Class B roof extensions and the cubic content allowance

Class B roof extension permitted development gives you an addition to the roof of a dwellinghouse of up to 40 cubic metres on a terrace house and 50 cubic metres on any other house, measured against the original roof space and counting everything anyone has added since. Nothing may go above the highest part of the existing roof, nothing may project beyond the plane of a roof slope that forms the principal elevation and fronts a highway, and none of it applies at all on designated land.

Those four limits decide almost every dormer case. The arithmetic is where people lose.

What Class B permits and what it does not

Class B of Part 1 of Schedule 2 to the General Permitted Development Order 2015 covers the enlargement of a house consisting of an addition or alteration to its roof. Rear dormers, side dormers on the flank of a semi-detached house, hip to gable conversions and roof enlargements that join the main roof to the roof of an existing extension all sit here.

Five things take a proposal outside it:

• Any part of the house exceeding the height of the highest part of the existing roof.
• Any part extending beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway.
• Cubic content above the allowance.
• A veranda, balcony or raised platform.
• The house being on article 2(3) land, which removes Class B entirely.

Rooflights are a separate matter. Adding a window to a roof slope generally falls under Class C, which has its own limits, including that it must not protrude more than 150 millimetres beyond the plane of the slope and must not be above the highest part of the roof. Solar panels on a roof slope sit under a different class again, with its own projection limit, discussed in the 200 millimetre projection limit for domestic solar panels.

The cubic content allowance and how it is measured

The allowance is 40 cubic metres for a terrace house and 50 cubic metres for any other dwellinghouse. A terrace house here means one in a row of three or more, and an end of terrace counts as a terrace house for this purpose. A semi-detached house does not.

Three points do most of the damage.

The allowance is cumulative. It counts every enlargement of the roof since the house was originally built, by anyone, including previous owners. A 1998 rear dormer of 22 cubic metres leaves you 18 or 28, depending on house type. If you cannot prove what the roof looked like originally, you cannot prove you are within the allowance, and the burden of proof sits with the applicant.

The measurement is of volume added to the roof space, taken externally. It is the difference between the cubic content of the resulting roof space and the cubic content of the original roof space, so it includes the thickness of the new walls and roof covering of the dormer, not just the usable room inside.

And it is volume, not floor area. A box dormer that looks modest on a floor plan can be large in cubic terms because it is full height across its width. Sketch the section and do the sum before you commit to a layout.

Present the calculation as a schedule with dimensions, a section drawing and the arithmetic set out, rather than as a bare figure. A statement that the dormer is 47 cubic metres is an assertion. A table showing length, width, height and the resulting volume is something the council can check.

The set back from the original eaves

Class B carries conditions as well as limits, and the eaves condition catches a lot of otherwise compliant dormers.

Other than for a hip to gable enlargement, or one joining the original roof to the roof of a rear or side extension, the edge of the enlargement closest to the eaves of the original roof must, so far as practicable, be at least 20 centimetres from the eaves, measured along the roof slope from the outside edge of the eaves.

The intent is that the dormer sits within the roof slope rather than sitting on the wall head. In practice it means detailing a strip of original roof below the dormer cheek, which is awkward on a shallow pitch and needs to be drawn rather than assumed. "So far as practicable" is not a general escape clause, and councils read it narrowly.

The other conditions apply throughout. The materials of the exterior must be of similar appearance to those of the existing house, which is why a rendered or fibre cement dormer on a brick and clay tile house is frequently the thing that fails. And any window in a wall or roof slope forming a side elevation must be obscure glazed, and non-opening unless the opening part is more than 1.7 metres above the floor of the room.

Why the principal elevation roof slope is off limits

Nothing under Class B may extend beyond the plane of an existing roof slope which forms the principal elevation and fronts a highway. So a front dormer on the street-facing slope needs planning permission.

Two definitions matter. The principal elevation is normally the front of the house as designed, which is usually but not always the elevation containing the main entrance and facing the road. Corner plots and houses turned side-on to the street produce genuine difficulty here, and it is worth asking the council rather than guessing.

"Fronts a highway" brings in the road, and a highway includes a public footpath or bridleway, not only a carriageway. A slope facing an adopted footpath at the side of the house can therefore be caught even though nobody would call it the front.

Where the front slope is off limits, the usual solutions are a rear dormer with rooflights to the front, or a full planning application. On a street with a consistent roofscape, expect the council to resist a front dormer on design grounds as well.

Hip to gable conversions

A hip to gable enlargement is permitted development under Class B where it stays within the limits, and it is exempt from the eaves set back condition, which is why it is often the simplest way of gaining usable roof volume.

But it consumes the allowance quickly. Squaring off a hipped end on a semi-detached house adds a triangular prism of volume that can run to 25 or 30 cubic metres before a dormer is added. Combine a hip to gable with a large rear box dormer and you are very often over 50.

There is also a party wall dimension on a semi-detached or terraced house that has nothing to do with planning. Building up to and along the boundary engages the Party Wall etc. Act 1996, with its own notice periods, and that is a separate process running to its own timetable.

Designated land and the loss of the right

On article 2(3) land, Class B does not apply. That covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. Any roof enlargement in those areas needs planning permission, however small.

Article 4 directions can also remove Class B in areas that are not otherwise designated, and a number of councils have made directions covering roof alterations in areas of consistent roofscape. Check both the designation mapping and the directions register before you draw anything. The way materials and finishes are treated differently on designated land also affects related work, including painting the exterior of a house.

Drawings that show compliance

A certificate of lawfulness of proposed development under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful, and on a Class B dormer it is worth the fee because the sum is contestable.

What a council needs to see:

1. Existing and proposed roof plans, elevations and a section through the dormer at a stated scale.
2. The original roof form, evidenced where the house has been altered, using historic mapping, old photographs or earlier permissions.
3. A dimensioned cubic content calculation, with any earlier enlargements identified and deducted.
4. A detail at the eaves showing the 20 centimetre set back measured along the slope.
5. A materials note comparing the proposed finish with the existing house.

Where the volume is close to the limit, design in a margin. A dormer at 49.6 cubic metres invites the council to measure it themselves, and a rounding disagreement at that point costs far more than the volume you gave up. Set out the position in a short Planning Statement accompanying the certificate application.

This is general guidance, not legal advice. Permitted development rights are removed in some areas by article 4 directions, so 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. Enter your postcode on PlanScope to check what your site needs.