Loft dormers under Part 1 Class B: volume and position
Class B of the GPDO 2015 allows a loft dormer without planning permission, inside a 40 or 50 cubic metre volume allowance, away from the principal elevation and set back from the eaves.
A rear dormer is usually permitted development under Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, provided the roof enlargement stays within 40 cubic metres on a terraced house or 50 cubic metres on any other house, sits away from the principal elevation, and does not rise above the existing ridge. Dormer permitted development is one of the few householder rights where the limit is a volume rather than a depth, and that single difference catches people out. The allowance is cumulative, it counts additions you never built yourself, and it is measured externally.
Flats and maisonettes have no Class B rights at all, because Part 1 applies only to a dwellinghouse. Houses in conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites lose Class B entirely under article 2(3). So the first two questions are what the building legally is, and where it sits.
What Class B allows and what it leaves out
Class B covers the enlargement of a dwellinghouse consisting of an addition or alteration to its roof. In practice that means a box dormer, a pitched dormer, a hip-to-gable enlargement, or a combination of the three on the same roof.
It does not cover rooflights. Those fall under Class C, which permits other alterations to the roof so long as nothing protrudes more than 150 millimetres beyond the plane of the existing slope and nothing rises above the highest part of the existing roof. Class C is the reason a conservation-area loft conversion can often still take conservation rooflights while the dormer itself needs an application.
Class B also excludes any verandah, balcony or raised platform. A Juliet balcony behind a dormer door, where nothing projects and there is no platform to stand on, is generally treated as acceptable. A cut-in terrace is not, and it is the single most common reason a loft scheme that looked like permitted development ends up as a householder application.
The 40 and 50 cubic metre allowances
The limit in B.1(d) is that the cubic content of the resulting roof space must not exceed the cubic content of the original roof space by more than 40 cubic metres for a terrace house, or 50 cubic metres in any other case.
Three words in that sentence do most of the work.
Original means the house as it was built, or as it stood on 1 July 1948 if it is older. Not as you bought it. If a previous owner added a side dormer in 1994, that volume comes off your allowance even though you did not build it and even if it has since been removed.
Resulting means the whole roof space after the works, measured externally, including the thickness of the walls and roof of the enlargement. Internal habitable volume is a smaller and different number, and quoting it is a reliable way to have a certificate refused.
Terrace house includes an end-of-terrace, and the technical guidance treats a house in a row of three or more as a terrace. A semi-detached pair gets the 50 cubic metre figure. Where a house has been extended sideways and now reads as part of a run, the categorisation is worth settling in writing with the council before the roof is designed, because the gap between 40 and 50 cubic metres is roughly the difference between a modest rear dormer and a full-width one. Our guide to how previous extensions count against your allowance covers the same cumulative logic on the Class A side.
| Limit | Terrace house | Any other house |
|---|---|---|
| Volume added to original roof space | 40 m3 | 50 m3 |
| Previous roof enlargements | Counted, including removed ones | Counted, including removed ones |
| Measurement basis | External, including walls and roof | External, including walls and roof |
Why front dormers almost never work
B.1(c) stops any part of the enlargement extending beyond the plane of any existing roof slope which forms the principal elevation and fronts a highway. Two conditions have to be met together for the bar to bite. The slope must form the principal elevation, and it must front a highway.
The principal elevation is normally the front of the house, the elevation with the main architectural treatment and usually the front door, whether or not that is where you actually enter. Corner plots complicate this, because a side elevation facing a road can also front a highway while not being the principal elevation, in which case Class B is not excluded on that slope. The related article on curtilage and principal elevation in Class A cases sets out how officers identify the principal elevation, and the same reasoning applies to roofs.
Highway includes public footpaths and bridleways as well as carriageways. A front roof slope facing an adopted footpath is still fronting a highway.
And where the front slope faces only a private drive or a rear service lane in private ownership, Class B may survive. That is a question of the status of the land, so check it on the highway authority's adopted highways record rather than assuming from appearance.
The eaves set back, the ridge and side windows
Condition B.2(b) requires that, other than in the case of a hip-to-gable enlargement, the edge of the enlargement closest to the eaves of the original roof is, so far as practicable, not less than 20 centimetres from the eaves, measured along the roof slope from the outside edge of the eaves. The measurement is along the slope, not vertically. Detailers routinely lose this when the dormer cheek is set out from the internal face of the wall rather than from the eaves line.
B.1(b) prevents any part of the enlargement exceeding the height of the highest part of the existing roof. Existing, not original, so a previously raised ridge sets the new ceiling. Flat-roof dormers that sit flush to a low ridge are where this is usually tested, and 50 millimetres of unallowed build-up in the roof construction is enough to break it.
Condition B.2(c) applies to any window installed in a wall or roof slope forming a side elevation of the dwellinghouse. It must be obscure glazed to at least level 3, and non-opening unless the opening parts are more than 1.7 metres above the floor of the room in which the window is installed. Level 3 glass is not frosted film, and a tilt-and-turn unit with a restrictor is still an opening part.
Materials, under B.2(a), must be similar in appearance to those of the exterior of the existing house. Tile-hung cheeks on a tile-hung house pass. A zinc box on a plain-tiled semi is a judgement call and the sort of thing worth resolving before the order is placed.
Evidencing the volume for a certificate
A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that a proposed dormer is lawful. It is not compulsory. It is also the document a conveyancing solicitor will ask for in five years, and the cost of obtaining one is a fraction of the cost of a retrospective application after a sale falls through.
A certificate application for a Class B dormer should carry:
- Existing and proposed roof plans, sections and elevations at a measured scale, with the eaves and ridge lines annotated.
- A cubic content calculation showing the original roof space volume, any previous additions with their volumes and dates so far as known, and the proposed addition, each set out as a dimensioned solid rather than a single figure.
- A plan identifying the principal elevation and the highway it fronts, with the adopted highway boundary shown.
- Confirmation that the property is not on article 2(3) land and is not subject to an article 4 direction removing Class B.
- Photographs of the existing roof from front and rear.
Illustrative example, for method only. A 1930s semi has an original roof space measured externally at 118 cubic metres. A previous owner added a side dormer of 11 cubic metres in the 1990s. The proposed rear box dormer measures 36 cubic metres externally. The cumulative addition is 47 cubic metres against a 50 cubic metre allowance, so the scheme is within Class B on volume, and the remaining tests are ridge height, the eaves set back and the side window condition. Change the house to a mid-terrace and the same scheme fails on volume alone.
Where Class B does not stretch far enough, the fall-back is a householder planning application with a short Planning Statement setting out the design response and the neighbour amenity position, which is a far easier conversation to have before the loft is boarded than after. PlanScope can draft that statement from your site details and the reports your project needs, listed at all 66 report types.
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
Does a rear dormer need planning permission?
Usually not on a house, if it stays within the 40 or 50 cubic metre Class B allowance, sits below the existing ridge, is set back 20 centimetres from the eaves and is not on the principal elevation fronting a highway. Flats have no such right, and article 2(3) land such as conservation areas loses Class B altogether.
How is the cubic content of a dormer measured?
Externally, including the thickness of the walls and roof of the enlargement, as a solid volume added to the original roof space. The original roof space is the house as built or as at 1 July 1948. Internal floor-to-ceiling volume is a different and smaller figure that councils will not accept.
Do previous loft alterations count against the allowance?
Yes. The test compares the resulting roof space with the original roof space, so every earlier enlargement counts, whether you built it or not. Additions that have since been removed still count. Deeds, historic aerial imagery and old applications on the council register are the usual way to evidence what was there.
Can I put a dormer on the front of my house?
Only with planning permission, in nearly all cases. Class B excludes anything extending beyond the plane of a roof slope that forms the principal elevation and fronts a highway. On a corner plot the side roof slope may still qualify, because it fronts a highway without forming the principal elevation.
Is a Lawful Development Certificate worth getting for a dormer?
It is the only formal confirmation that the works are lawful, and it is the document a buyer's solicitor will ask for. Applications are decided on the measured evidence rather than on planning merit, so the drawings and the volume calculation carry the whole case. The fee is set nationally and indexed, so check the current schedule.
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.