Extension in Flood Zone 3b: Why the Environment Agency Rejects Voids, and the Ground Lowering That Got Approved
An extension in Flood Zone 3b, the functional floodplain, can be approved, but only under the council's local policy for existing buildings, and only if the storage it takes from the floodplain is given back in the ground on the same plot. National policy treats a house as incompatible with Flood Zone 3b. On the Thames the Environment Agency now rejects a void or stilts under the floor as the way to keep that storage, and asks for compensation level for level. This guide explains how to tell whether a house is in Flood Zone 3b, the policy route and why voids fail. It then follows a Spelthorne householder application that AFC took to permission on 3 November 2026 by lowering the front driveway, and the conditions that make the owner dig before building.
Extending in Flood Zone 3b? Send us the address and the drawings for a fee proposal, and we will tell you whether compensation can work on your plot before you apply. The Staines case study is the job this guide is built on, and our flood risk assessment service covers the compensation design.
What Flood Zone 3b is, and how to tell if a house is in it
Flood Zone 3b is the functional floodplain. The Planning Practice Guidance (PPG) describes it as land "where water from rivers or the sea has to flow or be stored in times of flood", which will normally comprise "land having a 3.3% or greater annual probability of flooding", and asks councils to identify it in their Strategic Flood Risk Assessments in agreement with the Environment Agency (Flood risk and coastal change, Table 1, paragraph 078, Reference ID 7-078-20220825).
The Environment Agency's Flood Map for Planning does not show it. The map shows Flood Zones 2 and 3 only, and Flood Zone 3 on that map covers both 3a and 3b. The 3b line is drawn by the council in its Strategic Flood Risk Assessment (SFRA), usually from the river model's 3.3% annual exceedance probability (AEP) extent, the 1 in 30-year flood. The Spelthorne Level 1 SFRA (October 2024) defines Flood Zone 3b on the Thames at that 3.3% AEP extent.
Before the PPG was revised in August 2022, the usual starting point was the 1 in 20-year (5% AEP) flood, and that wording is still in circulation. The officer's report on the Staines application described Flood Zone 3b as "land with a 1 in 20 or greater annual probability of flooding", while the FRA worked to the SFRA's 1 in 30 extent. On a riverside plot that floods in both events the difference does not matter. On a plot near the edge it decides the policy that applies, so the starting point is the council's own SFRA map, not the national map or an older document.
Three checks settle it for a single plot:
the SFRA's Flood Zone 3b map, with the plot marked on it;
the river model's level for the 1 in 30-year flood against surveyed ground levels on the plot: if ground is below that level, the plot floods in the event that defines 3b;
the Environment Agency's Risk of Flooding from Rivers and Sea band: "High" means a 3.3% or greater chance each year.
At Staines all three agreed. The Thames (Datchet to Teddington) 2023 model gives a 1 in 30-year level of 14.43 m AOD at the extension, the surveyed ground there is about 14.00 m AOD, and the Agency's national assessment (NaFRA2) puts the whole plot in the High band.
Figure 1. The whole plot is in the High (1 in 30 or more each year) band of the Environment Agency's NaFRA2 river flooding data (AFC FRA, Figure 4).
Figure 2. Every level on the job, from AFC's FRA: the ground at the extension is 0.91 m below the design flood level, and the plot floods in the 1 in 30-year event that defines Flood Zone 3b.
Can you build an extension in Flood Zone 3b?
Not under the national rules alone. Annex F of the National Planning Policy Framework (2026) lists "Buildings used for dwelling houses" as More vulnerable in its Table 2, and its Table 3 marks More vulnerable development in Flood Zone 3b "no", with the key "Development should be refused" (NPPF Annex F). Table 2 of the PPG gives the same answer, "Development should not be permitted" (paragraph 079, Reference ID 7-079-20220825), and the Environment Agency's standing advice tells anyone with a minor extension in Flood Zone 3b to check that table. A bigger house in 3b therefore starts from a presumption against it, however good the Flood Risk Assessment.
The sequential and exception tests are not the obstacle. Policy F5.2 of the NPPF exempts "Householder development" from the sequential test, and Policy F6.2 says the exception test does not need to be applied where the sequential test is not required (NPPF Chapter 18). The exemption removes the tests; it does not make a house compatible with the functional floodplain. AFC's Staines FRA addressed both parts of the exception test anyway, and the case rested on local policy.
What makes an extension possible is local policy for existing buildings in the functional floodplain. Spelthorne's is Policy E3 of the Spelthorne Local Plan 2024 to 2039/40, adopted on 17 March 2026. The officer's report on the Staines application sets out the six criteria under which residential uses within Flood Zone 3b "will only be considered":
"the footprint of the building should not be increased unless level for level floodplain compensatory storage can be provided";
"finished floor levels shall not be lower than the existing and where possible they should be raised";
"surface water runoff rates and volumes from the site should be reduced";
"where possible, floodplain storage capacity should be increased and space created for flooding to occur by restoring functional floodplain";
"flood resistance and resilience measures be incorporated";
"inclusion of measures to ensure development remains safe for users in time of flood".
Policy E3(7) adds that "schemes which deliver a betterment will be supported". Spelthorne's Flooding SPD (2012), at Appendix E, puts the same test in two parts: no net loss of flood storage capacity, and no net increase in impedance to the flow of flood water. If either fails, the extension is unacceptable.
Other councils word their policy differently, and some have none, in which case the national presumption applies with nothing to set against it. Find the policy in the council's Local Plan, usually under flood risk or development management, before any design work. The first criterion is the one that decides most applications: no bigger footprint unless the storage is replaced level for level.
Why a permitted development fallback did not save the first FRA
The Staines application was first supported by a Flood Risk Assessment from another consultant. It argued that the extensions could be accepted because they would be built at first-floor level above the modelled flood levels, relying on a permitted development fallback: a Certificate of Lawfulness (26/00349/CPD) for a single-storey rear extension. It also proposed raising the existing floor to 14.91 m AOD, which is the design flood level itself, with no freeboard.
The officer's report records why that failed: "As the proposed scheme comprises two storeys, the fallback position was not considered valid, and the PD argument could not be relied upon." A fallback only counts if it is a real alternative that would be built, and a certificate for a single-storey extension is not a fallback for a two-storey scheme. The Environment Agency had also not accepted the first assessment, because the ground-floor footprint increased in Flood Zone 3b.
AFC replaced that assessment in full, and the new FRA did not rely on the fallback. It worked through each criterion of Policy E3(6)(c) and quantified the storage. Use a fallback, if there is one, as background; never as the main case for the flood risk.
Why the Environment Agency rejects voids and stilts in Flood Zone 3b
The traditional answer to an extension in the floodplain is to lift it on piles or a suspended floor and leave the space beneath open, so that flood water can pass under it and fill the void. Many web pages still recommend it, and Spelthorne's own Flooding SPD, written in 2012, provides at paragraph 4.43 (as summarised in AFC's FRA) that the required net position may be achieved by incorporating floodable voids at ground level, or by compensatory storage elsewhere.
The Planning Practice Guidance now says otherwise. Where floodplain storage is lost, "on-site level-for-level compensatory storage", allowing for climate change over the lifetime of the development, should be provided. Stilts and voids may protect the building itself, but "such techniques should not normally be relied upon for compensating for any loss of floodplain storage." The PPG gives the reasons: "voids do not allow water to freely flow through them, trash screens get blocked, voids get silted up", "they have limited capacity", "and it is difficult to stop them being used for storing belongings or other materials." (Flood risk and coastal change, paragraph 049, Reference ID 7-049-20220825).
In practice a void under a floor fills with garden furniture, bins and debris carried by the flood, and owners close the sides in for warmth or appearance. A council cannot easily see or enforce any of this over the life of the building. Storage dug out of the ground and left open to the river stays where it was put.
On the Staines application, AFC's FRA set out two options: Option A, the extension on an open void above the flood level, and Option B, built solid with the front driveway lowered to make the storage up. The officer's report records the Agency's answer:
"The Environment Agency has advised that voids would not constitute appropriate mitigation in this instance, in accordance with the Planning Practice Guidance, and that level-for-level flood storage compensation is required. While the FRA initially presented two options, the EA has confirmed that the piled-void option is not acceptable, leaving Option B; compensatory excavation at the front of the site, as the suitable approach".
Figure 3. The two options in AFC's FRA for 25/01183/HOU: the void was rejected and the lowered driveway approved.
It is not only the Thames team. On Ferry Lane, Alveston, in Flood Zone 3 on the River Avon, the Environment Agency's letter states that stilts and voids "should not normally be relied upon for compensating for any loss of floodplain storage", and AFC's addendum counted every new step, platform and post as solid (Alveston case study). On Poplars, Datchet, a Flood Zone 3b site on the Thames, the Agency's September 2026 objection asked for level-for-level floodplain compensation in height slices (Datchet case study). A void can still help the building itself, by keeping water off the floor. It does not count as compensation.
What got approved instead: lowering the front driveway (worked example)
The application. Spelthorne householder application 25/01183/HOU, a detached riverside house on Thames Side, Staines-upon-Thames, on the north bank of the non-tidal Thames at the Penton Hook reach. The proposal adds a habitable first floor under a raised roof, a part two-storey, part single-storey rear extension with a balcony, and a front porch on piers. The description of development ends "Involving excavation of front driveway for flood compensation". The existing 123.6 m² footprint is retained and the ground floor grows by a net 21.5 m² on the river side, in two parts of 10.5 m² and 11.0 m². The existing conservatory is demolished. AFC's FRA (project 120, received by the council on 12 October 2026) is on the public register.
The levels. Flood levels come from the Environment Agency's Thames (Datchet to Teddington) 2023 model, read at the extension:
| Event | Level (m AOD) | |---|---| | 1 in 30 (3.3% AEP), present day | 14.43 | | 1 in 100 (1% AEP), present day | 14.66 | | 1 in 100 plus climate change: the design flood level | 14.91 | | 1 in 1000 (0.1% AEP) | 14.94 |
The measured topographic survey (CD Surveys, drawing 2609021, September 2026) gives ground at the extension of about 14.00 m AOD and the existing damp-proof course at 14.496 m AOD. In the design event the existing house takes about 0.4 m of water.
The storage the extension takes. Built solid, the new footprint occupies the space between the ground and the design flood level: 21.5 m² × 0.91 m = 19.6 m³. That is the volume to replace. The method for slicing and counting is set out in our guide to flood compensation for extensions, and the floodplain compensation calculator runs the band check for any plot.
Why strict level for level could not be met. Level-for-level compensation replaces storage in each height slice between the ground and the design flood level, so that the floodplain fills and empties as it did before. Storage in a slice can only be recreated where there is ground in that slice to dig away. At Staines the surveyed ground across the whole plot lies between about 13.6 and 14.5 m AOD, all of it below 14.91 m AOD. No ground stands high enough to be lowered to recreate storage in the top slices, roughly 14.5 to 14.91 m AOD. The FRA therefore used a volume-for-volume balance for the excavation and set out that limit plainly for the Agency. The lowered area sits as close to the extension as the plot allows and fills from the same river.
The compensation. The granite-sett front driveway, at 14.2 to 14.5 m AOD, is removed and the ground lowered to a formation level of 14.06 m AOD over about 137 m². The lowered area stays clear of the house and of a 0.5 m set-back from its foundations. The mean depth is 162 mm and the maximum 365 mm. That creates 22.1 m³ of storage below the design flood level against 19.6 m³ displaced, a net gain of 2.6 m³ in the FRA's table. Demolishing the conservatory, 21.2 m² of solid structure standing 0.84 m into the flood (17.8 m³), adds to that.
Figure 4. The storage balance below 14.91 m AOD, from the FRA's table.
Figure 5. Option B in section: the driveway lowered to 14.06 m AOD (blue, 22.1 m³) against the volume the solid extension takes below the design flood level (red, 19.6 m³). AFC FRA, Figure 23.
Figure 6. Where the compensation goes: across the front of the plot, clear of the house and a 0.5 m foundation set-back, levels to the CD Surveys survey (AFC FRA, Figure 24).
Draining the lowered area. In August 2026 the council accepted the approach in principle, subject to detail on how flood water would drain from the excavated area. A dug-out area that holds water after the river falls is a pond, not floodplain. AFC graded the formation to fall towards the river, so that it fills and empties under gravity with no pipe or culvert. Sections through the lowered area went to the council with it. The officer's report accepted that runoff would "naturally drain towards the river and leave the site".
Figure 7. Sections through the lowered driveway: the formation opens downslope towards the river, with no enclosed low point (AFC FRA, Figure 25).
The water table. Groundwater was recorded at about 1.5 m below ground in the nearest British Geological Survey borehole (TQ07SW522, about 298 m away). The deepest cut is 365 mm, so the lowered area stays above the water table and its storage is not already full of groundwater when the river rises.
The survey. The planning office then asked for a measured topographic survey, commissioned in September 2026. The FRA was re-checked and reissued against it, and the survey drawing is one of the approved plans in condition 3.
The council's figures. The officer's report summarises the final FRA in its own words: the extension "would displace approximately 18 cubic m of floodplain storage", and the excavation "would measure 137 sqm with an average depth of 0.16 m, creating approximately 20.8 cubic m", a net gain "of around 2.7 m³". On that basis it found "no net loss of floodplain storage and no increase in impedance to flood flows, instead delivering a modest net betterment". The calculation figures in this guide are the FRA's.
The outcome. Spelthorne Borough Council granted planning permission on 3 November 2026, subject to conditions, with the compensation secured by conditions 5 to 9 below.
Floor levels, resilience and safety in Flood Zone 3b
The first scheme set the floor at 14.91 m AOD, level with the design flood. AFC's FRA raises the extension's finished floor level to 15.21 m AOD, 300 mm above the design flood level and above the 1 in 1000-year level of 14.94 m AOD. Criterion (ii) of Policy E3 asks that floors are not lower than existing and, where possible, raised. The Environment Agency's standing advice recommends floor levels "at least 600 millimetres (mm)" above the estimated flood level, reduced to 300 mm "if there is a high level of certainty about your estimated flood level"; the FRA used 300 mm on the modelled level and took the resistance measures to 600 mm. Sleeping accommodation is at first floor. Our guide to finished floor levels in Flood Zones 2 and 3 covers how the freeboard is chosen.
Illustrative AI visualisation generated from the planning drawings (25/01183/HOU); not a design drawing. The side door is reached by steps because the floor is raised above the design flood level.
Below the floor, the FRA sets flood resistance and resilience to at least 600 mm above the design flood level: low-permeability walls, renders and mortars; flood-resistant doors and sealed openings; automatic airbrick covers; non-return valves on all drainage; resilient plaster, boards and insulation; and electrics raised. The occupier registers with the Environment Agency's Floodline service for the Thames warning area and, on a Flood Warning, walks out about 645 m (about 10 minutes) to land outside Flood Zones 2 and 3. The first floor is a temporary refuge only, if an extreme event arrives before evacuation is complete. Our guide to what a council wants in a flood emergency plan covers that document.
Illustrative AI image; not a real site or an AFC project. Demountable door barriers and airbrick covers are typical of the resistance measures an FRA in Flood Zone 3b specifies.
After consent: the conditions that catch people out
The permission came with five conditions that bear on the flood scheme. Condition 9 is the one that changes the build programme. It reads:
"The development hereby permitted shall not commence until the excavation and floodplain compensation works identified within the approved Flood Risk Assessment have been completed in full. Within 14 days of the completion of those works, written notification shall be provided to the Local Planning Authority (LPA) confirming their completion, so that a site visit can be carried out to review this work. Written confirmation from the LPA that these works are satisfactory must be received prior to the development commencing."
The other four:
Condition 5: "There shall be no raising of existing ground levels on the site within the area liable to flood, other than in accordance with the approved details."
Condition 6: "All spoil and building materials stored on site before and during construction shall be removed from the area of land liable to flood upon completion."
Condition 7: the development is to be carried out in full accordance with the approved Construction Traffic Management Plan, which sets out how the excavated material is removed. The officer's report records that all excavated material, including hardstanding and topsoil, leaves the site "with no storage or stockpiling within the floodplain".
Condition 8: the development is to follow the approved FRA, "including all flood mitigation measures, compensatory flood storage provision, site level alterations, and excavation works detailed therein", retained and maintained "for the lifetime of the development".
Figure 8. The two-stage build under condition 9 of permission 25/01183/HOU.
In practice this means two phases. The groundworks contractor removes the driveway, lowers it to the approved levels, takes the spoil off site and lays the permeable pea shingle finish. The owner notifies the council and waits for the visit and the letter. Only then does the builder start on the extension. Build the council's response time into the programme, and have the finished levels surveyed so the notification can show the dig matches the approved plan. Spoil left on the drive is a breach of condition 6 and takes back the storage the scheme was approved on.
What the council asked for before it would decide
Beyond the FRA itself, the planning office asked for three things before deciding: how the lowered area would drain and what the regraded driveway would look like (August 2026), a measured topographic survey (commissioned September 2026), and a plan for taking the excavated material off site. Each is easier to supply at submission than mid-application. The application was validated on 1 October 2025 and decided on 3 November 2026, after an agreed extension of time.
The traps
Relying on the Flood Map for Planning. It does not show Flood Zone 3b. A plot shown as Flood Zone 3 may be 3b in the SFRA, and the policy that applies is different.
Offering a void as the compensation. The PPG and the Agency's advice on the Staines and Alveston applications point the same way. A void may help the building; it is not compensation. Count the extension as solid over its full footprint.
Leaning on an old SPD. Supplementary guidance written before the current PPG may still mention voids. The Agency's advice follows the PPG.
A fallback that does not match the scheme. A certificate for a single-storey extension does not support a two-storey one.
No freeboard. A floor set at the design flood level, as in the first Staines scheme, leaves no margin for error in the model.
Ground that is all below the design flood level. Strict level-for-level compensation needs ground in the upper slices to dig away. Check the survey early; if none exists, say so and agree the volume-for-volume basis with the Agency.
Digging into the water table. A deep excavation can sit full of groundwater. Check the nearest borehole or a trial pit before setting the formation level.
Forgetting the spoil and the sequence. Spoil kept on the plot cancels the storage the scheme relies on, and starting the extension before the council's sign-off breaches a condition like condition 9.
What to submit for an extension in Flood Zone 3b
An extract of the council's SFRA Flood Zone 3b map with the plot marked.
The design flood level in m AOD, from the Environment Agency's model data for the site (Product 4), with the climate change allowance stated.
A measured topographic survey to Ordnance Datum, covering the house and the area to be lowered.
The storage balance below the design flood level: volume displaced by the extension built solid, volume created by lowering ground, and any credit for structures demolished, by height slice where the ground allows it.
A compensation plan and existing and proposed sections, showing that the lowered area fills and drains by gravity to the same river.
The finished floor level in m AOD with its freeboard, on a section.
A schedule of flood resistance and resilience measures.
A flood warning and evacuation plan.
A plan for removing all spoil from the floodplain, and the construction traffic arrangements.
The council's policy criteria for Flood Zone 3b, each answered in the FRA. A permitted development fallback, if any, as background only.
To check whether the development is compatible with its flood zone at all, the flood risk vulnerability classification checker gives the class and the zone rule. For an extension outside Flood Zone 3b, the Environment Agency's standing advice for extensions may be all the application needs. If the Agency has already objected, our guide to an Environment Agency objection on flood risk sets out the response.
Can I build an extension in Flood Zone 3b?
Sometimes. National policy treats a house as incompatible with the functional floodplain, so an extension depends on the council's local policy for existing buildings in Flood Zone 3b. In Spelthorne, Policy E3 allows it if the footprint increase is compensated level for level, floors are not lowered and, where possible, raised, runoff is reduced, resistance and resilience measures are included and the house stays safe in a flood. The Staines extension AFC worked on was granted on that basis on 3 November 2026.
How do I know if my house is in Flood Zone 3b or 3a?
The Environment Agency's Flood Map for Planning shows only Flood Zone 3. Flood Zone 3b is drawn by the council in its Strategic Flood Risk Assessment, often at the 1 in 30-year (3.3% AEP) flood extent. Check the SFRA map, then compare the river model's 1 in 30-year level with surveyed ground levels on the plot.
Will the Environment Agency accept a void or stilts under my extension instead of compensation?
Not as compensation. The Planning Practice Guidance says voids and stilts should not normally be relied on to compensate for lost floodplain storage, and on the Staines application the Agency confirmed that the piled-void option was not acceptable. A void can still keep water off the floor; the storage has to be replaced in the ground. See our guide to flood compensation for extensions.
What is level-for-level compensation, and what if my garden is too low to do it?
Level for level means replacing lost storage in each height slice between the ground and the design flood level, by lowering ground that stands in that slice. If all the ground on the plot is below the design flood level, as at Staines, the top slices cannot be recreated. The FRA then has to show that, and offer a volume-for-volume balance as close as possible, for the Agency's agreement.
Can I rely on permitted development to get an extension through in the functional floodplain?
Only if the fallback matches what is proposed. At Staines the earlier FRA relied on a certificate for a single-storey rear extension, and the council found that "the PD argument could not be relied upon" for a two-storey scheme.
Do I need a topographic survey for an extension in Flood Zone 3b?
In practice, yes. The storage calculation depends on ground levels to a few centimetres, and at Staines the planning office asked for a measured survey before deciding.
What happens after planning permission? Can I start the extension straight away?
Not if a condition like Spelthorne's condition 9 applies. The compensation has to be dug first, the council notified within 14 days, the work checked on site, and the council's written confirmation received before the extension starts.
Does a householder extension in Flood Zone 3b need the sequential test or the exception test?
No. Policy F5.2 of the NPPF (2026) exempts householder development from the sequential test, and Policy F6.2 says the exception test is not needed where the sequential test is not. The incompatibility of a house with Flood Zone 3b remains, so the case rests on the council's local policy and the compensation.
Sources
National Planning Policy Framework (2026), Chapter 18 and Annex F, gov.uk
Planning Practice Guidance: Flood risk and coastal change, gov.uk
Flood risk assessment: standing advice, Environment Agency
Spelthorne Local Plan 2024 to 2039/40, Policy E3
Spelthorne Flooding SPD (2012), Appendix E
Application 25/01183/HOU, Spelthorne Borough Council: FRA, officer's report and decision notice
Thinking of extending a house in the functional floodplain? Send us the address and the drawings through the quote form and we will tell you whether compensation can work on the plot, and what the council is likely to ask for, before you apply.
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