Do you need planning permission for a loft conversion?

The short answer

A rear dormer that stays below the ridge, keeps off the front roof slope, and adds less than your volume allowance is permitted development. You do not need planning permission and nobody has to approve it.

A hip to gable conversion is usually permitted development too, and it uses volume in a way people rarely expect, because filling in a hipped end adds a great deal of roof space.

The part worth understanding is that permitted development is not a decision. There is no application, no approval and no paperwork. Your builder saying it is within permitted development is your builder's opinion. It is usually right, and it is not proof.

A Lawful Development Certificate turns it into proof. The council checks the facts against Class B and, if the conversion qualifies, issues a certificate saying so. It is the document a buyer's solicitor is asking for when they ask what approval you have for the loft.

The volume allowance, and why it is the number that matters

Class B allows the roof space to grow by a fixed amount:

• 40 m³ if you live in a terraced house, including an end of terrace.
• 50 m³ for a detached or semi-detached house.

Three things about that allowance catch people out.

It is cumulative

The cap applies to the resulting roof space compared with the original roof space, taking in every enlargement that has ever happened. If a previous owner put a small dormer on the back in 1998, that volume is spent. Your new dormer has to fit in what is left. It does not matter that the earlier work was somebody else's, or that it had planning permission, or that you did not know about it.

It is measured externally

The rule tests cubic content, which means the external volume of the enlargement, not the room you end up with inside. Internal measurements understate it, typically by around a quarter once you allow for the thickness of the walls and the roof. A dormer that measures comfortably inside the allowance from the inside can be over it from the outside.

This is the single most dangerous error in a Class B case, because it points the wrong way. Underestimating the volume produces a certificate for a conversion that actually breaches the cap, and a certificate obtained on wrong figures is not worth having.

Hip to gable spends a lot of it

Filling in a hipped roof end to make a gable adds a substantial wedge of volume before you have built any dormer at all. On a semi it is common for the hip to gable work to use half the 50 m³ on its own. If you are doing both, they are added together.

The other Class B rules

Volume is the one that decides most cases, but it is not the only condition.

• Nothing may go higher than the existing ridge. The highest part of the enlargement must not exceed the highest part of the existing roof. This is a comparison, not a measurement against a fixed height.
• Nothing on the front roof slope. No part may extend beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway. In practice this is the no front dormer rule. Rooflights on the front are treated differently and are generally fine, provided they project no more than 0.15 m beyond the plane of the roof.
• Set back 0.20 m from the eaves. The edge of the enlargement nearest the eaves must be at least 0.20 m from them, measured along the roof slope. This does not apply to a hip to gable enlargement or to one joining the roof to a rear extension.
• Materials similar in appearance. The finish has to be similar to the existing house. It does not have to be identical, and the test is appearance rather than an exact match.
• Side windows obscure glazed. Any window in a side elevation of the enlargement must be obscure glazed, and non opening below 1.7 m above the floor of the room. Dormer cheek windows are the usual case.
• Not on a house created by a prior approval conversion. If the house itself was made from an office or a shop under Class M, N, P or Q, Class B does not apply at all.

What it costs

A loft conversion alters the house itself, which puts it in the higher of the two statutory fee bands.

The council fee is fixed by regulation and identical at every council in England. We pass it through at cost and add nothing to it. If the council refuses, we refund our £349 in full. The council fee is not refundable, because the council keeps it whether the answer is yes or no.

Getting one before you build, or after

Before you build is a Section 192 certificate. It costs half as much, it needs no historical evidence, and it settles the volume question while you can still change the design. If your dormer comes out at 43 m³ on a terrace, finding that out on paper is a great deal cheaper than finding it out afterwards.

After it is built is a Section 191 certificate. This is the common case, and it is usually a sale that prompts it. It needs the conversion described exactly as built, with photographs and whatever documentation you have, and a statement of truth that you sign in front of a solicitor.

On a retrospective case there are two possible arguments. The first is that the work was permitted development when it was done, which does not depend on how long ago that was. The second is that enough time has passed for the council to have lost the ability to enforce. The first argument is nearly always the stronger one, and it is the one we lead with.

What the council actually looks at

A Lawful Development Certificate application is not a planning application. The case officer is not deciding whether your loft conversion is a good idea, whether it fits the character of the street, or whether the neighbours mind. None of that is relevant and none of it can be taken into account.

They are checking facts against a list of conditions. Is the volume within the cap. Does anything exceed the ridge. Is there anything on the front slope. Are the materials similar. That is the whole exercise.

Which is why refusals are usually self inflicted. The commonest cause is an application that does not state its figures precisely enough for the officer to check them. If the description says "rear dormer" without dimensions, there is nothing to certify, and the officer refuses rather than guessing on your behalf.

Questions people actually ask

My builder said I do not need planning permission. Is that enough?

It is enough for the work to be lawful, if the builder is right. It is not enough to prove it later. Builders are usually right about this and they are not the ones a buyer's solicitor will be asking in five years' time.

Does a rooflight count towards the volume?

A rooflight fitted in the plane of the roof does not enlarge the roof space, so it adds nothing to the volume. It has its own rule instead: it must not project more than 0.15 m beyond the plane of the roof slope.

What about a Juliet balcony?

A balcony takes the work outside permitted development. A Juliet balcony, meaning glazed doors with a guard rail across the opening and nothing to stand on outside, is generally treated as a window rather than a balcony. Anything you can stand on is a different matter.

The previous owner converted the loft. Can I still get a certificate?

Yes. The certificate is about the building, not about who built it. You will need to describe the work accurately and provide what evidence you have, which for an inherited conversion often means photographs, the sale particulars from when you bought, and any paperwork that came with the house. If you genuinely do not know the dimensions they will need measuring.

Do I need building regulations approval as well?

Almost certainly, and it is a separate system with a separate department. A loft conversion involves structural work, means of escape and fire separation, all of which are Building Regulations matters. A Lawful Development Certificate says nothing about them. If your conversion was done without building control sign off, that is resolved with a regularisation certificate from the same council's building control team, and the two processes can run at the same time.

What if my conversion is over the volume limit?

Then it is not permitted development and a certificate application will fail. Your options are a retrospective planning application, which is a different thing with a different fee and no guaranteed outcome, or planning indemnity insurance if a sale is the pressing issue. We will tell you if we think you are in this position and we will not take the case.

How we handle it

1. You answer the questions. Ten minutes or so. We ask for the dormer's width, depth and height rather than a volume, and do the arithmetic ourselves against the cap for your house type.
2. We check it before you pay. If the volume is over, or the work touches the front slope, or a chimney was moved, we say so on screen and you pay nothing.
3. We draft the application. A statement working through B.1 and B.2 condition by condition with your figures, the application form, and the covering letter. You review everything before it goes.
4. We file it and handle the council. Eight weeks is the statutory period. We deal with any questions the officer raises.

The honest position

If your loft was converted years ago, nobody has ever asked about it, and you are not selling, you can leave it alone. Permitted development work is lawful whether or not you have a certificate.

Get one if you are selling, if you are remortgaging, if the conversion is anywhere near the volume cap, or if you want the question settled permanently instead of raised again by every future buyer. And get one before you build if you have the choice, because it is half the price and you can still change the design.

Related reading

• What is a Certificate of Lawfulness? The full explanation of the document.
• Garden offices and Class E for outbuildings and the 50% curtilage cap.
• Rear extensions and Class A for the depth and height limits.
• Retrospective planning permission or an LDC? Which one you actually need.
• What an LDC costs with the fee bands set out by project type.