7(2)(d)). Floor levels are the main way a flood risk assessment shows the first of those, and the resistance and resilience measures below a floor that cannot be raised show the second.

When 300 mm is accepted, and when 600 mm is expected

The 300 mm figure depends on certainty about the flood level. A level read from a detailed Environment Agency model at the site, with the right climate change allowance, is the usual basis for it. A level estimated from the Flood Map outline, from historic records or from LiDAR alone is not, and the 600 mm default applies.

Local policy and the Environment Agency's area teams can set their own figure. Two recent applications AFC has worked on show both ends:

• Greater Nottingham, 300 mm. The Greater Nottingham SFRA (2017) specifies that minor development is positioned at least 300 mm above the 1% AEP plus 30% climate change modelled flood level. The Environment Agency accepted 300 mm on that basis at Waterside Apartments, the worked example below.

• West Midlands, 600 mm. On a replacement house beside a main river in the West Midlands, for which AFC is assessing floodplain compensation, the Environment Agency accepted floors set 600 mm above the design flood level, in line with its West Midlands area guidance. The floor levels are in another consultant's flood risk assessment. The Environment Agency had not withdrawn its objection on floodplain storage, and the application had not been decided, when this guide was written.

Before fixing a level, check the SFRA for the council and any pre-application advice from the Environment Agency. The freeboard that applies, and the reason, belongs in the flood risk assessment.

Worked example: Waterside Apartments, West Bridgford (25/01976/FUL)

Waterside Apartments is a high-rise residential building on Pavilion Road, West Bridgford, in Flood Zone 3 beside the River Trent. The application to Rushcliffe Borough Council, 25/01976/FUL, was for a diesel standby generator in the car park to power the firefighting lifts and life-safety systems if the mains supply fails.

AFC's first flood risk assessment, of October 2025, treated the generator as plant that would not be needed in a flood and gave no modelled flood level. The Environment Agency objected. Its later email explained that the National Planning Policy Framework then in force (December 2024, Annex 3) classed "electricity supply including generation" as essential infrastructure, and that a flooded generator could compromise fire safety. The objection was about how flooding, including extreme events, would affect the unit. Annex F of the NPPF 2026 now describes essential utility infrastructure as "including for electricity generation and storage".

Revision 2 of the flood risk assessment used the Environment Agency's Product 4 data (EIR/2025/36206, dated 20 October 2025). The River Trent at Nottingham model gives a design flood level of 23.83 mAOD for the defended 1% AEP plus 30% climate change event at the model point closest to the generator, a flood depth of about 0.22 m on the site (Figure 2). Revision 2 of the flood risk assessment, issued on 14 January 2026, set the generator at 24.13 mAOD, 300 mm above the design flood level, as the Greater Nottingham SFRA requires for minor development. The base raised the unit about 0.54 m above the ground, and the overall height of the generator became 2,440 mm.

Figure 2: The design flood level and the raised level as the AFC flood risk assessment states them (Revision 2, 14 January 2026, page 11).

The Environment Agency withdrew its objection the next day by email and formally on 18 February 2026: "We rescind our previous objection and are satisfied that the proposal now meets the requirements of being safe from flooding for the lifetime of the development." Rushcliffe granted permission on 10 July 2026 (application record). Condition 4 fixes the level: "The generator shall be installed 24.13 metres above Ordnance Datum (AOD);".

Figure 3: The levels at Waterside Apartments in mAOD. The 300 mm freeboard is measured from the design flood level, not from the ground.

Three points carry over to any scheme. The freeboard was measured from a modelled level in mAOD, which is what made 300 mm acceptable. The figure came from the SFRA, cited in the report. And the higher unit needed a separate check with the case officer, who had raised no visual concern up to about 2.1 m; the officer accepted 2.44 m.

Extensions: what the officer checks

For a householder extension in Flood Zone 2 or 3, the council usually assesses flood risk against the standing advice itself, without consulting the Environment Agency. The officer looks for the estimated flood level, the existing floor level and the proposed floor level of the extension, all in mAOD.

The Royal Borough of Windsor and Maidenhead's report on a rear extension at Partridge Mead, Maidenhead (26/01427/FULL, granted 6 August 2026) set the test out: "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". AFC's flood risk assessment for that extension gave no flood levels. The officer accepted it only because of where the building stood: "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." (paragraph 8.9). Condition 3 requires the mitigation measures in the report to be maintained for the lifetime of the development. The case study is on our Partridge Mead project page.

Figure 4: Resistance and resilience measures with no levels attached (AFC flood risk assessment, Partridge Mead, page 15). Accepted here only because the extension footprint lay outside Flood Zone 2.

The lesson AFC took from that job: where the extension footprint itself is in Flood Zone 2 or 3, give the modelled flood level and the proposed floor level in mAOD, then show either the floor at least 300 or 600 mm above the flood level or resistance and resilience measures to at least 600 mm above it. If a Product 4 has been bought, quote its levels. More on when an extension needs a flood risk assessment is on our page on flood risk assessments for extensions.

When the floor cannot be raised

An extension usually meets the existing floor, because a step up inside the house is impractical. Where the existing floor is below the flood level, the extension floor will be too, and the flood risk assessment has to show how the building is protected instead.

An AFC job in Datchet in October 2026 shows the numbers. AFC's assessment places the house in Flood Zone 3a. The design flood level from the Environment Agency's Product 4 is 19.40 mAOD (1% AEP plus 35% climate change). The existing ground floor, from LiDAR and step heights measured at both doors, is 19.22 mAOD, 0.18 m below the design level, and the extension floor is kept at the same level. The flood risk assessment sets out:

• flood resistance to 19.65 mAOD, 0.25 m above the design flood level and about 0.6 m above the patio ground at 19.06 mAOD: the new slab waterproofed, the existing solid slab sealed, air bricks covered or self-closing, and door barriers and air-brick covers on the existing house;

• flood 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.

The resistance level stops short of the 600 mm line at 20.00 mAOD. Excluding water to that line would mean holding back about 0.94 m of water against the patio ground, and the standing advice asks for advice from a structural engineer where a design "aims to keep out a depth of more than 600mm of water". The flood risk assessment sets out that departure from the 600 mm figure for resistance, and carries resilience and services above the line instead (page 43 of the issued report).

AFC's flood risk assessment was issued to the client on 28 October 2026. An earlier application for the extension was withdrawn, and AFC's assessment had not yet been submitted to the council when this guide was written. Neither the council nor the Environment Agency has commented on it.

Figure 5: An extension floor kept at the existing level, below the design flood level: water excluded to 19.65 mAOD, with resilience and services above the 600 mm line.

The measures themselves, from door barriers to resilient plaster, are covered on our page on flood resistance and resilience measures. Temporary or demountable barriers are not appropriate for new buildings; for an existing building the standing advice allows them only where raising floors with passive resistance is clearly inappropriate and a flood warning is available. Resistance on new work has to be built in.

Raising the floor has knock-on effects

A higher floor solves one problem and can create three others:

• Access. Steps or a ramp to a raised floor take space, and level access may be required for new dwellings.

• Floodplain storage. In Flood Zone 3, a raised floor, raised patio or new steps below the design flood level remove floodplain storage, which has to be replaced level for level. Our guide to flood compensation for extensions sets out the method, and the floodplain compensation calculator checks it band by band.

• Height. Every millimetre added to the floor goes onto the ridge, and the case officer may have a view on it, as at Waterside Apartments.

These are reasons to agree the level early, before the design is fixed, not reasons to leave it out of the application.

What to put on the drawings for finished floor levels

Officers and the Environment Agency check levels on the drawings and in the report together. A submission that sets floor levels in a flood zone normally shows:

• the design flood level in mAOD, with the event, the climate change allowance and its source;

• the existing ground level in mAOD, stating whether it is from a topographic survey or LiDAR;

• the existing floor level in mAOD, for an extension or a change of use;

• the proposed finished floor level in mAOD and its height above the design flood level;

• the basis for the freeboard: the standing advice, the SFRA or the Environment Agency area's guidance;

• where the floor is below the flood level, each resistance and resilience measure and the level it reaches;

• a section through the building with ground, floor and flood levels drawn on;

• the refuge level and the access route, where floors are below the flood level.

Figure 6: What the officer looks for, with the Waterside Apartments values.

For a borough where floor levels come up often, see our Windsor and Maidenhead guide, and for the zones themselves, Flood Zones 1, 2 and 3 explained.

Frequently asked questions

How high above the flood level should my floor be?

The Environment Agency's standing advice recommends at least 600 mm above the estimated flood level, measured in mAOD from the design flood level (normally the 1% AEP level plus the climate change allowance). It may be reduced to 300 mm where there is a high level of certainty about the flood level, usually a modelled level for the site. Some SFRAs set their own figure, so check the council's Strategic Flood Risk Assessment too.

Is it 300 mm or 600 mm freeboard?

600 mm is the default in the standing advice, since its update of 24 May 2024. 300 mm applies only with a high level of certainty about the flood level or where local policy sets it, as the Greater Nottingham SFRA does for minor development. Environment Agency area teams can expect 600 mm even with modelled levels.

Can my extension floor be the same level as my house in Flood Zone 3?

It can, if the flood risk assessment shows that the floor cannot reasonably be raised and that the building is protected by flood resistance and resilience measures to at least 600 mm above the estimated flood level. The current standing advice has no option to set the floor no lower than the existing floor without that case.

What if I can't raise the floor level?

Raise it as far as practicable, then protect the building: resistant materials, doors and openings, resilient finishes, and services raised, all to at least 600 mm above the estimated flood level. For more vulnerable development in Flood Zone 2, the standing advice says to "consider moving vulnerable uses to upper floors" (in a house, the bedrooms), and adds: "Internal flooding of new vulnerable development like residential dwellings is unlikely to be considered appropriately flood resistant and resilient." Our page on flood resistance and resilience measures covers the options.

Does raising the floor mean I need flood compensation?

In Flood Zone 3, anything raised or built below the design flood level removes floodplain storage, and the Environment Agency normally expects it replaced level for level. The floodplain compensation calculator gives a first check.

What does mAOD mean on a drawing?

Metres above Ordnance Datum: a height above the national datum, roughly mean sea level at Newlyn. Flood levels, ground levels and floor levels are all given in mAOD so that they can be compared directly.

Do I need a topographic survey to set floor levels?

LiDAR is enough for a first check, but the levels on the submitted drawings should come from a topographic survey tied to Ordnance Datum, with the existing floor levels measured. A difference of 100 mm can decide whether a floor is above or below the flood level.

Ask us about your floor levels

AFC prepares flood risk assessments that set floor levels from the Environment Agency's modelled data, with the resistance and resilience case where the floor cannot be raised, for extensions, new dwellings and commercial schemes. Send your drawings through the fee proposal form and we will tell you what level the council and the Environment Agency will expect.