Retrospective Planning Permission: The 2026 Guide

Retrospective planning permission is planning permission applied for after building work has already started or finished. It is made under Section 73A of the Town and Country Planning Act 1990. It is then judged in exactly the same way as any other application: the same policy, the same case officer, the same consultees. That the building already exists is not a reason to grant it.

That is the part people underestimate. It is a full planning application that happens to arrive late, and where the site sits in a flood zone, or the works changed how water drains off it, the flood risk and drainage evidence still has to be produced. Only now the levers that would normally solve the problem are set in concrete.

Planning guidance is explicit that a person who has undertaken unauthorised development has only one opportunity to obtain planning permission after the event.

This guide covers whether you can get one, what it costs, how long it takes, the four-year and ten-year enforcement rules as they now stand, and what happens if it is refused. Then it covers the part no general planning guide does: what the flood risk consultees actually require, and why an as-built case is the hardest kind of flood risk assessment for planning to make stand up.

What is retrospective planning permission?

It regularises development already carried out without the permission it needed. The statutory basis is Section 73A of the Town and Country Planning Act 1990. That section lets an authority grant permission for development carried out before the date of the application. It covers a completed building, an extension built differently from the approved drawings, an unauthorised change of use, and works begun before a pre-commencement condition was discharged. Four very different situations, one route.

The decisive point is that it is determined normally. Planning Practice Guidance puts it plainly. Although an authority may invite an application, "it cannot be assumed that permission will be granted", and the authority "should take care not to fetter its discretion" before determining it. The Planning Portal says the same. It is not a formality.

A further trap sits in the same enforcement guidance: where a Community Infrastructure Levy charge applies, retrospective permission can cost you relief granted under an earlier permission.

Can you get retrospective planning permission?

Usually, yes. But the sequence matters more than almost anything else in this article. Apply before the council issues an enforcement notice and you keep both routes open: the Section 73A application, and an appeal against any notice that follows.

Wait until after a notice has been issued and the first route can close before anyone has looked at the merits of your case, because the council may decline to determine your application at all under Section 70C of the Town and Country Planning Act 1990. There is no appeal against that.

Once an enforcement notice has been issued, your retrospective application can be declined without ever being considered on its merits.

Some authorities issue an enforcement warning notice first. It formalises the invitation to apply where unauthorised development has a reasonable prospect of being acceptable. These stay rare. District authorities in England issued ten of them nationally in the first quarter of 2026, and there is no right of appeal against one either.

Retrospective application or certificate of lawfulness?

The two answer different questions. A retrospective application asks whether the development is acceptable in planning terms. A certificate of lawful existing use or development, under Section 191, asks something narrower. It asks only whether the development is now immune from enforcement because the time limit has expired.

That has real consequences on a flood-affected site. A certificate is decided on fact and law, on the balance of probabilities, and the guidance on lawful development certificates confirms that the lawfulness of whatever is certified is then conclusively presumed, which means the merits of the development are never examined and nobody asks for a flood risk assessment at all. A retrospective application reopens everything: flood risk, drainage, amenity, design. And it is your single shot.

Where the enforcement period has run, the certificate is the safer route, because it never exposes an as-built scheme to a flood risk objection from the Environment Agency or the Lead Local Flood Authority. Where it has not, the retrospective application is the only route. The evidence has to be right first time.

How to apply for retrospective planning permission

The mechanics are the same as any application. What differs is the evidence base, because you are describing something that exists rather than proposing something that does not, and a case officer who can walk round the thing you are describing will notice any gap between the drawings and the building.

1. Establish what was actually built. Survey, levels and photographs of the as-built development, not the drawings you meant to build to. Where levels matter that means a topographical survey, not an estimate.
2. Check the constraints properly. Flood zone, surface water risk, critical drainage area, proximity to a watercourse, and any local plan policy the works engage.
3. Commission the reports those constraints trigger: a flood risk assessment, a drainage strategy, or both.
4. Write the planning statement honestly: what was built, when, and why permission was not obtained. Officers read a great many of these, and the defensive ones read as defensive.
5. Submit before a notice is issued, wherever that choice is still yours, for the Section 70C reason above.
6. Engage the consultees. On flood risk that is the Environment Agency; on surface water it is the Lead Local Flood Authority, whose objection can stall the whole application.

A missing report is the commonest reason an application never reaches an officer at all: validation checklists bite hardest on technical documents, and an invalidated application loses weeks before it starts.

How much does retrospective planning permission cost?

The same as the equivalent ordinary application. No more. The widely repeated claim that retrospective applications attract a double fee is wrong, and it is worth killing, because a myth that makes people delay is a myth that costs them the very sequencing advantage described above.

The doubling is real but lands elsewhere. Under regulation 10(3) of the 2012 fees regulations, the deemed application arising when you appeal a notice on ground (a) is charged at twice the normal fee. If it is not paid in time, that ground lapses and cannot be restarted. Current figures sit on the gov.uk planning fees guidance, indexed each April.

Applying voluntarily costs the ordinary fee. Being enforced against and then appealing costs double, on top of everything else.

The fee is rarely the significant number. The reports, the survey work and any remediation the assessment identifies are where these cases get expensive, and what drives the price of a flood risk assessment applies with more force once a site is built out.

How long does retrospective planning permission take?

Eight weeks. The statutory determination period is the same as for any application, being eight weeks for non-major development and thirteen for major development under article 34 of the Development Management Procedure Order 2015, or whatever longer period the applicant and the authority agree in writing between them.

The published figures show how often that extension is used. In the first quarter of 2026, 93 per cent of householder decisions in England were made within eight weeks or an agreed extended period. Only 38 per cent of minor applications were decided inside the statutory period itself, on the MHCLG planning statistics.

Add the lead time on the evidence and the real programme is longer. Groundwater is the sharpest example: the winter monitoring window many sites need runs roughly November to May, and missing it adds a season before the report can be written.

What is the time limit for retrospective planning permission?

There is no deadline for making one. None at all. The limits that matter are the enforcement limits, and once they expire the development becomes immune and the council can no longer act on it. Those limits changed on 25 April 2024, and the change is mis-stated more often than stated correctly.

Section 115 of the Levelling-up and Regeneration Act 2023 amended Section 171B of the Town and Country Planning Act 1990 so that England now has a single ten-year limit. It did not sweep away the four-year rule for everything, and it did not reach work already finished.

A council in England now has a full decade to act on most breaches, so waiting out the clock is a far weaker strategy than it was.

Two riders matter: concealment can defeat the limit altogether, and where the authority has already taken enforcement action it gets a further four years to act again. On a flood-affected site the transitional date decides which route is open. A building substantially completed in 2023 may already be immune under the four-year rule. The same building finished in 2025 has nine years still to run.

What flood risk and drainage evidence does a retrospective application need?

The same evidence as any other application, on the same triggers. Nothing is waived because the work is finished. If anything the scrutiny is tighter, because the consultee is no longer asking whether a drawing could be made safe but whether a real, standing, occupied thing already is.

The framework it sits within was rewritten on 17 August 2026. Flood risk policy now sits in a dedicated chapter as Policies F1 to F9, and the Framework states its policies are material considerations from the day of publication. There is no transitional carve-out for applications already in the system. The detail sits in our explainer on the August 2026 NPPF changes.

The Environment Agency's flood risk assessment guidance requires an applicant to show development is safe for its lifetime and does not increase flood risk elsewhere. A completed building meets that test or it does not, and no amount of explaining why it was built the way it was built changes the answer. Our guide on when you need a flood risk assessment sets out the full trigger list, and the 2025 national standards for sustainable drainage explain what a compliant design has to achieve.

Works that added no buildings can still trigger a strategy. Hardstanding, a filled ditch, raised levels and paving over a garden all change how water leaves a site.

Why a policy-compliant flood risk assessment is harder after the build

Because a flood risk assessment is designed to shape a development, not defend one. Done at the right time it influences where the building sits, how high the floor levels are and how surface water is managed, and each of those choices is still cheap to make. Done retrospectively, all of them are fixed, and the assessment has to work backwards from what exists.

• The baseline is gone. A drainage strategy has to show post-development runoff is no worse than the greenfield runoff rate or the pre-existing rate. Once a site is surfaced that can no longer be measured directly, and reconstructed evidence is easier for a Lead Local Flood Authority to challenge.
• Finished levels constrain the mitigation. Raising floor levels, setting the building back, or grading ground to fall away from it are the reliable fixes, and each depends on being decided before construction.
• Ground investigation is compromised. Infiltration testing for soakaways assumes undisturbed ground and access; on a completed site it is disturbed or built over.
• Below-ground features cannot be inspected. Where attenuation, pipework or a soakaway should exist, the assessment often cannot confirm what is in the ground without excavation.
• The goalposts have moved. The application is judged against current policy, not the policy in force when the work was done. That includes the widened Flood Zone 1 trigger and updated climate change allowances.

Every lever that normally solves a flood risk problem is, on a finished site, either fixed, hidden or already spent.

A box-ticking or AI-drafted assessment is even less likely to survive here than usual. What the consultee is testing is whether an existing thing is genuinely safe. That is an evidence problem, and no amount of drafting fixes it. Where the mapping itself is the obstacle, site-specific flood modelling is sometimes the only way to replace a coarse dataset with defensible levels.

How does the sequential test apply to a development that already exists?

Awkwardly, and it is often the point the whole application turns on. The sequential test steers new development towards land at the lowest risk of flooding, by comparing the site with reasonably available alternatives elsewhere in the area of search. Applied to something already built, that logic strains: the applicant is being asked whether the development should have gone somewhere else, when it is already there.

Under the August 2026 Framework the test sits at Policy F5, and two features cut both ways. The area of search is capped at the development's own catchment, which helps a genuinely minor as-built scheme resist a district-wide comparison. A site at risk from surface water only can avoid the test, but only where the flood risk assessment shows the development is safe for its lifetime without increasing risk elsewhere. On a finished site that is the hardest demonstration to produce.

The exemption that reads like relief is the one that lands straight back on the flood risk assessment.

Policy F5 exempts householder development and small non-residential extensions under 250 square metres, which covers a fair proportion of retrospective domestic cases. Anything larger needs the test addressed properly, in a sequential and exception test report built on the evidence the consultees will interrogate. As the Yatton judgment showed, these tests turn on evidence and the planning balance, not on how the case is worded.

Refused, or served with an enforcement notice: what happens next?

Refusal is not the end. But the options narrow fast, and where retrospective planning permission is refused you are left with two of them: appeal the decision to the Planning Inspectorate, or accept it and remove or alter what was built. If the council issues an enforcement notice, it will specify a date the notice takes effect, no less than 28 days after service, and a separate compliance period. An appeal must be lodged before the notice takes effect.

The grounds run from (a) to (g) under Section 174 of the Town and Country Planning Act 1990. Ground (a), that permission ought to be granted, carries a deemed planning application, the double fee above, and a hard restriction: for notices issued on or after 25 April 2024 it is barred for two years where the notice followed a related application that has ceased to be under consideration.

1. Diarise both dates from the notice. The date it takes effect and the compliance deadline are not the same date.
2. Decide the ground or grounds on advice. Ground (a) is the one that reopens the planning merits and the one that costs.
3. Get the technical evidence in order before the appeal, not during it. Recent changes to the appeal process reward applications that put their full case to the authority first.
4. Where the objection is a flood risk objection, deal with it at source: our guides on an Environment Agency objection and an LLFA objection set out what each consultee is asking for.

Enforcement is not a remote prospect. District planning authorities in England issued 1,435 enforcement notices in the first quarter of 2026 alone, up from 1,005 in the preceding quarter, and the trend across the last four published quarters runs one way.

Remediation, not paperwork: what a flood risk failure actually costs

Here is the part that catches people out. Retrospective planning permission is refused on flood risk grounds when the evidence cannot be made to work, and the evidence is physical. Suppose the assessment shows the completed development increases flood risk, or that surface water is not managed to standard. The remedy is not a better-worded report. It is physical remediation of what has been built, on a site that is finished, landscaped and usually occupied, by people who did not expect a digger.

• Excavation. Digging out finished ground to install the attenuation tanks, crates or soakaway that should have been built in from the start.
• Floor levels. Lifting or rebuilding finished floors to achieve a safe freeboard above the flood level.
• Surfacing. Replacing impermeable surfaces with permeable construction, or re-grading so water falls away from the buildings.
• Compensation. Providing level-for-level compensatory storage where the works displaced floodplain volume.
• Footprint. In the worst cases, reducing or removing part of what has been built, because the site cannot accommodate compliant drainage as built.

On a constrained finished site there is sometimes no room for the storage the runoff calculations demand, and full compliance cannot be achieved at any price.

Each is disruptive, expensive and far slower than it would have been in the original build, which is why the retrospective route is rarely as low-risk as it looks from the outside. The flood and drainage dimension is where the gamble usually fails. Where a condition rather than a refusal is the mechanism, it becomes live whether a condition can make you fix a problem you did not cause.

Works near a watercourse: the permit planning permission cannot cure

Some works need a second consent altogether, and it is the one consent in this article that cannot be sorted out afterwards. Where they involve a main river, a flood defence or the land beside one, building, excavating or placing a structure in a watercourse or its byway, they may have needed a flood risk activity permit from the Environment Agency before anything started.

The Agency's guidance is unambiguous that permission must be obtained before work starts, and that you are breaking the law if you operate without the permit you need. Doing so is an offence under regulation 38 of the Environmental Permitting Regulations 2016, and for a flood risk activity the penalty on indictment runs to two years' imprisonment.

A permit and a planning permission are separate consents, and a later grant of planning permission does not cure a missing permit.

If you are in that position, take advice on the permit first. The two consents run on different timescales, against different tests, and are decided by people who will not consult each other about your site.

How Unda approaches retrospective flood risk and drainage work

Unda prepares flood risk assessments and drainage strategies for retrospective applications most months. The approach differs from a pre-construction commission, because instead of describing a proposal the work reverse-engineers the evidence for something that is already standing, from whatever record of the original ground the site still holds.

1. An honest as-built appraisal: what has actually been constructed, what its levels and surfaces are, and how surface water behaves now compared with before.
2. Reconstruction of the pre-development baseline as robustly as the evidence allows, from historic mapping, records and site investigation.
3. Modelling of the site as it stands, screening every source of flooding the current Framework requires.
4. The mitigation genuinely achievable on a finished site, rather than the mitigation a blank plot would have used.
5. Any remediation set out clearly and proportionately, so the applicant can weigh the cost before committing.

Where remediation is unavoidable we say so plainly, with the cost attached. Where a development can be shown to be safe and compliant as built, we make that case with the site-specific evidence the consultees actually test. A recent retrospective new dwelling in a River Ems floodplain is a good example:

If you are facing a retrospective planning application, by choice or by enforcement notice, the flood risk and drainage evidence is usually the hardest part to get right. It is also the part most likely to require physical works. Unda has delivered more than 5,000 flood risk assessments and drainage strategies since 2014, and prepares flood risk assessments for planning and surface water drainage strategies for retrospective applications across England and Wales. Get in touch for a no-obligation quote and one of our flood risk consultants will respond within 60 minutes.

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