If your extension falls within permitted development, you do not need planning permission. You can build it. Nobody has to approve the design.
We still recommend applying for a certificate of lawfulness, and we recommend it often enough that it is worth explaining why in full. The short version: permitted development is a right, not a decision. A certificate turns the right into a written decision from the council, and a written decision is what you will need years later when somebody asks you to prove it.
What a certificate of lawfulness actually is
It is a formal statement from your local planning authority that a particular building or use is lawful. It is not planning permission and it does not work like planning permission. The council is not being asked whether your extension is a good idea. It is being asked a narrower question: given the facts and the law, is permission required or not.
That distinction changes everything about how the application is handled. There is no assessment of design. Neighbours have no say on whether they like it. The officer reads your drawings, applies the legislation, and answers yes or no.
There are two types, and people mix them up constantly.
| Certificate of lawfulness for a proposed use or development | Certificate of lawfulness for an existing use or development | |
|---|---|---|
| When you apply | Before you build | After something has already been built or a use established |
| What it confirms | That what you intend to do would be lawful | That what already exists is lawful and immune from enforcement |
| What you submit | Drawings and a description of the proposal | Evidence: photographs, invoices, statutory declarations, dated records |
| Typical reason | Certainty before you spend money | A conveyancing solicitor has raised a query on a sale |
| Burden of proof | On you | On you, and it is a much heavier burden |
The first is the one to aim for. The second is where people end up when they did not do the first.
Why we recommend one even when nothing requires you to have it
Because you will sell the house eventually
This is the practical reason, and it is the one clients come back and thank us for. When you sell, the buyer's solicitor will ask what authorised the extension. "It was permitted development" is an assertion. A certificate is a document. Without one, the solicitor may require indemnity insurance, a retrospective application, or a price reduction, and all of this happens at the point in a sale where you have the least leverage and the least time.
The cost of a certificate now is a fraction of the cost of a stalled sale later.
Because permitted development is narrower than almost everybody assumes
Permitted development rights are set nationally, but they are removed or reduced in a long list of circumstances. Listed buildings. Conservation areas and other designated land. Flats and maisonettes, which have no householder rights at all. Article 4 directions, which a council can make over a specific area to withdraw specific rights. Planning conditions on newer housing estates, which very commonly strip permitted development from every house on the development. And the allowance already consumed by a previous owner's extension, which most buyers know nothing about.
Any one of those can turn a scheme you believed was permitted into a scheme that needed permission. A certificate application forces that question to be answered by the authority, in writing, before you have poured a foundation.
Because enforcement is not a fee
If a council concludes that development was carried out without the permission it required, it can take enforcement action, and that can mean removing what you built. The existence of a certificate closes the argument off. There is no realistic route by which an authority enforces against development it has certified as lawful, provided you build what the certificate describes.
That last point matters. A certificate covers the scheme on the drawings. Build something different and the certificate does not cover it.
Because it settles disputes before they start
Extensions upset neighbours. A neighbour who believes your building work is unlawful can and will report it. If you hold a certificate, that conversation ends in a single email. If you do not, you may find yourself assembling evidence under pressure while the scaffolding is up.
Because lenders, insurers and building control ask
Remortgaging, changing insurer, or dealing with a building control completion query can all surface the same question. Having the answer already filed is worth more than the modest effort of filing it.
What the council actually decides, and what it will not
An officer determining a certificate application is deciding a point of fact and law. They will consider:
- The drawings and dimensions you have submitted, against the relevant class of permitted development
- Whether the property benefits from those rights at all
- Whether previous works have used up any part of the allowance
- Whether any condition, direction or designation removes the rights
They will not consider whether the extension overlooks the neighbours, whether the materials are appropriate, or whether the design is any good. If those things concern you, they belong in a planning application, not here.
This is why the accuracy of the drawings is the entire application. A certificate stands or falls on whether the submitted scheme is measurably within the rules. Ambiguity gets refused. We prepare certificate drawings to the same standard as planning drawings for that reason.
What it costs you
There is an application fee, paid by you directly to the local planning authority, in the same way as a planning application fee. Fees are set nationally and change, so check the current figure with your authority before you submit. There is our fee for preparing measured drawings and the supporting statement, which sits within our Stage 1 and Stage 2 work rather than being an extra exercise, because those drawings are needed to build from in any case.
We do not state permitted development limits in our articles, and that is deliberate. The rules were amended in 2024 and continue to move. Any dimension we published today could be wrong by the time you read it, and a page selling planning expertise is the worst possible place to publish an out-of-date number. The position has to be checked against your specific property, on the day, which is the first thing we do at the feasibility stage.
When a certificate is not worth it
We do not recommend one in every case. If you are already applying for planning permission for part of the scheme, a separate certificate for the rest is usually unnecessary duplication. If the works are genuinely trivial and leave no lasting trace on the building, the sale risk is low. And if you are close enough to the edge of the rules that we are uncertain, the honest advice is often to make a full planning application instead, because a refused certificate is a written record that the scheme was unlawful, which is a considerably worse document to hold than no document at all.
That is the judgement worth paying for: not the form-filling, but knowing which of the three routes your project belongs on.
How this fits into the process
A certificate sits naturally at the end of our Stage 1 and Stage 2 work. We survey the property, prepare existing and proposed drawings, establish whether permitted development rights survive on your particular site, and then either submit the certificate application or advise you that a planning application is the safer route. On a straightforward domestic project such as our rear extensions at Cringleford, that assessment happens before any detailed design work begins, because it changes what the design can be.
You can see how we approach applications generally on our planning applications page, and how extension projects are structured on our house extensions page.
Talk to us before you build
If somebody has told you your extension is permitted development, that is a starting point, not a conclusion. We offer a free initial consultation: we review the site constraints, identify the planning or technical issues, and tell you plainly which route your project should take.
Call the Norwich studio on +44 (0) 1508 830 310 or email info@archidite.com.