Class B roof extensions and the 20cm eaves set-back

Class B of Part 1, Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 lets you add to or alter the roof of a house without applying for permission, within a volume allowance of 40 cubic metres for a terraced house and 50 cubic metres for anything else. The condition people trip over is the set-back: the edge of the enlargement nearest the eaves of the original roof has to be, so far as practicable, at least 20 centimetres from those eaves, measured along the roof slope. Class B loft dormer permitted development stands or falls on that detail and on the volume sum.

The 20 centimetre rule is why box dormers on permitted development schemes stop short of the gutter rather than running down to it.

What Class B allows on a roof slope

Class B covers the enlargement of a house consisting of an addition or alteration to its roof. In practice that means rear or side dormers, a hip to gable widening, and raising a roof within the limits. Development is outside the class if:

• any part of the works would be higher than the highest part of the existing roof
• any part would extend beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway
• the cubic content of the resulting roof space exceeds the original roof space by more than 40 cubic metres (terraced) or 50 cubic metres (other houses)
• the works include a veranda, balcony or raised platform
• the house is on article 2(3) land

That last exclusion matters more than any of the measurements. On article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, Class B does not apply at all. A dormer in a conservation area needs planning permission, whatever its size.

Flats and maisonettes are outside Part 1 entirely, so Class B is no help in a converted building.

The volume allowance and how it is measured

The comparison is between the roof space you end up with and the roof space of the original dwellinghouse, meaning the house as built or as it stood on 1 July 1948. The allowance is cumulative across all roof additions since then.

Two consequences follow. A 1990s rear dormer already on the house eats into what is left, so the available volume for a new side dormer may be far smaller than the headline figure. And a terraced house gets 10 cubic metres less than a semi or detached house, which on a narrow London terrace is often the difference between a usable dormer and a token one.

Have the volume calculated and recorded on the drawings, with the original roof form shown. A set of plans that simply asserts compliance is worth very little if the council later takes a different view.

The 20 centimetre set-back from the original eaves

The condition applies to the edge of the enlargement closest to the eaves of the original roof, and the 20 centimetres is measured along the roof slope from the outside edge of the eaves. The wording includes "so far as practicable", which gives a little room where the roof geometry makes it impossible, but do not treat that as a general excuse.

There is an exception. The set-back does not apply to a hip to gable enlargement, because the new gable necessarily runs out to the edge of the roof.

In design terms, the set-back produces a strip of original roof below the dormer cheek. That strip has to be detailed properly, with the tiling, flashing and gutter arrangement resolved, because a poorly executed junction there is the most common defect on permitted development dormers.

No extension beyond the plane of the principal elevation

A front dormer on a roof slope that forms the principal elevation and fronts a highway is outside Class B. This is the clause that rules out most front dormers.

Two points of detail. The exclusion applies to the plane of the roof slope, so a rooflight sitting within the slope is a different matter and falls under Class C rather than Class B. And "fronts a highway" is not simply the front of the house: a corner plot can have more than one elevation fronting a highway, and a public footpath can be a highway.

Where a front dormer is what the conversion needs, the route is a householder application, and a short Planning Statement addressing the local plan design policies and any householder design guide is what the council will look for.

Materials and side-facing window obscuring

Two conditions attach to any Class B enlargement:

1. The materials used must be of similar appearance to those of the exterior of the existing house
2. Any window in a wall or roof slope forming a side elevation must be obscure-glazed, and non-opening unless the opening parts are more than 1.7 metres above the floor of the room

The second is a condition, not a limitation, so a clear glazed side window does not simply make the dormer unlawful in part. It puts the development outside the class. Fit the obscure glazing from the outset, and record it on the drawings. The same requirement appears as a condition on many householder permissions, which is explained in upper floor side windows and the obscure glazing condition.

Rooflights and window replacements in existing openings follow different rules again, covered in replacement windows and doors: when they are development.

Hip to gable and the volume it consumes

A hip to gable conversion is attractive because it creates standing height across the width of the roof. It is also the hungriest use of the allowance, because filling in the hip adds a large solid volume before you have added a single dormer.

On a semi-detached house with a 50 cubic metre allowance, a full hip to gable on one side can take a substantial share of it, leaving little for a rear dormer. Calculate both together before committing to either. Where the sum does not work, the options are a smaller dormer, a partial hip to gable that retains part of the hip, or a planning application for the scheme you actually want.

The set-back exemption for hip to gable work is a genuine advantage, so where the volume is available it usually produces a tidier result than a dormer squeezed into a shallow slope.

Where Class B does not apply

Keep a short list in mind before you measure anything: article 2(3) land, flats and maisonettes, houses created under the prior approval classes in Part 3, and any property where an Article 4 direction has removed householder rights. Read the Article 4 direction itself, because its schedule defines exactly which rights have gone.

Building Regulations apply separately and independently, and a loft conversion engages means of escape, stair geometry, structure, insulation and fire separation. A lawful development certificate for proposed works under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that a loft scheme is permitted development.

If the conversion needs an application, PlanScope drafts the Planning Statement and Design and Access Statement from a postcode and a short set of answers, flagging site facts it does not hold rather than guessing. Enter your postcode on PlanScope to see what your application needs.

This is general guidance, not legal advice; 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.