Class B roof enlargements and the 50 cubic metre limit

The Class B roof extension volume limit is 50 cubic metres for most houses and 40 cubic metres for a terrace house, measured as the amount by which the resulting roof space exceeds the original roof space. That one sentence decides most loft conversions. If your dormer fits inside the allowance, sits off the front roof slope, keeps below the ridge and your house is not in a conservation area, you are probably building under Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 and you need no planning application.

The conditions attached to it are where people come unstuck.

What Class B covers

Class B permits the enlargement of a dwellinghouse consisting of an addition or alteration to its roof. In plain terms, that is a dormer, a roof extension, a hip to gable widening, or a combination of the three.

It does not cover flats or maisonettes. Part 1 rights belong to dwellinghouses, so a top floor flat converting its loft needs planning permission whatever the volume. It also excludes a verandah, balcony or raised platform, which is why a Juliet balcony set into a new dormer usually fails Class B even when the volume works.

Rooflights are a different class. Where the work is limited to velux style windows that project no more than 150 millimetres beyond the plane of the roof slope and stay below the ridge, Class C is the relevant one and no volume allowance is consumed.

Measuring the 40 and 50 cubic metre allowances

The test compares the cubic content of the resulting roof space with the cubic content of the original roof space. Two words in that sentence do most of the damage.

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. Any earlier roof enlargement by a previous owner counts against your allowance, so a 1990s rear dormer on a semi leaves you about 20 cubic metres of headroom rather than 50.

Resulting means the roof space after the works, measured externally. External measurement includes the thickness of the walls and roof covering of the new dormer, which is why a cheek measured internally at 1.8 metres wide can push the volume past the limit.

A terrace house gets 40 cubic metres. The Order treats an end of terrace house as a terrace house for this purpose, which surprises people with a house at the end of a row of four.

An illustrative example. A rear box dormer on a 1930s semi, 4 metres wide, 2.4 metres high and 3 metres deep, measured externally, gives roughly 29 cubic metres. Add a hip to gable enlargement on the same roof and the combined figure will usually sit in the high 40s. That leaves almost nothing in reserve, so the drawings need to be measured rather than estimated. Treat the arithmetic as illustrative and have your designer calculate the actual figure from dimensioned sections.

The front roof slope exclusion

No part of the enlargement may extend beyond the plane of any existing roof slope that forms the principal elevation and fronts a highway. A front dormer on a house facing the road is therefore not permitted development, no matter how modest.

Highway is wider than it sounds. It includes public footpaths, bridleways and byways, not just carriageways. A house whose front elevation faces a public footpath across a green still has a highway-fronting principal elevation.

The principal elevation is normally the one with the main architectural emphasis and the front door, which on a corner plot may not be the one facing the busier road. Where the answer is arguable, that argument is worth having with the council before you build rather than afterwards.

The 0.2 metre set-back from the eaves

A condition most builders have never read. The edge of the enlargement closest to the eaves of the original roof must, so far as practicable, be not less than 0.2 metres from the eaves, measured along the roof slope from the outside edge of the eaves.

The only exception is an enlargement that joins the original roof to the roof of a rear or side extension. So a dormer carried down to meet the roof of an existing rear extension is fine, and a box dormer taken flush to the gutter on an unextended house is not.

Two further limits sit alongside it. No part of the works may exceed the height of the highest part of the existing roof, which rules out raising the ridge to gain headroom. And the materials must be similar in appearance to the existing house, which is a condition rather than guidance.

Side-facing windows and obscure glazing

Any window inserted on a wall or roof slope forming a side elevation must be obscure glazed, and must be non-opening unless the openable parts are more than 1.7 metres above the floor of the room in which the window is installed.

Obscure glazing means glazing you cannot see a clear image through, usually specified at level 3 or above on the standard obscuration scale. Frosted film applied to clear glass does not satisfy the condition, because the condition attaches to the window rather than to its current finish.

The 1.7 metres is measured from the internal floor level, which in a converted loft with a raised floor is not the same as the structural floor. Check it on the section drawing.

Designated land stops Class B entirely

Class B is not permitted on article 2(3) land. That means conservation areas, National Landscapes (formerly areas of outstanding natural beauty), National Parks, the Broads and World Heritage Sites.

There is no partial allowance and no smaller volume. In a conservation area every dormer and every hip to gable needs a planning application, which is a different exercise and usually needs a design rationale and, where the building contributes to the character of the area, a Heritage Statement addressing the effect on that character.

Permitted development rights can also be removed in areas that are not designated. An Article 4 direction, or a condition on the original permission for a newer estate, will do it. Check both before you design.

Evidence worth keeping, and the certificate route

Relying on Class B means you carry the burden of proving the works were lawful if anyone asks later, typically a buyer's solicitor eight years on. Keep dimensioned existing and proposed sections showing the volume calculation, photographs of the roof before work started, the obscure glazing specification for any side window, and a note of the eaves set-back as built.

If the margin is tight, or an earlier enlargement makes the sums arguable, apply for a lawful development certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990. It costs half the equivalent planning fee and it produces a formal determination that the works would be lawful. That certificate is the only way to be certain, and it costs a fraction of an enforcement dispute.

Where Class B does not work, the fallback is a householder planning application supported by a short Planning Statement explaining the design response to the roof form and to neighbouring amenity. The same volume discipline helps there too, because a dormer that nearly complies reads as a proportionate addition to an officer.

Compare this with the separate volume and area tests that apply to outbuildings, which work on curtilage coverage rather than cubic content, as our note on the 50 per cent curtilage rule sets out.

This is general guidance, not legal advice, and a lawful development certificate is the only formal way to confirm that works are lawful. Check the position with your local planning authority or a qualified professional before relying on it. If Class B does not cover your loft conversion, you can browse the report types by category to see what a householder application will need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.