Flood Compensation for Extensions: Level-for-Level and Volume-for-Volume
Flood compensation for extensions has quietly become the issue that decides householder applications. A rear extension that would have been waved through five years ago is now being refused because nobody calculated how much floodwater the new footprint displaces, or showed where that volume goes instead.
It is no longer confined to Flood Zone 3. We are being asked for compensatory flood storage on Flood Zone 2 sites and, increasingly, on Flood Zone 1 sites with a high surface water flood risk — a combination that catches applicants completely off guard. This page explains what compensatory flood storage is, the difference between level-for-level and volume-for-volume, why voids and stilts are being turned down, and what a council actually wants to see. It includes a worked scheme of ours that was granted permission in August 2026 on exactly that Flood Zone 1 surface water basis.
What compensatory flood storage actually is
When a flood happens, water occupies a volume of space. Put a solid object into that space — an extension, a raised patio, imported fill — and the water that used to sit there has to go somewhere else. It goes sideways and upwards, onto your neighbours' gardens, into their homes, and across the access roads people need to leave by.
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Compensatory storage is the correction. You put back, below the design flood level, at least as much storage as the development takes out, so the floodplain holds the same volume it held before. The shorthand planners use is no net loss of flood storage.
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For fluvial sites the design flood level is normally the 1% annual exceedance probability (1% AEP) event — the 1 in 100-year flood — plus an allowance for climate change over the lifetime of the development. Planning Practice Guidance takes that lifetime as 100 years for residential development and 75 years for non-residential. For surface water sites the design level is derived differently, and we come to that below.
Why this now catches minor householder extensions
The exemption most applicants rely on does not do what they think it does.
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Under the August 2026 National Planning Policy Framework, householder development is exempt from the sequential test at policy F5(2)(c)(i), and small non-residential extensions with a footprint of less than 250 m² are exempt at F5(2)(c)(ii). Because F6(2)(a) disapplies the exception test wherever the sequential test is not required, those same proposals escape the exception test as well.
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None of that touches policy F7. F7 has no householder exemption, and it is worded as a refusal. F7(2) provides that where development is proposed in a location known to be at risk from any form of flooding, now or in the future, "it should be refused unless" five limbs are satisfied. The one that bites here is F7(2)(e): "It can be demonstrated that flood risk will not be increased elsewhere."
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An extension in a flood extent displaces water. Displaced water increases flood risk elsewhere. Unless you show where the volume goes, F7(2)(e) is not satisfied and the default under F7(2) is refusal.
Flood Zone 1 is not a defence
F7(2) says "any form of flooding", not river or sea flooding. Annex F, Table 3 — the vulnerability and flood zone matrix — deals only with risk from rivers and the sea, but F7 is not limited that way. Planning Practice Guidance is drawn equally widely: where flood storage from any source of flooding is to be lost as a result of development, on-site level-for-level compensatory storage, accounting for the predicted impacts of climate change over the lifetime of the development, should be provided.
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That is how a site sitting comfortably in Flood Zone 1 on the Environment Agency Flood Map for Planning ends up being asked for compensation. The flood zones describe rivers and the sea. Surface water is mapped separately, and many boroughs now classify it separately too. A site can be Flood Zone 1 for fluvial purposes and Surface Water Flood Zone 3a in the borough's strategic flood risk assessment at the same time.
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A site-specific flood risk assessment is still required regardless. F4(1) requires one for all proposals in Flood Zones 2, 3a and 3b with no householder carve-out, and F4(2)(c) pulls in Flood Zone 1 land identified in a strategic flood risk assessment or on the Flood Map for Planning as at risk of flooding from any source, now or in the future.
One drafting note if you are reading consultee letters.
Environment Agency and council correspondence still routinely cites "paragraph 181 of the NPPF", which was the December 2024 wording — development "should only be allowed" where certain things could be demonstrated. The current equivalent is F7(2), which says it "should be refused unless". The substance is similar; the starting position is not. Our guide to flood risk assessments under the NPPF 2026 covers the wider changes.
Level-for-level: the slice rule, and the trap
Level-for-level is the preferred method, and the Environment Agency says so in terms. Local plan policy often words it directly — Spelthorne's Policy E3, for example, provides at 6(c)(i) that within the functional floodplain "the footprint of the building should not be increased unless level for level floodplain compensatory storage can be provided".
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The method is not simply "dig out as much as you build". The flooded depth is cut into horizontal slices, typically 100 mm to 300 mm bands, and the storage you gain by excavation must equal or exceed the storage you lose to the building in each individual slice. The reason is timing. Floodwater arrives at a level and rises through it. If you take storage out at 14.6 mAOD and put it back at 14.1 mAOD, the floodplain behaves differently at 14.6 mAOD than it did before, and water goes to a neighbour at exactly the level where you failed to compensate.
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This is where the most common mistake happens. Adding surplus volume from a low slice to cover a shortfall in a high slice, then quoting the total as balanced, does not work. A balanced total with a failed upper slice still displaces water at that upper level. Any compensation case reporting only a single total volume, with no slice-by-slice table, should expect to be challenged.
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The controlling constraint is physical: you can only recreate storage where ground exists at that level to be lowered. On a low-lying garden that already sits below the design flood level, there is no ground in the upper bands to dig out.
We modelled exactly this on a Thames-side garden in Surrey. The design flood level was 14.91 mAOD and a proposed rear extension of 21.53 m² displaced 18.09 m³.
Sliced into 200 mm bands, the excavated volume all sat in the lowest one or two bands, because the garden ran only about 14.0 m to 14.4 mAOD — the whole plot was already below the design flood level. The extension needed compensating volume in every band up to 14.91 mAOD. The totals could be made to balance; the upper slices could not.
That is a genuine finding rather than a design failure, and it is the evidence that justifies moving to volume-for-volume with the authority's agreement.
Volume-for-volume: the proportionate fallback
Where strict level-for-level is precluded by site topography and the development is minor, volume-for-volume is the accepted fallback: provide a total replacement volume below the design flood level at least equal to the volume displaced. It meets the underlying policy aim of no net loss and no increase in risk elsewhere.
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The argument is won by explaining plainly why level-for-level was not achievable, with the slice analysis behind it. Authorities are alert to volume-for-volume being reached for as a shortcut, so the honesty of that reasoning is what carries it.
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Local practice is converging on a fairly specific design. Harrow, for example, has directed compensatory flood storage "on volume for volume basis by gradually lowering ground levels between 150mm and 300mm, with flood water flowing freely out of the lowered area when flooding recedes."
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Three things are being asked for there, and all three matter:
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Gradual, shallow lowering. A broad, shallow re-grade, typically 150 mm to 300 mm, feathered at the edges — not a pit.
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Free drainage. The lowered area must fall towards lower ground so water drains back out as the flood recedes. An enclosed hollow that traps water is not compensation; it is a pond, and it will be refused.
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Buildability and permanence. The formation has to tie into existing levels in a way a contractor can build and a homeowner will not fill back in.
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Case study: Flood Zone 1, high surface water risk, permission granted
196 Exeter Road, South Harrow — application PL/0951/26, granted 19 August 2026.
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A householder scheme for a front porch and a single-storey side-to-rear extension. The site sits in Flood Zone 1 on the Environment Agency Flood Map for Planning, and would once have been treated as unremarkable. It is also confirmed within Surface Water Flood Zone 3a in the West London Level 2 Strategic Flood Risk Assessment (October 2024), with surface water risk assessed as high. That combination is what put compensatory storage on the table.
Flood sources and probability
The assessment identifies every realistic source of flooding for your site — rivers, the sea, surface water, groundwater, sewers and any nearby reservoirs or watercourses — and sets out the likelihood of each, both now and over the lifetime of the development with an allowance for climate change.
How the volume was calculated
Harrow's SFRA general requirement 4.3 asks that where development decreases the volume of a surface water flood area, compensatory storage equal to or exceeding the volume lost is provided, so there is no net loss, accounting for the 3.3% and 1% AEP surface water depths. For minor development the volume is taken as a constant depth — the upper limit of the relevant depth band — over the area of the structure within the flood extent.
The Council confirmed a design depth of 0.20 m. The extension measured 10.6 m × 1.7 m, giving 19.1 m², so:
19.1 m² × 0.20 m = 3.82 m³ of displaced surface water flood storage.
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Ground under the proposed extension was 44.29 mAOD, giving a design surface water flood level of 44.49 mAOD (ground plus 0.20 m). As a cross-check we integrated flood depth over the actual ground surface across the footprint, which gave 3.5 m³. The higher constant-depth figure of 3.82 m³ was retained as the conservative design value — a small point that removes an easy line of challenge.
The compensation design
Level-for-level was tested first and failed on the upper slices, because the rear garden falls away to the south-east from about 44.4 mAOD behind the extension to 44.2 mAOD at the rear boundary. That failure was reported, and it became the topographic justification for the volume-for-volume basis the Council had directed.
The compensation is 33 m² of the rear garden immediately adjacent to and contiguous with the extension — hydraulically linked to the point of loss, on the same flood frontage rather than in some unconnected corner of the plot. Ground is regraded to a formation level of 44.09 mAOD, a lowering of up to approximately 260 mm, inside the Council's 150–300 mm envelope, feathered at the edges and graded to fall with the natural slope so flood water drains freely with no permanent ponding. The area is reinstated as lawn, and excavated material is removed from site rather than re-spread within the flood extent.
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Storage balance
Volume
Displaced by extension (19.1 m² × 0.20 m, worst case)−3.82 m³
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Compensatory excavation (33 m² to 44.09 mAOD, mean 116 mm / max 258 mm)+3.83 m³
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Net+0.01 m³ — no net loss
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Importantly, no voids were relied upon. The Council had directed on 6 August 2026 that voids, stilts and undercroft parking should be avoided, and the revised assessment removed them entirely.
No topographic survey was required
The ground model was built from the Environment Agency LiDAR 1 m composite DTM, with the site boundary from the Flood Map for Planning and building footprints digitised from the architect's drawings. The assessment stated the vertical tolerance openly, at the order of ±0.15 m, and confirmed the excavation extent would be adjusted to maintain no net loss once levels were confirmed.
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Harrow accepted that. No measured topographic survey was needed, which on a householder scheme is a meaningful saving in both fee and programme. Being straightforward about the tolerance, and about how the design responds to it, is what makes a LiDAR-based ground model acceptable rather than a weakness to be picked at.
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Drainage betterment alongside the compensation
The scheme also demonstrated a reduction in surface water runoff. Modelled against the existing situation, and including a 40% climate change allowance, peak discharge at the 1 in 100-year event fell from 5.30 l/s existing to 3.16 l/s proposed — roughly 40% betterment — with negligible flooding from the drainage system itself. Greenfield rates for the 0.04 ha site were estimated by the IH124 method, but at 1.13 l/s for the 1 in 100-year event they sit well below the practicable limit of conventional flow control, so the operative test became no increase on the existing developed rate, with betterment where practicable.
The outcome
Permission was granted on 19 August 2026. Condition 2 lists the Flood Risk Assessment as an approved document, which is the point made in the next section: the compensation was approved as part of the permission, not deferred to a condition. No flood or drainage condition was attached to the decision at all, only advisory informatives on SuDS and drainage connections.
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The timeline is worth noting. The Council confirmed the earlier submission was unsatisfactory on 6 August 2026. The revised assessment was issued on 13 August 2026. Permission followed on 19 August 2026 — six days later.
Why voids, stilts and undercrofts are being refused
There is a respectable engineering argument that an open void beneath an extension displaces nothing at any level, and therefore satisfies level-for-level by construction without any excavation. On the physics, that argument is sound.
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It is also, right now, being rejected. Two positions we have seen in writing:
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"The use of voids, stilts, or under croft parking as mitigation for loss of floodplain storage should be avoided as they can become blocked over time by debris or domestic effects."
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"Whilst voids may be an acceptable method of mitigating the risk of flooding for a building, they are not considered appropriate for providing floodplain storage."
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The reasoning is durability rather than hydraulics. A void works on the day it is built. Ten years later it holds bikes, bins, garden waste and silt, and nobody is auditing it. Because a permission cannot realistically police the inside of a void in perpetuity, consultees are treating the storage as unreliable.
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Our advice, and it is advice rather than policy: do not build a compensation case on a void, a stilted floor or an undercroft unless you hold written pre-application agreement that it will be accepted on your site. Design for excavation, and treat a void as a supporting measure for the building's own flood resilience under F7(2)(d) rather than as flood storage.
What has to be counted, and what gets missed
Compensation is calculated against the total built footprint within the flood extent, not just the new room. The items most often left out of a first submission:
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Land raising. Imported fill, raised patios and terraces, and made-up levels all displace water.
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Resurfacing. Replacing a lawn with a raised paved area changes ground level as well as permeability.
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Previously raised land. Where levels were raised without consent, or under an earlier permission, an authority may require the land to be returned to its previous level or the raising counted in the calculation. We have seen exactly that point taken in writing.
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Outbuildings and ancillary structures. Garden rooms, stores, plant enclosures and containers are built footprint.
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The whole polygon, not a point. Displacement is an area calculation across the full footprint against the terrain, not a single spot level.
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Structures also cannot simply sit low in the flood extent. Where ancillary buildings were proposed with levels beneath the design flood level, the Agency's position was that this "is not appropriate", that the structures would obstruct flood flows and reduce storage capacity, and that the underside of those structures must be set to the design flood level.
Finished floor levels and the drawing councils want
Two separate requirements travel together, and applicants regularly conflate them.
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Compensation deals with the volume the building takes out of the flood extent. Finished floor level deals with keeping water out of the building. The Environment Agency's standard ask on fluvial sites is that finished floor levels are set above the 1% AEP plus climate change flood level, to reduce the risk of flooding to people and property. Meeting one does not discharge the other.
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Both belong on the same drawing. A council has asked us, in terms, to "mark the Finished Floor Level (FFL) on the CFS cross-section drawing" before it would move an application forward. A compensation cross-section showing existing ground, proposed formation level, the design flood level and the FFL, all on one section with levels in mAOD, answers most of what a case officer needs in a single sheet.
The procedural trap: compensation is not a condition
This is the point that costs the most time.
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Compensatory flood storage details are generally not something an authority will condition. One council put it as directly as this: "we would not normally condition an application for the Compensatory Flood Storage (CFS) details requested." The sequence described was that the CFS drawing has to be submitted and approved first, and only then would drainage details — surface water and foul disposal, soakaway design — be conditioned.
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The Exeter Road decision bears that out. The Flood Risk Assessment is named in Condition 2 as an approved document, and the drainage matters that the officer had said could be conditioned were ultimately dealt with by informative rather than condition.
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The practical consequence is that the compensation design has to be resolved before determination, not deferred to a pre-commencement condition. If you are programming a householder application on the assumption that flood compensation can be sorted out later, that assumption is wrong in most flood-risk authorities, and it is the most common reason we are asked to rescue a stalled application.
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At pre-application stage authorities are also declining to give a view in advance of the evidence. As one officer put it, the outcome "is entirely dependent on the FRA" — some assessments successfully address flood risk by providing appropriate on-site compensation and others do not, and the position can only be assessed once the assessment is in.
Net positive compensation, and why it helps
Providing more storage than the development displaces is not wasted effort. Where a scheme demolishes an existing structure in the flood extent, or lowers previously raised ground, the restored volume can be brought into the balance as betterment, and a net gain is looked on favourably in the planning process.
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Keep it as a separate line in the storage balance. A compensation option should stand on its own excavation, with any demolition shown as additional betterment on top, so options are compared on the same footing. Folding a demolition credit into the excavation margin to make the numbers work invites a challenge that the excavation alone is inadequate.
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The same logic applies to drainage. At Exeter Road the compensation achieved no net loss of storage and the drainage strategy separately delivered around 40% betterment on peak discharge, including climate change. Two independent improvements, each quantified, are a far stronger case than one marginal balance. Where a scheme can show a net gain, say so explicitly and quantify it — it engages F7(2)(e)'s wider aim, which is not merely to avoid increasing risk but, where possible, to reduce it.
What to submit
A compensation case that gets approved usually contains all of the following:
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The design flood level in mAOD with its source stated — EA modelling or a Product 4 request for fluvial sites; for surface water, the borough SFRA method and the confirmed design depth band.
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The ground level basis — LiDAR or measured survey. LiDAR is frequently acceptable for minor development provided the vertical tolerance is stated and the design responds to it.
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A displacement calculation in m³ across the whole footprint within the flood extent, including land raising, resurfacing and outbuildings.
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A level-for-level slice analysis in 100 mm to 300 mm bands showing lost and gained volume per slice, and whether each slice passes.
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Where level-for-level is not achievable, a plain statement of why, and the volume-for-volume design offered in its place.
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A storage balance showing displacement, excavation and any betterment as separate lines, with the net figure.
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A cross-section showing existing ground, proposed formation, design flood level and finished floor level in mAOD, plus an excavation plan with extents and set-backs.
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Confirmation the lowered area drains freely and creates no enclosed low point, and that excavated material leaves the flood extent.
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A note on maintenance and permanence — what stops the volume being filled back in.
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We produce this as a set: the cut-and-fill table, the storage balance, the section and excavation plan, and an interactive three-dimensional sliced-volume model showing displacement and excavation band by band. That model tends to settle the level-for-level argument quickly, because a case officer can see which bands balance and which do not.
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For the wider assessment this sits inside, see our flood risk assessment service, our explainer on flood zones 1, 2 and 3, and, where surface water drainage is also in play, our SuDS page.
Frequently asked questions
Do I need flood compensation for a small rear extension?
If any part of the extension sits within a flood extent, then in most cases yes. Being householder development exempts you from the sequential test under F5(2)(c)(i), and from the exception test under F6(2)(a), but not from F7. F7(2)(e) still requires you to demonstrate that flood risk will not be increased elsewhere, and displaced volume is exactly what that limb is about.
My site is in Flood Zone 1. Why is the council asking about compensation?
Flood Zone 1 describes risk from rivers and the sea only. F7(2) applies to any form of flooding, and Planning Practice Guidance expects compensation where flood storage from any source is lost. Our Exeter Road scheme was Flood Zone 1 on the Flood Map for Planning and Surface Water Flood Zone 3a in the borough SFRA at the same time, and compensation was required on the surface water basis.
Do I need a topographic survey?
Not always. At Exeter Road the ground model was built from the Environment Agency LiDAR 1 m composite DTM and accepted without a measured survey, on a householder scheme. The conditions for that are stating the vertical tolerance openly and showing how the excavation extent would be adjusted to maintain no net loss once levels are confirmed. On larger or more marginal schemes a measured survey is still the safer route.
Can I use a void or build on stilts instead of excavating?
On the engineering, an open void displaces nothing and satisfies level-for-level by construction. In current practice, both councils and the Environment Agency are refusing voids, stilts and undercrofts as flood storage, on the basis that they become blocked over time by debris and domestic use. Unless you hold written agreement for your site, design for excavation.
What is the difference between level-for-level and volume-for-volume?
Level-for-level requires the storage you gain to match the storage you lose within each horizontal slice of the flooded depth, so the timing of flooding is preserved. Volume-for-volume requires only that the total replacement volume below the design flood level equals or exceeds the volume displaced. Level-for-level is preferred; volume-for-volume is the proportionate fallback for minor development where topography makes level-for-level impossible.
Can flood compensation be dealt with by planning condition?
Usually not. Authorities we deal with will not condition compensatory flood storage details and expect the drawing to be submitted and approved before determination. At Exeter Road the Flood Risk Assessment was listed as an approved document in the permission itself. Plan for the compensation design to be resolved before the decision, not after it.
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Get in Touch
If your application has attracted a flood storage objection, or you want the compensation designed properly before you submit, send us the site address, the architect's drawings and any correspondence from the council or the Environment Agency. We will tell you whether level-for-level is achievable on your plot, size the excavation, and produce the section, the cut-and-fill table and the storage balance in a form your case officer can approve.
Get a quote and we will come back with a fixed fee and a programme.
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