Dormer windows under Class B: the volume allowance

The dormer permitted development volume allowance is 40 cubic metres for a terrace house and 50 cubic metres for any other house, measured as the increase in the cubic content of the roof space over the original roof. Those figures come from Class B of Part 1 of Schedule 2 to the General Permitted Development Order 2015, and they are cumulative: every earlier roof enlargement counts against them, including one carried out by a previous owner forty years ago.

Most loft conversions that fall outside permitted development do so on volume or on the set-back from the eaves, not on anything exotic. So the arithmetic is worth doing properly before the drawings are finished.

What Class B permits on a roof

Class B permits the enlargement of a house consisting of an addition or alteration to its roof. In practice that covers rear and side dormers, hip to gable enlargements, and raising a roof within the height limit.

It does not cover everything on a roof. Rooflights and other alterations that do not enlarge the roof space sit in a separate class with its own limits, chiefly that the glazing must not protrude more than 150 millimetres beyond the plane of the roof slope. And Class B does not apply to flats or maisonettes at all, only to houses.

The limitation that stops most front dormers is simple. No part of the enlargement may extend beyond the plane of any existing roof slope which forms the principal elevation and fronts a highway. A dormer on the front roof slope facing the street is therefore outside Class B, whatever its volume.

The cubic content allowance, and why terraces get less

The definition of terrace house in Part 1 is wider than people expect. It covers a house in a row of three or more buildings used as dwellings which shares a party wall with, or has a main wall adjoining the main wall of, the building on either side, and an end of terrace house with that relationship on one side only. So an end of terrace gets 40 cubic metres, not 50, and that catches out a great many applicants.

The comparison is against the original roof space. Original means as the house was built, or as it stood on 1 July 1948 for older properties. Not as you bought it.

Measuring the enlargement, including earlier work

The measurement is of cubic content, taken externally. That means the added volume includes the thickness of the dormer's walls and roof construction, not just the usable space inside.

Illustrative example. A full width rear box dormer on a 5 metre wide terrace house, rising 2.3 metres from the new flat roof to the floor of the loft and projecting around 3 metres back from the eaves line, adds roughly 30 cubic metres measured externally. That leaves about 10 cubic metres of the 40 cubic metre allowance. A modest side dormer on the same roof will use most of what remains, and an earlier rear extension with its own roof enlargement may have used it already. The figures here are illustrative and depend on the actual geometry, the pitch and the construction thickness, so measure the real building.

Earlier work is the trap. Three categories count against the allowance:

• Any previous dormer, however small.
• A previous hip to gable enlargement, which adds a large volume for no habitable gain.
• A previous roof raise or a rear addition that enlarged the roof space.

None of that is recorded anywhere convenient. The practical approach is to compare the current roof against the original terrace form, using the neighbours as the control. If the house next door has an unaltered hipped roof and yours is gabled, someone has already used volume.

Where the allowance has run out, the route is a householder planning application, usually supported by a short Planning Statement addressing the design and the effect on neighbouring amenity. That is not a bad outcome. Applications for rear dormers on unremarkable streets are routinely approved, and an application removes the risk of an enforcement dispute later.

The 20 centimetre set-back from the eaves

Class B carries a condition that, other than for a hip to gable enlargement, the edge of the enlargement closest to the eaves of the original roof must be set back, so far as practicable, not less than 20 centimetres from the eaves. The distance is measured along the roof slope from the outside edge of the eaves.

This is the condition that causes the most trouble on site, for two reasons. It removes the option of running the dormer face down to the gutter line, which is what most box dormers want to do. And the phrase "so far as practicable" invites argument about whether a particular construction could have achieved it.

Do not rely on impracticability. Design the set-back in, show it dimensioned on the section, and build it. A 200 millimetre strip of original roof slope below the dormer cheek costs almost nothing to construct and removes the whole issue.

The set-back applies to the eaves of the original roof. If an earlier owner lowered or altered the eaves, the measurement still runs from where the original eaves were, which requires some judgement about what was original.

The highest part of the existing roof

No part of the house, as a result of the works, may exceed the height of the highest part of the existing roof. For a conventional pitched roof that means the ridge.

Two points follow. A box dormer whose flat roof sits level with the ridge fails, because the parapet or upstand almost always finishes a little above it. Keep the dormer roof clearly below ridge level and dimension it.

And the comparison is with the existing roof, not the original one. Where the roof has previously been raised under a planning permission, the raised ridge is the reference point for Class B purposes, even though the volume comparison still runs back to the original roof space. Those two different baselines in the same class are a genuine quirk of the drafting.

Side-facing windows and obscure glazing

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

Three practical notes. Obscure glazing means glazing through which a person cannot see, not tinted or reflective glass. The 1.7 metre measurement is from the floor of the room in which the window sits, so a window in a sloping ceiling can satisfy it where a vertical dormer window cannot. And a side dormer serving a bathroom is usually straightforward, while a side dormer serving a bedroom that needs a view is not.

A rooflight in a side roof slope is subject to the same obscure glazing condition under the separate rooflight class, which surprises people fitting a conservation style rooflight to a flank slope.

Designated land, flats and Article 4 directions

Class B does not apply on article 2(3) land. That means conservation areas, National Parks, the Broads, National Landscapes (formerly areas of outstanding natural beauty) and World Heritage Sites, where any roof enlargement needs planning permission regardless of volume.

In a conservation area that application will be assessed on the effect on the character and appearance of the area, and a Heritage Statement describing the significance of the area and the effect of the dormer is normally expected. A front dormer in a conservation area is a hard case. A rear dormer on a roof slope not visible from any public vantage point is a much easier one, and the submission should say which it is with photographs to prove it.

Two further restrictions. Flats and maisonettes are outside Class B entirely, so a top floor flat owner wanting a loft conversion needs permission as well as the freeholder's consent. And an Article 4 direction can withdraw Class B for a defined area even where the land is not designated, which is increasingly common in streets with uniform roofscapes.

Check both before you draw anything. The constraint layers on a site, in the way described for the 50 per cent curtilage rule for garden outbuildings and for permeable surfacing under Class F, tend to interact, and a house with one restriction often has others.

Enter your postcode on PlanScope to see which documents a loft conversion application is likely to need if the volume allowance has run out.

This is general guidance, not legal advice. A Lawful Development Certificate is the only formal way to confirm that works are lawful, so check the position with your local planning authority or a qualified professional before relying on any of it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.