Class B roof enlargement and the volume allowances

Class B of Part 1 of Schedule 2 to the GPDO 2015 allows you to enlarge the roof of a house by up to 40 cubic metres on a terrace house and up to 50 cubic metres on any other house, without planning permission, provided every limitation and condition is met. That allowance is the whole of the question for most loft conversions. Get the volume calculation wrong, or forget that the loft was already extended in 1998, and the dormer you have just built is unauthorised development rather than permitted development.

What counts as an enlargement of the roof

Class B covers an addition to or alteration of the roof of a dwellinghouse that enlarges the roof space. A rear dormer is the obvious case. So is a hip-to-gable conversion, which fills in the triangular hip with a vertical gable wall and adds the volume behind it. So is raising a rear roof slope, or joining the main roof to the roof of an existing rear outrigger.

What Class B does not cover sits in other classes. Alterations that do not enlarge the roof, such as rooflights and re-tiling, fall under Class C. Chimneys, flues and soil vent pipes fall under Class G. Solar equipment falls under Part 14.

Class B rights apply only to a dwellinghouse. Flats and maisonettes have no Part 1 permitted development rights at all, as set out in the explainer on why flats and maisonettes have no Part 1 rights. And Class B is not available on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites.

The cubic content allowances

Two figures, and the distinction between them turns on a definition rather than on appearance.

The Order defines a terrace house as a dwellinghouse in a row of three or more buildings used as dwellinghouses, where it shares a party wall with, or has a main wall adjoining the main wall of, the building on either side. An end-of-terrace house meets the same definition with the adjoining building on one side only. So an end terrace gets 40 cubic metres, not 50, which catches people out regularly.

A house in a row of two, meaning an ordinary pair of semis, is not a terrace house and takes the 50 cubic metre allowance.

Measuring volume correctly, including earlier additions

The allowance is measured against the original roof space, and original means as the house was built, or as it stood on 1 July 1948 if it is older than that.

This is cumulative. Any previous enlargement of the roof space counts against the allowance, whether you did it or a previous owner did, and whether it was done under permitted development or with planning permission. A 1998 rear dormer of 22 cubic metres leaves 18 cubic metres on a terrace house, which is not enough for a full-width box dormer on most plans.

Measure the added volume externally, to the outside faces of the new construction. The figure that matters is the increase in the volume of the roof space, so the calculation is the volume of the resulting roof space minus the volume of the original roof space.

Two traps in the arithmetic. People measure internally, which understates the volume by the thickness of the walls and roof build-up and can turn a 42 cubic metre dormer into a 38 cubic metre one on paper. And people forget that a hip-to-gable adds volume in a wedge shape that is easy to underestimate by eye. Have the designer produce the calculation as a dimensioned sketch with the volumes set out, not as a bare number.

Class B also has an absolute height limit that sits alongside the volume allowance. No part of the enlargement may exceed the height of the highest part of the existing roof. A gable wall built up level with the existing ridge is fine. A raised ridge is not.

The principal elevation restriction

No part of the enlargement may extend beyond the plane of any existing roof slope which forms the principal elevation of the house and fronts a highway.

Two elements have to be satisfied together. The slope has to form the principal elevation, and it has to front a highway. The principal elevation is normally the front of the house, the one with the main architectural treatment and usually the front door, and it does not change because the owner reorients the garden. A highway includes an adopted road, and in practice also footpaths and bridleways that are highways, so a house facing a public footpath can be caught.

The consequence is that a front dormer almost never falls within Class B. On a corner plot, where two elevations front highways, only the elevation that is the principal one triggers the restriction, though the side elevation window conditions still apply. This is the point where a lot of householders should stop and take advice rather than assume.

Set back from the eaves and the 200 millimetre rule

Where the enlargement is not a hip-to-gable, and does not join 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.

Measure that 200 millimetres along the roof slope, from the outside edge of the eaves, not vertically and not in plan. It is a small dimension with a visible purpose, which is to keep a strip of original roof slope below the dormer so the addition reads as sitting within the roof rather than replacing it.

The phrase "so far as practicable" gives a little room where the original eaves detail makes it impossible, but do not rely on it. Councils and their enforcement teams treat the 200 millimetres as a requirement, and the set-back is one of the first things measured when a complaint comes in.

Side windows, materials and the other conditions

Three conditions apply to any Class B enlargement.

Materials used in any exterior work must be of a similar appearance to those used in the construction of the exterior of the existing house. Similar appearance, not identical, which allows a lead-effect finish or a tile-hung cheek where the context supports it, though grey render on a brick house has been treated as failing it.

Any window installed in a wall or roof slope forming a side elevation of the house must be obscure-glazed, and non-opening unless the parts that open are more than 1.7 metres above the floor of the room in which the window is installed. Obscure glazing here means glass that cannot be seen through, and councils commonly refer to level 3 or above on the standard obscuration scale.

The enlargement must not consist of or include a veranda, balcony or raised platform. A Juliet balcony fitted to a dormer door, where the guarding sits in the plane of the wall and there is no standing area outside it, is generally accepted. A cut-out terrace in the roof slope is not.

Proving it later

Permitted development is a right, not a certificate. Nothing in the process creates a document confirming that what you built was lawful, which is a problem at the point of sale or when a lender's solicitor asks.

A Lawful Development Certificate for proposed use or development, under section 192 of the Town and Country Planning Act 1990, is the only formal way to confirm that works are lawful. Apply before you build, with the volume calculation, the section through the eaves showing the 200 millimetre set-back, and the elevations showing the obscure glazing. The fee is half the equivalent householder application fee, and the council either issues the certificate or tells you what fails, which is useful either way.

If the scheme falls outside Class B, the application that follows will need drawings and a short Planning Statement addressing the local plan amenity and design policies. You can check which other documents a householder application is likely to need by entering the postcode.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Permitted development rights can be removed by an Article 4 direction or by a condition on an earlier permission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.