The short answer
A lawful development certificate is the council's written confirmation that a project is permitted development and needs no planning application. It is optional. For an extension it costs half the householder planning fee and is usually decided within eight weeks. If your kit will be made to drawings that rely on permitted development, get the certificate before you pay for the kit.
What the certificate does
Most single-storey rear extensions in England never go near a planning committee. They are built under permitted development, a national set of rights that lets a householder extend without applying, provided the extension stays inside fixed limits on depth, height, position and materials.
Nobody checks that you have stayed inside those limits before you build. The rights simply apply or they do not. A lawful development certificate turns that judgement into a document: you send the council your drawings, and it confirms in writing that what you propose is lawful without planning permission. The formal name for the version you want is a certificate of lawfulness of proposed use or development.
It is not planning permission, and the council does not weigh up whether it likes the design. The only question is whether the drawings meet the rules. Neighbours are not normally consulted, because they have no say over a permitted development.
When it is worth having
The certificate is optional. People get one for three reasons.
Certainty before money is spent. The depth limit for a rear extension on a semi or terrace is 3 m, measured from the original back wall of the house. “Original” means as it stood in July 1948, or as first built if later. If a previous owner added a kitchen in the 1980s, that counts against your allowance, and plenty of people only find out when the enforcement letter arrives.
Selling the house. A buyer’s solicitor will ask how the extension was authorised. “It was permitted development” is an opinion. A certificate is an answer.
A kit supplier or lender who asks. Some suppliers will not start production on a permitted development scheme without one, because a refused or altered scheme leaves them holding panels cut to the wrong size.
Why the timing matters with a kit
A builder working in brick can absorb a late change. If the council says the extension must be 300 mm shallower, the bricklayer sets out a different line. Factory-made walls cannot do that. The panels for extension kits are cut from the drawing, and a change after they are made means paying for new ones.
So the order is drawings, then the certificate, then the kit. Send the supplier the certified drawings and ask them to confirm the external dimensions of their frame match, including the thickness of any cladding or brick skin. Permitted development limits are measured to the outside face, and a wall that grows by 100 mm in the factory can take a 3 m extension over the line.
The limits the council will measure
For a single-storey rear extension under the householder rules in England, the main tests are:
- no deeper than 3 m from the original rear wall on an attached house, or 4 m on a detached one;
- no higher than 4 m, and no more than 3 m to the eaves if within 2 m of a boundary;
- exterior materials similar in appearance to the house;
- together with other buildings, covering no more than half the garden;
- not on a flat or maisonette, which have no householder permitted development rights.
There is also a larger extension route, allowing up to 6 m on an attached house and 8 m on a detached one. That uses a different process, prior approval, in which neighbours are notified and can object. It is not the same as a lawful development certificate.
Conservation areas, listed buildings and houses where the council has removed permitted development rights, by an Article 4 direction or a condition on an earlier permission, follow tighter rules or none at all. Your council’s planning team can tell you whether any apply before you draw anything.
What you send and what it costs
The application goes to the council, usually through the Planning Portal. It needs a site location plan, existing and proposed drawings with dimensions, and a short statement of why the extension is permitted development. The drawings your designer prepares for the kit will usually serve, provided they show the house as well as the new room.
The fee for a proposed extension is half the householder planning application fee. Councils aim to decide within eight weeks. If they refuse, the letter explains which limit you have broken, and you can either redesign or apply for planning permission instead.
Scotland and Wales
Wales has its own permitted development rules, broadly similar in shape but with different details, and its own certificate. Scotland has separate householder permitted development rights and a certificate of lawfulness of its own. In Scotland a building warrant is needed for the work whatever the planning position, so the certificate there settles planning only.
The short version for a kit build
If your extension relies on permitted development and you are about to pay a deposit on a frame cut to the millimetre, a certificate is cheap insurance. Apply with the same drawings the supplier will build from, wait for the decision, then place the order.