Class B roof extensions: the 40 and 50 cubic metre limits
The volume allowance is cumulative, measured against the original roof, and a hip-to-gable can spend most of it before the dormer is drawn. Do the arithmetic first.
Class B of Part 1 of Schedule 2 to the General Permitted Development Order 2015 allows the roof space of a house to be enlarged by up to 40 cubic metres on a terrace house and 50 cubic metres on any other house. Those figures are the whole of the allowance, they are measured against the original roof rather than the current one, and they are cumulative.
Most loft conversions that get into trouble do so because someone drew the dormer first and calculated the volume afterwards.
How the volume allowance is measured
The measurement is of the roof space, taken externally, comparing the resulting roof with the original roof. Original means the house as first built, or as it stood on 1 July 1948 for anything older.
Three consequences follow.
A dormer added in 2004 by a previous owner counts against your allowance, even though you never had the benefit of it. So does an earlier hip-to-gable, and so does a roof enlargement carried out as part of a rear extension where it added to the original roof space.
The calculation includes the thickness of the construction, because it is an external measurement. A dormer measured to the inside face of the plasterboard will come out smaller than the figure the council uses.
And the comparison is with the original roof, so a house whose roof has already been raised or altered is measured against what was there at the start, not against the roof you can see today.
Terraced against other houses and the two figures
The Order defines a terrace house as one in a row of three or more houses which shares a party wall with, or has a main wall adjoining, a house on either side. A house at the end of such a row is also a terrace house where it adjoins a house that meets that description.
So the practical position is:
| House type | Allowance |
|---|---|
| Mid-terrace | 40 m³ |
| End of terrace | 40 m³ |
| Semi-detached | 50 m³ |
| Detached | 50 m³ |
End-of-terrace catches people. A house that looks and feels like a semi, with a side garden and a gable end, is a terrace house for these purposes if it sits at the end of a row of three or more, and it gets 40 cubic metres.
Hip to gable and what it consumes
A hip-to-gable enlargement fills in the sloping hipped end of a roof to create a vertical gable. It is the standard first move on a semi-detached loft conversion, because it turns an unusable triangular void into floor area with headroom.
It also uses volume. The added space is essentially a wedge running from the original hip up to the new gable, and its size depends on the roof pitch, the span of the house and the depth of the hip. On a modest semi that wedge can account for a large share of the 50 cubic metres before any dormer exists.
Work it out before designing the dormer, not after. The arithmetic is straightforward geometry and any competent designer can do it in an hour, and it determines whether you are drawing a full-width rear dormer or something considerably smaller.
Illustrative example: a 1930s semi with a hipped roof at 40 degrees. The hip-to-gable is drawn first and calculated. What remains is enough for a rear dormer across part of the roof but not the full width. The owner faces a choice between a smaller dormer under permitted development and a planning application for the scheme they wanted, and that is a much better position than discovering it after the roof is open.
Set back from the eaves and the side elevation rule
Two conditions shape the appearance of the finished conversion.
The eaves of the original roof must be maintained or reinstated, and the enlargement must be set back at least 20 centimetres from the eaves where practicable, measured along the roof slope. That condition does not apply to a hip-to-gable enlargement, nor to an enlargement joining the original roof to the roof of a rear or side extension.
The set-back is why properly executed permitted development dormers have a strip of original roof below them rather than running straight down to the gutter. Builders skip it, and it is one of the easiest breaches for an officer to spot from the street.
Windows in a wall or roof slope forming a side elevation must be obscure-glazed, and non-opening unless the opening parts are more than 1.7 metres above the floor of the room. That applies to the bathroom rooflight in the flank as much as to a side window in a dormer cheek.
Nothing under Class B may exceed the height of the highest part of the existing roof, and nothing may extend beyond the plane of a roof slope forming the principal elevation and fronting a highway. Verandahs, balconies and raised platforms are excluded outright, which is what catches a Juliet balcony detail on a rear dormer.
Article 2(3) land and why Class B disappears
On article 2(3) land, Class B is not available at all. That land comprises conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites.
There is no reduced allowance in those areas and no prior approval route. Any roof enlargement needs planning permission, and in a conservation area the council will apply its own design guidance on dormer proportions, materials and position, which is usually more restrictive than the volume rules would have been.
Check the designation before anything else. Council mapping shows conservation area boundaries, and boundaries follow property lines in ways that are not always obvious from the street. An article 4 direction can have the same effect outside a designated area, removing Class B rights street by street, and those are published on the council's planning policy pages rather than shown on the main constraint map.
Recording the calculation for a certificate
Where the works are permitted development, apply for a lawful development certificate for proposed development under section 192 of the Town and Country Planning Act 1990. It is the only formal way to confirm that works are lawful.
The application should include existing and proposed roof plans, elevations and sections at a stated scale, and a volume calculation showing the original roof volume, any previous enlargements, the proposed enlargement and the running total against the allowance. Set the arithmetic out so the officer can follow it rather than asking them to derive it from the drawings.
Add evidence for the original roof where any earlier work has been done: the council's planning history for the address, historic mapping, or dated photographs. A short Planning Statement tying the evidence to each limit in the class turns a contested calculation into a straightforward decision.
Keep the certificate with the deeds when it comes. Loft conversions are the works most often queried on sale, partly because they are visible from the street and partly because building control approval and planning lawfulness are separate questions that buyers' solicitors ask about together. Having both documents in one folder saves a fortnight at the worst possible moment.
To see which supporting documents your scheme is likely to need, browse the report types or enter your postcode on PlanScope.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How much can I extend my roof under permitted development?
By up to 40 cubic metres of additional roof space on a terrace house, including an end-of-terrace, and 50 cubic metres on a semi-detached or detached house. The figures are cumulative against the original roof, so any earlier dormer or roof enlargement comes out of the same allowance.
Does a hip-to-gable count towards the volume?
Yes. Filling in a hipped end to create a gable adds roof space, and that addition is measured in the same way as a dormer. On a typical semi-detached house the hip-to-gable element alone can account for a substantial share of the 50 cubic metres, which is why the dormer then has to be modest.
Can I build a dormer on the front of my house?
Not under Class B. The Order prevents any part of the dwellinghouse extending beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway. A front dormer therefore needs planning permission, whatever the volume figures say.
What is the 20 centimetre eaves rule?
A condition on Class B. Other than for a hip-to-gable enlargement or one joining the original roof to the roof of a rear or side extension, the eaves of the original roof must be maintained or reinstated, and the enlargement must be set back at least 20 centimetres from the eaves where practicable, measured along the roof slope.
Do I need permission for rooflights?
Rooflights are dealt with by a separate class covering other roof alterations, which has no volume allowance but restricts how far a rooflight may protrude beyond the plane of the roof slope and requires side-facing windows to be obscure-glazed. Check the class that applies before assuming the volume allowance is engaged.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.