Is your rear extension permitted development?

The depth limits

For a single storey rear extension, measured from the rear wall of the original house:

• 4 m if the house is detached.
• 3 m for any other house, which means semi detached, terraced and end of terrace.

There is also a height cap of 4 m for a single storey rear extension, and a separate rule for anything within 2 m of a boundary, covered below.

Two words in that first sentence do a lot of work. "Original" means the house as it stood on 1 July 1948, or as first built if it was built later. So the measurement runs from the original rear wall, not from the back of a conservatory somebody added in 2005. If your house already has a rear extension, your allowance is measured from where the wall used to be, and it may already be used up.

The conditions that go with it

Nothing forward of the principal elevation

No part of the extension may extend beyond a wall forming the principal elevation of the house, which is normally the front. It also may not extend beyond a side wall that fronts a highway. A side extension on a corner plot facing the road is caught by this.

Not higher than the existing roof, and not higher at the eaves

The extension may not exceed the highest part of the existing roof, and its eaves may not be higher than the existing eaves.

Within 2 m of a boundary, the eaves must be 3 m or lower

If any part of the extension is within 2 m of any boundary of the curtilage, its eaves height must not exceed 3 m. Terraced and semi detached houses often have a boundary close to the flank wall, so this applies more often than people expect.

The 50% rule

Buildings within the curtilage, excluding the original house, must not cover more than half the curtilage. Everything counts: the new extension, any earlier extension, the detached garage, the shed and the garden office. They pool against a single limit.

This is the same allowance that Class E outbuildings draw on. If you have a large garden office already, your extension has less room, and the reverse is also true. The two classes cannot be considered in isolation.

Side extensions have their own limits

A side extension must be single storey, no more than 4 m high, and no wider than half the width of the original house.

Materials similar in appearance

The exterior materials must be similar in appearance to the existing house. This is a condition rather than a limit, which matters: breach it and the whole extension falls outside permitted development, not just the part with the wrong brick.

Upper floor side windows

Any upper floor window in a side elevation must be obscure glazed and non opening, unless the opening parts are more than 1.7 m above the floor of the room.

Not on a house created by prior approval

If the house itself was created by converting an office, shop or agricultural building under Class M, N, P or Q, Class A does not apply to it at all.

Permitted development is not a document

If your extension satisfies all of the above then it is lawful, and it was lawful the day it was finished. Nobody approved it because nobody had to.

The difficulty is proving it. There is no automatic record. When you sell, a buyer's solicitor will ask what consent exists for the extension, and "it did not need any" is an assertion. They will want it evidenced, and the thing that evidences it is a Lawful Development Certificate.

The certificate is the council confirming, in writing, that the extension is lawful. It is not planning permission and it is not retrospective planning permission. It is an official statement that permission was never required.

What it costs

An extension alters the house itself, which puts it in the higher of the two statutory fee bands. An outbuilding sits in the lower one.

The council fee is set nationally and is the same wherever you are in England. We pass it through at cost. If the council refuses, we refund our £349 in full. The council keeps its fee either way, so that part is not recoverable.

A planning consultant preparing the same application typically charges £600 to £1,500, and an architect more than that.

Before you build, or after

A Section 192 certificate confirms a proposed extension would be lawful. It costs half as much as the retrospective version and it settles the argument while the design can still change. If your plans come out at 3.4 m on a semi, that is much better discovered on paper.

A Section 191 certificate confirms an extension that already exists. This is what most people need, and a sale is usually what prompts it. It requires the extension described exactly as built, photographs, whatever documentation you have, and a statement of truth signed by you.

For a retrospective case, the strongest argument is almost always that the work was permitted development when it was carried out. That argument does not depend on when it happened. The alternative argument, that too much time has passed for the council to act, does depend on dates, and the dates changed on 25 April 2024 when the four year enforcement rule in England was replaced by a uniform ten year rule.

Measuring from the right wall

This is the mistake that costs people certificates, so it is worth its own section.

The depth is measured from the rear wall of the original dwellinghouse. If a conservatory was added in 2005 and then replaced with a solid extension in 2021, the 3 m or 4 m runs from the original 1948 rear wall, and the conservatory's footprint has already spent part of it. Two 2 m extensions built fifteen years apart are a 4 m extension for these purposes.

The same principle runs through Class A. "Original" is a fixed point in the past, not the state of the house when your builder arrived.

What a case officer is actually doing

A certificate application is a factual test, not a planning judgement. The officer is not weighing whether the extension is attractive, whether it overlooks a neighbour, or whether it suits the street. Those considerations belong to a planning application and are irrelevant here. If the facts meet the conditions, the certificate must be issued.

That cuts both ways. It means a well documented application on a compliant extension is close to a formality. It also means there is no room for argument if a measurement is over, and no discretion for the officer to be generous.

The most common reason for a refusal that did not need to happen is an imprecise description. An application that says "single storey rear extension" without dimensions gives the officer nothing to certify. It needs the depth, the width, the height at the eaves and at the highest point, the distance to any boundary within 2 m, the materials, and the curtilage arithmetic with the figures shown.

Questions people actually ask

Does a conservatory count as an extension?

For the 50% curtilage cap and for measuring depth from the original wall, yes. A conservatory is a building within the curtilage and its footprint counts. Some conservatories are separately exempt from Building Regulations, which is a different system and does not affect the planning position.

My extension is 3.2 m on a semi. What are my options?

It is not permitted development, so a certificate application would fail and we would not take it. The routes open to you are a prior approval application under the larger home extension scheme if it has not been built, a retrospective planning application if it has, or planning indemnity insurance if the pressing problem is a sale rather than the extension itself. A planning consultant is the right person for the first two.

Do I need a certificate if I have building regulations sign off?

Yes, if you need one at all. Building Regulations completion certificates cover construction standards: structure, insulation, drainage, fire safety. They say nothing about planning. A buyer's solicitor asking about planning status will not be satisfied by a building control certificate, and vice versa.

What if the extension was there when I bought the house?

That is fine and it is common. The certificate concerns the building, not who built it. You will need to describe it accurately and supply what evidence you have, which for an inherited extension usually means photographs, your own purchase paperwork, and anything the previous owner left behind. If the dimensions are unknown they will need measuring.

How long does it take?

The council has eight weeks. That period is statutory and nobody can shorten it. What we control is everything before filing, which is where delay usually happens in practice. We draft within two working days of payment, and the same day on the Express tier.

Will the neighbours be consulted?

No. A certificate application is not publicised or consulted on, because neighbours' views are not a relevant consideration. That is one of the practical differences between this and a planning application. The prior approval route for a larger extension is different again, and does involve neighbours, which is part of why we do not offer it.

How we handle it

1. You answer the questions. About ten minutes, with measurements you can take yourself. We check the postcode against national planning data for conservation areas, Article 4 directions and listed buildings as you go.
2. We tell you if it fails. Over 3 m on a semi, or forward of the principal elevation, or over the curtilage cap, and we say so on screen before you pay anything.
3. We draft the application. A statement working through A.1 and A.3 one condition at a time with your figures against each, the completed application form, and the covering letter. You approve all of it before it is sent.
4. We file it and manage the council. Including answering the case officer's questions during the eight week period.

The honest position

A compliant extension is lawful with or without a certificate, and if you are not selling and nobody has asked, there is no urgency.

Get one if a sale is in progress or coming, if any measurement sits close to a limit, or if you would rather answer the question once than have every future buyer raise it. And if you have not built yet, get the Section 192 version: it is half the cost, and it tells you whether the design works while changing it is still cheap.

Related reading

• What is a Certificate of Lawfulness? The full explanation of the document.
• Loft conversions and Class B for the volume allowance rules.
• Garden offices and Class E which share the 50% curtilage cap with extensions.
• Your conveyancer asked for an LDC for when a sale is already under way.
• What an LDC costs with the fee bands by project type.