Compensatory flood storage explained: level for level design for planning

Compensatory flood storage is the replacement of floodplain storage lost to development, provided at the same flood levels so that floodwater is not displaced onto neighbouring land. It is a standard Environment Agency requirement wherever development raises ground levels or places buildings within the floodplain, and it most commonly arises on sites in Flood Zones 2 and 3.

Put plainly: if a development takes space away from the floodplain, that space has to be given back in a way that matches how flooding actually happens. That is the principle behind level for level compensation, and a well-prepared flood risk assessment for planning applications should identify any loss of storage and show exactly how it will be replaced.

Level for level means replacing lost storage at the same flood levels, not simply the same total volume somewhere lower down the site.

Why flood storage matters when building in the floodplain

During a flood, water spreads across low-lying land and is held temporarily within the floodplain. That natural storage moderates flood levels both upstream and downstream, which is one of the main reasons floodplains matter to developers and why planning policy protects them.

When development raises ground levels or introduces buildings into that space, some of the storage is lost. The displaced water does not disappear; it is pushed elsewhere, raising flood levels or altering flow paths on neighbouring land. Planning policy is therefore concerned with more than whether a site itself floods. It asks whether the development could make flooding worse for someone else.

The starting point for understanding a site's risk is the Flood Map for Planning, read alongside the flood zones in England. National guidance is explicit on the point: as the government's flood risk and coastal change planning guidance puts it, the cumulative impacts of development "may result in an increase in flood risk elsewhere as a result of impacts such as the loss of floodplain storage".

What does "level for level" actually mean?

Level for level means replacing lost flood storage at the same flood levels, rather than replacing the same total volume elsewhere. Flooding is progressive: as water rises, different parts of a site are occupied in turn. If development removes storage at several levels, that storage must be replaced at those same stages of a flood.

In practice this is demonstrated through a cut and fill exercise. The floodplain is divided into level bands, often at 100 mm or 200 mm increments, and the volume of fill placed in each band is matched by an equal volume of excavation, or cut, in the same band. The diagram below, from Environment Agency guidance, shows the idea.

A simple way to understand it: Archimedes' "Eureka" moment

The idea behind compensatory storage is the same one Archimedes is said to have grasped in his bath. When he stepped in, the water rose because his body displaced a volume of water equal to the space it occupied. That is the observation that produced the famous "Eureka".

Floodplain development behaves in the same way. When buildings or raised ground take up space in the floodplain, they displace water that would otherwise sit there during a flood. Level for level compensation is the answer to that displacement: if space is taken away at certain levels, it must be given back at those same levels so the floodplain keeps working as it did before.

Why total volume alone is not enough

Replacing the same total volume of storage is rarely sufficient. If the replacement is provided at lower levels than the storage that was lost, it fills too early in a flood. As water rises further, the site still has reduced capacity at exactly the levels where it matters most, and the result can be higher flood levels elsewhere.

How much storage is replaced matters less than where in the flood range it is replaced.

Direct and indirect compensatory flood storage

Flood compensation schemes fall into two categories, and the Environment Agency treats them very differently.

If a layout cannot deliver direct, level for level compensation, that is usually a signal to revisit the layout rather than to assume an indirect scheme will be accepted. Indirect schemes are the exception, not a fallback.

Can underground tanks provide compensatory flood storage?

In almost all cases, no. An underground or subsurface system, whether a buried tank, a geocellular crate system or a below-ground vault, cannot fill and drain naturally with the floodplain as water rises and falls, so it cannot demonstrate level for level performance. At best it is treated as indirect compensation, and it carries long-term maintenance and blockage risks that count further against it.

• No natural connection: a sealed tank does not flood and drain in step with the river, so it does not replace storage at the levels where it was lost.
• Rate of fill is wrong: water enters through pipes or controls, not by the floodplain rising over it, so it behaves as attenuation rather than compensation.
• Maintenance and blockage: inlets and outfalls can silt or block, and the capacity relies on someone maintaining it for the life of the development.
• Hard to evidence: proving equivalent performance across the flood range usually needs modelling the EA is unlikely to accept for a buried void.

Floodable voids beneath buildings are a different concept and should not be confused with buried tanks. A building raised on stilts or over an open undercroft lets floodwater keep occupying the space beneath it, which retains floodplain storage rather than replacing it. We cover this in our guide to floodable voids, stilts and undercrofts.

Is a flood storage area the same as compensatory storage?

Not quite, and the two are often confused. A flood storage area is any land, natural or engineered, that holds floodwater. Compensatory storage is the specific, like-for-like replacement of storage that a development removes, provided at the same levels. A balancing pond or attenuation basin built for surface water drainage is different again: it manages rainfall run-off from the site, not the loss of fluvial floodplain storage, so it does not automatically count as compensation.

Attenuation manages your rainfall run-off; compensatory storage gives back the floodplain volume your development displaces. A scheme usually needs both, and one cannot stand in for the other.

When is compensatory flood storage likely to be needed?

Compensatory storage is typically required wherever development reduces existing floodplain storage, most often within Flood Zone 3 and increasingly within the climate change extents of Flood Zone 2. Whether it is needed on a given site depends on the layout, the proposed levels and how the scheme interacts with flood risk, which is why the assessment has to be site-specific.

• Raised ground levels: importing fill to lift a platform above flood level removes storage that must be replaced.
• Buildings in the floodplain: footprints placed within the flood extent displace water directly.
• Flood Zone 3 and functional floodplain: the highest-probability land, where loss of storage is scrutinised most closely.
• Climate change extents: land that is dry today but within the modelled future floodplain can still trigger the requirement.

The quality of that judgement is often decisive, which is one of the reasons flood risk assessment expertise matters.

How compensatory flood storage is worked out

The calculation compares existing and proposed ground levels against the design flood level. For most sites in England that is the 1 in 100 year (1% annual probability) fluvial flood level, plus an allowance for climate change over the development's lifetime. The Environment Agency's climate change allowances set those uplifts, and because residential development is assumed to last at least 100 years, housing schemes are held to the longer epochs, and we set out how to apply them in our guide to climate change allowances for flood risk assessments.

That comparison shows where storage is lost across the flood range. Replacement storage is then designed so equal space is available at the same levels, and the result is presented in a cut and fill table. The principle is set directly in national planning guidance:

The calculation relies on accurate inputs: a site topographical survey and Environment Agency flood data products. For more complex sites, flood modelling to support compensatory storage design is often needed to demonstrate that flood behaviour is not made worse.

1. Establish the design flood level (usually 1 in 100 year plus the relevant climate change allowance).
2. Compare existing and proposed ground levels to find where storage is lost, band by band.
3. Design the compensation area so equal volume is returned in each band.
4. Present the result as a cut and fill table proving the balance level for level.

Why hydraulic connectivity matters

Replacement storage only works if it behaves as part of the floodplain. Floodwater must be able to flow into and out of the compensation area naturally during a flood. If the area is disconnected from the flood mechanism, it provides no effective compensation, however large it is.

A storage volume that floodwater cannot reach is not compensation; hydraulic connectivity matters as much as the volume itself.

What the Environment Agency will usually look for

As a statutory consultee, the Environment Agency reviews the flood risk evidence on development in flood risk areas, and it responded to over 9,600 planning consultations in 2024–25. On compensatory storage, it will assess whether the scheme has been properly identified and designed, typically checking:

• The amount of storage lost: how much floodplain volume the development removes.
• The levels at which it is lost: where in the flood range the loss occurs.
• How it is replaced: that compensation is provided band for band, not as a lump.
• Hydraulic connectivity: that the replacement storage is connected to the flood mechanism.
• No detriment elsewhere: that flood risk is unchanged for third parties.

Understanding the Environment Agency's role in flood risk planning helps ensure a proposal lands with the evidence the caseworker expects.

What to submit with a planning application

Where level for level compensation is proposed, the supporting flood risk assessment will normally need to include a clear, evidenced package:

• Topographical survey: accurate existing ground levels across the site.
• Design flood level: usually the 1 in 100 year plus climate change level.
• Displacement plan: where the development displaces floodwater.
• Compensation plan: the location and levels of the flood compensation area.
• Cut and fill table: demonstrating that compensation is achieved level for level.

Common mistakes that cause problems

Most difficulties arise where compensatory storage is misunderstood or left too late in the design. The recurring problems are avoidable:

• Chasing total volume: matching the overall volume but not the levels it was lost at.
• Incomplete level data: designing on survey data that is too coarse or out of date.
• Assuming excavation counts: treating any dug-out space as storage without checking the levels.
• Proposing buried tanks: offering an underground system in place of open, connected storage.
• Ignoring connectivity: failing to show how the compensation area links to the floodplain.

Why this should be considered early in site design

This is a layout question, not a reporting afterthought. Compensatory storage can shape levels, access and the developable area of a site. Addressing it early lets the design respond properly, rather than forcing changes, or a fresh objection, late in the process.

Handled early, compensatory storage is a clear technical step within the planning process; overlooked, it is a common cause of delay, redesign and Environment Agency objection.

Frequently asked questions

Compensatory flood storage is a technical requirement, but the principle is straightforward: if development removes flood storage, it must be replaced in a way that works at the same levels and within the same flood system. Unda's chartered flood risk consultants can advise on compensatory storage as part of a flood risk assessment for planning, and can flag it early enough to protect your developable area.