Floodplain Compensation Calculator: Level-for-Level Volume Check by Height Band

This floodplain compensation calculator shows how much floodplain storage an extension, patio, decking or set of steps takes below the design flood level, and whether the ground you plan to lower gives it back level for level. It slices the loss and the gain into height bands, 200 mm by default, and checks each band on its own, because that is how the Environment Agency (EA) reads a compensation table. A scheme whose total gain beats its total loss can still fail in the top band, and the calculator shows where.

Need the calculation done for the EA? Send your drawings through our fee proposal form and we will set out what the compensation scheme needs. The method is in our guide to flood compensation for extensions, with a granted Harrow extension, where level-for-level failed and volume-for-volume was accepted, as its worked example. Volume-for-volume is the fallback when the ground levels on a site make level-for-level impossible.

How to use the floodplain compensation calculator

Design flood level. Enter the 1% annual exceedance probability (AEP) flood level for the site plus the climate change allowance, in metres above Ordnance Datum (mAOD). It comes from the EA's modelled flood levels (a Product 4 data request) or from the site's flood risk assessment. The allowance depends on the management catchment and the epoch, and is set out in the EA's flood risk assessments: climate change allowances guidance; our page on climate change allowances in a flood risk assessment explains which one applies.

Band depth. 200 mm is the default. EA West Midlands asked for the gain and loss in 200 to 250 mm bands (as quoted) on the 2026 job in the worked example below, and slices of 100 to 300 mm are usual. Check the EA's letter for your own site and use the bands it asks for.

Loss items. Add one row for each new structure that sits below the design flood level: the extension, a raised patio or terrace, decking, planters, steps and landings, or ground raised for any reason. Enter its plan area, the existing ground level under it and, if it stops below the design flood level, the level of its top or finished surface. Each item is counted solid over its full footprint, from existing ground up to the top of the item or the design flood level, whichever is lower. Leave the top blank and the item is counted up to the design flood level. Decking and steps are counted solid below their top, with no credit for the space under them.

Lowered areas. Add one row for each area of ground to be dug down, with its plan area, existing level and proposed level. Only the part below the design flood level counts.

Removed structures. An existing building, enclosed terrace or shed that is demolished and not replaced frees the volume it occupied below the design flood level. Enter its footprint, the ground level and its top level.

Levels should come from a topographic survey. Levels read from LiDAR or from an architect's spot heights are a starting point for a screening check, and the EA will expect surveyed levels in the submission.

How to read the result

The table lists each band from the top down, with the volume lost, the volume gained, the difference and a pass or fail. A band passes when the gain in that band equals or exceeds the loss in that band. The verdict says "Level-for-level achieved" only when every band passes. While any row has an error, the calculator shows no result until the row is corrected.

The bands start at the lowest level entered, rounded down to the nearest 0.1 m, and step up by the band depth. The top band ends at the design flood level. If the last slice would be less than half a band deep, it is merged into the band below, so a design flood level of 10.33 mAOD with 200 mm bands from 9.50 gives a top band of 10.10 to 10.33.

The top band decides most schemes. A structure that rises to the design flood level on ground below the top band loses the whole of its footprint times the band depth in that band. Only ground that is now above the bottom of the top band can replace it, and in a garden that slopes down to a river that ground is usually scarce.

Figure 1: Level-for-level: each slice lost is replaced at the same height. The lawn starts at 9.90 mAOD, so lowering it gains nothing in the top band, although the totals balance (the calculator's example).

What counts as lost floodplain storage

Every new structure below the design flood level takes storage over its full footprint: extensions, raised patios and terraces, decking, planters, steps and landings, bin and cycle stores, and any land raising. The volume is the plan area times the depth from existing ground to the top of the structure or the design flood level, whichever is lower, split by band.

Voids, stilts, open-sided decking on posts and open-tread steps earn no credit. The Planning Practice Guidance on flood risk and coastal change (paragraph 049, Reference ID 7-049-20220825) is direct on the point, and the EA quoted the same words in its objection on the worked example below: "whilst the use of stilts and voids below buildings may be an appropriate approach to mitigating flood risk to the buildings themselves, such techniques should not normally be relied upon for compensating for any loss of floodplain storage." The calculator therefore has no "void" or "on posts" option.

Figure 2: The full footprint of decking, steps and planters counts, not just the posts.

What counts as gain

The same paragraph of the guidance asks for "on-site level-for-level compensatory storage, accounting for the predicted impacts of climate change". Gain comes from ground lowered within the floodplain, at the heights where storage is lost, and from structures removed below the design flood level. The lowered ground has to connect freely to the floodplain, so that it fills as the flood rises and drains back as the flood falls. A hollow surrounded by a wall or a raised edge does not.

A buried tank or crate system is not level-for-level storage. It fills through its inlets rather than with the rising flood, and the water it holds is not at the heights where storage was lost. The guidance (paragraph 049) sets voids under buildings aside as compensation for similar reasons: water does not flow freely through them, trash screens get blocked, they silt up, they have limited capacity, and it is difficult to stop them being used for storing belongings or other materials. Councils publish their own notes on the method, for example Hart District Council's Technical Note 1, Level for Level Flood Compensation.

Worked example: a replacement house beside a main river

The figures below are from an AFC job in 2026: a replacement house and an annexe beside a main river in the West Midlands, in Flood Zone 3. The EA objected on floodplain storage. It asked for level-for-level, volume-for-volume compensation on site up to the design flood level (the 1% AEP level plus the climate change allowance), with calculations, and for the gain and loss in a table in 200 to 250 mm bands (as quoted).

As first drawn, the scheme had about 143 m² of new decking, planters and steps on ground below the top band. AFC's band check, with every structure counted solid, found a loss of 62.4 m³ below the design flood level, 32.8 m³ of it in the top band. The higher ground in the garden, dug out entirely, could have supplied about 20 m³ in that band, so the scheme as drawn could not balance.

The client then removed the decking and planters and kept open-tread steps and small platforms at each door and a short terrace, about 32 m², still counted solid. The loss fell to 15.5 m³. About 89 m² of ground lowered in three zones gives 22.9 m³ below the design flood level, and every band passes, with the smallest margin 30% in the band below the top (Figures 3 and 4). These are AFC's figures as at 30 October 2026, from a calculation not yet issued, and they may change before it is. The application had not been decided, and the redesign and AFC's calculation had not gone to the EA, when this page was written.

Figure 3: Loss and gain by band, as first drawn and as redesigned (AFC band check as at 30 October 2026).

Figure 4: The band table in the form the EA asks for.

The lesson for any scheme in Flood Zone 3: a smaller footprint below the design flood level does more than any amount of digging elsewhere, because the top band can only be replaced from ground that is now higher than it.

When the calculator is not enough

The calculator is a screening check. A submission to the council and the EA needs:

• a topographic survey of the site, with levels across the areas to be built on and lowered;

• the design flood level and its source, with the climate change allowance stated;

• a plan of existing and proposed ground showing every loss area and every lowered area;

• cross-sections of existing and proposed ground, with the design flood level drawn on;

• the band table, in the bands the EA's letter asks for (200 to 250 mm bands, as quoted, in the worked example), with loss, gain and the difference in each band;

• how the lowered ground connects to the floodplain and drains back after a flood.

AFC prepares the calculation, the plan and sections and the flood risk assessment that carries them. The full method is in our guide to flood compensation for extensions, with a granted Harrow extension where level-for-level failed and volume-for-volume was accepted. Whether an extension needs a flood risk assessment at all is covered on our page on flood risk assessments for extensions, and the zones on Flood Zones 1, 2 and 3 explained. For a borough where compensation comes up often, see our Windsor and Maidenhead guide.

Frequently asked questions

How do I calculate floodplain compensation for an extension?

Work out the volume the extension takes below the design flood level: its footprint times the depth from existing ground to that level, or to its top if it stops lower. Split the volume into height bands, then show ground lowered nearby that gives back at least the same volume in each band. The calculator above does the arithmetic; the method is set out in our flood compensation guide.

What is level-for-level flood compensation?

Storage lost at a given height is replaced at the same height, band by band, not just in total. Flood water rises through each level in turn. Storage removed at one height and replaced lower down is already full by the time the flood reaches that height, so the floodplain holds less at that height and the water goes elsewhere. Volume-for-volume compensation, equal in total but not band by band, is the fallback when the ground levels on a site make level-for-level impossible.

What band depth does the Environment Agency want in a compensation table?

EA West Midlands asked for 200 to 250 mm bands (as quoted) on a 2026 job; check your own EA letter or pre-application advice for the site, and use the same bands for loss and gain.

Does decking on stilts or posts count as lost floodplain storage?

Yes. The full footprint counts from ground to the top of the decking or the design flood level, whichever is lower. The Planning Practice Guidance says stilts and voids should not normally be relied upon for compensating for any loss of floodplain storage.

Can I use a void under the floor instead of digging out the garden?

Not as compensation. A void can help protect the building itself, but it does not replace lost storage, so the ground lowering is still needed.

Can floodplain compensation go in an underground tank?

Not normally. The Planning Practice Guidance (paragraph 049, Reference ID 7-049-20220825) says voids should not normally be relied upon for compensation, because water does not flow freely through them, trash screens get blocked, they silt up and they have limited capacity. A buried tank shares those problems: it fills through its inlets, not with the rising flood, and does not hold water at the heights where storage was lost. Compensation is normally provided by lowering ground that connects freely to the floodplain.

What design flood level should I use?

The 1% annual exceedance probability (1 in 100-year) flood level for the site plus the climate change allowance for its management catchment and epoch, in mAOD. The level comes from the EA's modelled data (a Product 4 request) or the site's flood risk assessment, and the allowances are in the EA's climate change allowances guidance.

Do I need a topographic survey for a compensation calculation?

For the submission, yes. The EA checks the band table against surveyed levels, plans and sections. LiDAR or spot heights are enough for a first screening check.

Ask us about your compensation scheme

AFC prepares floodplain compensation calculations, plans and sections, and the flood risk assessments that carry them, for extensions, replacement houses and new buildings. Send your drawings and any EA letter through the fee proposal form and we will tell you what the scheme needs. More on the service is on our flood risk assessment page.