Roof extensions and dormers under Class B

A loft conversion with a rear dormer can be permitted development under Class B of Schedule 2 Part 1 to the Town and Country Planning (General Permitted Development) (England) Order 2015, but the volume allowance is tight: 40 cubic metres for a terrace house and 50 cubic metres for anything else, measured as the increase on the original roof space. Nothing may go above the height of the highest part of the existing roof, nothing may break the plane of the front roof slope where that slope fronts a highway, and the dormer must be set back at least 20 centimetres from the eaves.

On designated land, Class B does not apply at all. Conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites all need a householder application for any roof enlargement.

The volume allowance and how it is counted

The allowance is an increase in cubic content, not a floor area and not a percentage.

An end-of-terrace counts as a terrace house for this purpose, which surprises people. The volume is the increase in the cubic content of the roof space, measured externally, so the thickness of the dormer walls and roof counts towards it. A dormer measured internally will understate the figure by a cubic metre or two, which is enough to matter on a terrace.

Crucially the allowance is cumulative against the original roof. Original means the roof as first built, or as it stood on 1 July 1948 if the house is older. A previous owner's dormer, a hip-to-gable done in 2004, even a modest box dormer over a bathroom, all count against your 40 or 50 cubic metres. On a house that has already had roof work, the remaining allowance can be close to nothing, and the article on measuring the volume allowance for a loft dormer works through the arithmetic.

Measure before you design. A rear box dormer across the full width of a modest terrace will often come out at 25 to 35 cubic metres on its own.

The front roof slope and the highway

Class B does not permit any part of the enlargement to extend beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway.

Two words do the work here. Principal elevation is usually the front of the house, but not always, and on a corner plot or a house set at an angle to the road the identification is a judgement. Fronts a highway includes a footpath, a bridleway and an unmade adopted road, so a house whose front faces a pedestrian alley may still be caught. The piece on roof extensions and the elevation fronting a highway goes through the hard cases.

Rooflights in the front slope are a different matter, dealt with under Class C rather than Class B.

The eaves set-back and the ridge limit

Two dimensional rules catch more dormers than the volume allowance does.

The first is the eaves set-back. 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. That strip of original roof has to remain. A dormer taken down to the gutter line fails, and this is the single most common breach on built dormers.

The second is the ridge. No part of the enlargement may exceed the height of the highest part of the existing roof. On a house with a hipped roof the existing ridge is the reference, and a dormer with a flat roof that sits level with the ridge is at the limit, so any upstand or parapet takes it over. Where you want more head height, the answer is a lower dormer floor or a planning application, not a few extra centimetres at the top.

Verandahs, balconies and raised platforms are excluded from Class B entirely. A Juliet balcony with no walkable platform is generally accepted. A terrace cut into the roof is not, and that distinction has caught out a lot of loft schemes designed around an outdoor space.

Side-facing windows and the glazing condition

Any window in a wall or roof slope forming a side elevation of the enlargement must be obscure-glazed, and non-opening unless the opening part is more than 1.7 metres above the floor of the room it serves.

That applies to the window, not just to the view. Obscure glazing means glazing through which you cannot see, commonly specified at level 3 or above on the standard obscuration scale, and clear glass with an applied film does not satisfy the condition on a long view. The condition applies from the moment the work is done, so replacing the obscure unit with clear glass two years later is a breach.

Materials are the other standing condition: the exterior must be of a similar appearance to the existing house. Tile-hung and lead-clad dormers on a slate roof are usually fine. A render finish on a brick-and-tile terrace is where arguments start.

Hip to gable conversions

Filling in a hipped roof to form a gable is permitted under Class B, subject to the same volume allowance, the same ridge limit and the same exclusions.

The problem is arithmetic. A hip-to-gable on a typical semi consumes a large share of the 50 cubic metre allowance before any dormer is added, and a double hip-to-gable on a detached house will usually exceed it. Where the scheme needs both a gable and a rear dormer, run the numbers properly before committing, because discovering the overrun after the scaffold is up is expensive.

The eaves set-back condition is relaxed for a hip-to-gable enlargement, and the side-elevation glazing condition does not apply to it in the same way, which is a point worth checking in the current text of the Order rather than taking on trust.

Rooflights under Class C compared

Class C covers any other alteration to the roof of a dwellinghouse, which in practice means rooflights and re-roofing.

The limits are different and much simpler. The alteration must not protrude more than 150 millimetres beyond the plane of the roof slope, must not be higher than the highest part of the roof, and side-facing windows must be obscure-glazed and non-opening below 1.7 metres. Class C is available on designated land, unlike Class B, which is why a conservation area loft conversion lit entirely by conservation rooflights is often the only permitted development route available.

Flush-fitting conservation rooflights are designed to sit within that 150 millimetre limit. Standard rooflights on an upstand frequently do not.

When to apply instead

Apply where the volume allowance is used up, where the house is on designated land, where the design needs a terrace or a front dormer, or where the remaining allowance is so close to the limit that a site measurement could tip it.

For anything marginal, a lawful development certificate for proposed development under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful. It costs less than a householder application and it gives you a document a future buyer's solicitor will accept. A Planning Statement setting out how the dormer sits against the rhythm of the terrace makes a householder application easier to recommend.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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