Class B roof extensions and the highway-facing roof slope

The Class B roof extension dormer rules allow you to enlarge a dwellinghouse by altering its roof, but not on the roof slope that forms the principal elevation and fronts a highway. That single exclusion, in paragraph B.1 of Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, is why rear dormers are usually permitted development and front dormers almost never are. The wording rewards close reading, because a front roof slope that does not front a highway is a different case from one that does.

What Class B permits and what it excludes

Class B permits the enlargement of a dwellinghouse consisting of an addition or alteration to its roof. Dormers, hip to gable conversions and the raising of a roof over a rear outrigger all sit here.

Paragraph B.1 then removes that permission in five situations. Where any part of the dwellinghouse would exceed the height of the highest part of the existing roof. Where any part would extend beyond the plane of a roof slope forming the principal elevation and fronting a highway. Where the cubic content of the resulting roof space would exceed the original by more than 40 cubic metres for a terrace house or 50 cubic metres in any other case. Where the works would include a verandah, balcony or raised platform. And where the dwellinghouse is on what the Order calls article 2(3) land.

That last exclusion catches more houses than people expect. Article 2(3) land covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, so a dormer anywhere in a conservation area needs planning permission as a starting point.

Class B applies to dwellinghouses only. A flat or maisonette has no Part 1 rights at all, which rules out roof extensions to a converted building however modest they are.

The restriction on roof slopes fronting a highway

Two conditions have to be met together for the exclusion to bite. The slope must form the principal elevation, and that elevation must front a highway.

"Principal elevation" is defined in article 2(1) of the Order, and a building has only one. It is normally the front, identified by its design and setting rather than by which way the front door faces, so a house whose entrance is at the side still has a principal elevation at the architectural front.

"Highway" takes its ordinary meaning, which under the Highways Act 1980 includes the footway, the verge and the land up to the highway boundary. An unadopted road over which the public has a right of way is still a highway.

So the useful question is whether the principal elevation actually fronts one. A house at the end of a long private drive, with no public highway in front of it, can have a front dormer as permitted development because the second limb fails. A house on a corner plot whose side roof slope faces the second road is also outside the exclusion, because a side slope is not the principal elevation, though the other limitations still apply.

Get this wrong in either direction and the consequences differ. Build a front dormer that was not permitted and you have unauthorised development. Apply for one you did not need and you have paid a fee and lost eight weeks.

Volume allowances for terraced and other houses

The allowance is measured as the increase in the cubic content of the roof space, not as the volume of the dormer, and the comparison is against the original roof space rather than the roof as it stands today.

"Terrace house" is defined in the interpretation provisions for Part 1 and includes end of terrace properties, so an end house in a row of three or more takes the 40 cubic metre figure rather than 50. Semi-detached houses take 50.

Earlier enlargements eat the allowance. A 1990s rear dormer of 25 cubic metres leaves 15 for a terrace house, whatever the planning history says about who built it. The measurement and the common errors are set out in our note on Class B roof extensions and the cubic content allowance.

Measure externally and show the calculation on a section drawing. An officer who can follow the arithmetic will not ask for a measured survey.

The 20 centimetre set back from the eaves

Paragraph B.2 imposes conditions, and the eaves set back is the one most often missed on site.

The edge of the enlargement closest to the eaves of the original roof must, so far as practicable, be not less than 20 centimetres from the eaves, measured along the roof slope from the outside edge of the eaves. The phrase "so far as practicable" gives a little latitude and should not be read as optional.

Two exceptions apply. A hip to gable enlargement is outside this condition, as is an enlargement joining the original roof to the roof of a rear or side extension. Those are the cases where a set back would be geometrically meaningless.

The practical effect is that a dormer cannot be built off the eaves line. On a shallow roof that 200mm costs headroom at the critical point, which is usually where the design has to change rather than where the condition can be argued away.

Hip to gable conversions and side dormers

A hip to gable enlargement is permitted under Class B subject to the same volume and height limits. The new gable must not exceed the height of the highest part of the existing roof, which in practice means the gable rises to the existing ridge and no further.

Side-facing windows attract their own condition. Any window inserted in a wall or roof slope forming a side elevation must be obscure-glazed and non-opening, unless the openable parts are more than 1.7 metres above the floor of the room the window serves. That applies to a side dormer cheek window and to a window in a new gable.

Double hip to gable conversions on a semi-detached pair raise a practical point rather than a legal one. Each house is assessed on its own rights, so one owner can proceed without the other, and the result can be an awkward junction at the party line. Worth a conversation before the scaffolding goes up.

Materials and the similar appearance condition

The materials used must be of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse. Similar, not identical, and the comparison is with the existing house rather than with the street.

In practice that means tile or slate cheeks matching the roof covering, or lead or zinc where that is the established local detail, and a window frame section comparable to the existing. A dormer clad in uPVC panels on a brick and clay tile house will not satisfy the condition, and a breach of a B.2 condition takes the whole development outside Class B rather than merely requiring remedy of the cladding.

Keep evidence. Photographs of the existing roof and a delivery note naming the tile match are what you need if a neighbour queries it later.

When a rear dormer still needs an application

Even a modest rear dormer needs planning permission in any of these situations:

• the property is in a conservation area, National Park, National Landscape, the Broads or a World Heritage Site
• an Article 4 direction has removed Class B
• the building is a flat or maisonette
• a planning condition on an earlier permission removed permitted development rights, which is common on new estates and on conversions
• the volume allowance is already used up
• the design includes a balcony or raised platform
• the dormer would rise above the existing ridge

Converted dwellings are the trap that catches investors. Rights are frequently restricted or absent, as explained in permitted development rights on a converted dwelling, and the general limitations in article 3 apply on top of everything in Class B, which we cover in Article 3(5), limitations and conditions in the GPDO.

Where the answer is an application, a short Planning Statement addressing the council's design guide criterion by criterion is worth more than a long one. And remember that permitted development says nothing about Building Regulations, which apply to the structure, insulation, stairs and means of escape regardless, as set out in permitted development and building regulations compared.

A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful without permission. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.