Measuring the volume allowance for a loft dormer
Class B allows 40 cubic metres of added roof space on a terrace house and 50 on anything else, measured outside the structure. Earlier roof additions count, including ones a previous owner built.
The dormer volume allowance is 40 cubic metres for a terrace house and 50 cubic metres for anything else, measured as the increase in the cubic content of the roof space above the original roof, externally. It comes from Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. Two things trip people up: the measurement is taken outside the structure rather than inside, and any earlier roof addition counts against your total even if a previous owner built it.
Work out the number before the design, not after. A rear dormer on a modest terrace can use the whole allowance.
What the volume allowance covers
Class B permits an addition or alteration to a roof, which covers rear dormers, side dormers, hip to gable enlargements and combinations of those. The allowance is expressed as a limit on how much larger the resulting roof space can be than the original roof space.
Alongside the volume limit, Class B imposes several other restrictions that bite independently:
- no part of the enlargement may exceed the height of the highest part of the existing roof
- nothing may extend beyond the plane of a roof slope that forms the principal elevation and fronts a highway
- no veranda, balcony or raised platform
- no chimney, flue or soil and vent pipe work under this class
- the class does not apply at all on article 2(3) land, which includes conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites
Two conditions also apply. Materials must be similar in appearance to the existing house. And other than for a hip to gable enlargement or a rear enlargement, the edge of the enlargement nearest the eaves must be set back at least 20 centimetres from the eaves, measured along the roof slope. Any window in a side elevation roof slope has to be obscure-glazed and non-opening below 1.7 metres above the floor.
Exceed any one of those and the whole scheme needs planning permission, regardless of the volume.
Terraced, semi-detached and detached figures
The 40 cubic metre figure applies to a terrace house and the 50 cubic metre figure to everything else. The definition of terrace house is where the money is.
A terrace house means a dwellinghouse in a row of three or more houses that shares a party wall with, or has a main wall adjoining the main wall of, the house on either side. Critically, the definition extends to a house at the end of such a row that shares a party wall with, or has a main wall adjoining, a house which meets that test. So an end-of-terrace house gets 40 cubic metres, not 50.
| House type | Allowance |
|---|---|
| Mid-terrace | 40 m³ |
| End-of-terrace in a row of three or more | 40 m³ |
| Semi-detached | 50 m³ |
| Detached | 50 m³ |
| Detached house in a row of two | 50 m³ |
Owners of end-of-terrace houses regularly design to 50 cubic metres because the house feels semi-detached on one side. It is the row that matters, not how the house looks from the street.
Measuring the added volume correctly
Measure externally. The volume is the cubic content of the resulting roof space compared with the cubic content of the original roof space, and both are taken to the outside faces, so the thickness of the roof structure, insulation and covering is included. Measuring internally, from plasterboard to plasterboard, understates the figure by a useful margin and is the single most common error.
For a flat-roofed rear dormer the calculation is straightforward. Take the external width, the external projection from the original roof plane, and the external height of the dormer box, and work out the volume of the resulting space added above the original roof slope. The part of the dormer that sits below the original roof plane is not an addition, because that space was already inside the roof.
For a pitched-roof dormer, break the shape into a rectangular prism and a triangular prism and add them.
Show the working. A drawing with the original roof profile shown as a dashed line, the new profile shown solid, and dimensioned sections through the dormer, demonstrates the calculation in a way a paragraph of text never will.
Where the figure lands within a couple of cubic metres of the limit, redesign it. A calculation challenged two years later, with a building already up, is not a position anyone wants to be in.
Earlier roof additions counting against you
Original means the house as it was built, or as it stood on 1 July 1948 if it is older than that. Every roof addition made since then reduces what remains.
That includes a hip to gable conversion carried out in 1995, a small dormer added in the 1970s, and a rear roof extension built by the previous owner under permitted development. All of it counts, and nobody keeps records.
Practical steps when the history is unclear:
- Order the historic Ordnance Survey mapping and any available aerial photography for the property.
- Search the council's planning history online, including applications that were refused or withdrawn, which often reveal what was built anyway.
- Look at the neighbours. In a terrace of identical houses, an unaltered neighbour shows you the original roof profile.
- Check the deeds pack and any building regulations completion certificates.
If the history cannot be established with confidence, treat the allowance as uncertain and apply for planning permission or a certificate rather than relying on an assumption. The council's enforcement position does not soften because the previous owner kept no paperwork.
Hip to gable and rear dormer combinations
The common loft conversion on a 1930s semi is a hip to gable enlargement plus a rear dormer, and the two together consume volume fast.
A hip to gable enlargement fills in the sloping hip end with a vertical gable, and the volume added is the wedge between the old hip slope and the new gable.
Illustrative example. Take a semi with a 7 metre span, a 35 degree pitch and a hip that runs back 3.5 metres to the end of the ridge. The gable triangle is 7 metres wide and about 2.45 metres high, giving roughly 8.6 square metres, and the solid filled in behind it works out at around 10 cubic metres. That leaves about 40 of the 50 cubic metre allowance for the dormer.
The order of construction makes no difference to the arithmetic. Both count.
Where both are proposed and the total exceeds the allowance, the usual resolutions are to reduce the dormer width, to set the dormer down from the ridge, or to accept that a planning application is needed. On a terrace house whose roof has already been altered once, a full-width box dormer often takes the total past 40 cubic metres, and the fact that half the street has one does not make it lawful.
Also remember the height restriction. Neither element may rise above the highest part of the existing roof, so a gable wall carried above the ridge line fails on that ground alone.
The principal elevation restriction
Nothing under Class B may extend beyond the plane of a roof slope that forms the principal elevation and fronts a highway. In practice this means no front dormer on a house whose front faces a road.
Two points of nuance. The restriction applies to the principal elevation, which is usually the front but is a question of fact about the building's architectural treatment rather than about which way the front door faces. And it requires the elevation to front a highway, so a house set behind other buildings, with its principal elevation facing a private drive that is not a highway, may fall outside the restriction. That is a narrow argument and worth checking rather than assuming.
Rooflights in the front slope are a different class. Class C permits roof alterations including rooflights, provided they project no more than 150 millimetres beyond the roof plane and do not rise above the highest part of the roof. Solar panels have their own measurement rule, and the 200 millimetre projection limit works in a similar way.
Recording the calculation for a certificate
If you are relying on Class B rather than applying for permission, document it properly at the time.
A lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works would be lawful. The application needs existing and proposed plans and sections, a volume calculation showing the original roof space and the resulting roof space with the method stated, evidence of the property's alteration history, and confirmation of the house type against the terrace house definition. A short supporting statement addressing each restriction in Class B in turn makes the officer's job straightforward, and the structure of a Planning Statement works well for it.
Where the house sits in a conservation area, Class B does not apply at all, so the route is a planning application, and a short Heritage Statement explaining how the dormer responds to the roofscape usually helps. Building regulations approval is needed for a loft conversion in any event, covering structure, fire escape and stairs, and that is a separate process from planning.
Enter your postcode on PlanScope to see which documents a loft conversion application is likely to need.
This is general guidance rather than 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
Is the dormer volume measured inside or outside?
Externally. The comparison is between the cubic content of the resulting roof space and the original roof space, taken to the outside faces, so the thickness of structure, insulation and roof covering is included. Measuring from plasterboard to plasterboard understates the figure and is the commonest calculation error.
Does an end-of-terrace house get 40 or 50 cubic metres?
Forty. The definition of terrace house extends to a house at the end of a row of three or more that shares a party wall with, or has a main wall adjoining, a house meeting the mid-terrace test. It is the row that matters, not how detached the house feels on one side.
Do previous roof alterations reduce my allowance?
Yes. Original means the house as built, or as it stood on 1 July 1948 if older. A hip to gable conversion, an old dormer or a rear roof extension built by a previous owner all count against the allowance. Check historic mapping, aerial photography, planning history and unaltered neighbours.
Can I build a dormer on the front of the house?
Not under Class B, where the front is the principal elevation and it fronts a highway, because nothing may extend beyond that roof plane. Rooflights in the front slope fall under Class C instead, provided they project no more than 150 millimetres and do not rise above the highest part of the roof.
Does Class B apply in a conservation area?
No. Class B does not apply on article 2(3) land, which includes conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. A dormer or hip to gable enlargement in those areas needs planning permission whatever its volume.
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.