Rear dormers on a semi detached house: the rules

A rear dormer on a semi detached house is often permitted development under Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, but only if it stays below the existing ridge, keeps back at least 0.2 metres from the eaves, uses materials of similar appearance to the house and adds no more than 50 cubic metres to the original roof space. Fail any one of those and the dormer needs a householder application. The set back is the condition most often lost on site, because a roofer building for headroom will happily run the dormer cheek straight down to the gutter.

The conditions a dormer has to meet

Class B allows an addition or alteration to the roof of a dwellinghouse. It does not apply to a flat or a maisonette, which is why extending a flat works differently from extending the house next door.

The limits that matter on a two storey semi are these.

The volume figure catches people out. A terrace house gets 40 cubic metres and everything else gets 50, and a semi detached house is not a terrace house: the Order defines a terrace house as one in a row of three or more. A pair of semis is a row of two. So a semi gets the 50 cubic metre allowance, and an end of terrace in a row of three gets 40.

That allowance is measured against the original roof space, meaning the roof as built or as it stood on 1 July 1948. If a previous owner added a small side dormer in 1996, its volume comes off your figure first. Nobody keeps a record of this, which is why volume calculations on older houses are worth doing properly before the drawings are finalised.

Class B is also switched off entirely on article 2(3) land: conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. An Article 4 direction can remove it street by street anywhere else.

Set back from the eaves and the 0.2 metre rule

The condition reads that the edge of the enlargement closest to the eaves must, so far as practicable, be not less than 0.2 metres from the eaves, measured along the roof slope from the outside edge of the eaves. Hip to gable enlargements are the one exception.

Two things follow. The measurement runs along the slope, not vertically, so on a 40 degree pitch a 0.2 metre set back along the rafter is a much smaller distance in plan than people assume. And the words "so far as practicable" are not a general escape hatch. They cover the odd case where the existing eaves detail makes the set back physically impossible, not the common case where the builder wants another 200 millimetres of standing height.

In practice the set back is what keeps a strip of original roof tile visible below the dormer. Lose it and the dormer reads as a flat wall rising straight out of the gutter, which is both a breach and the thing that makes these conversions look wrong from the garden.

Full width dormers and why they are refused

Once you are making an application rather than relying on Class B, the test changes from arithmetic to design. Councils' householder design guides almost universally ask for a dormer that is subordinate to the roof slope: set down from the ridge, set in from both the party wall and the flank, and narrower than the elevation below it.

A full width box dormer fails all three. It removes the visible roof plane, it usually butts hard against the party wall so the two halves of the pair stop reading as a pair, and it reads as an extra storey rather than a roof alteration. On a matched semi detached pair this matters more than on a detached house, because the symmetry of the pair is the thing the officer is being asked to protect.

The fix is rarely a smaller room. Splitting one wide dormer into two, holding the cheeks 500 millimetres in from each end of the slope, and dropping the flat roof 300 millimetres below the ridge usually keeps the floor area while changing the judgement entirely.

Side cheeks, materials and cladding

Under Class B the materials must be of similar appearance to the existing exterior. That is a lower bar than "matching", and it is why lead, zinc, and slate or tile hanging on the cheeks all pass, while white uPVC boarding on a red clay tile roof generally does not.

On an application the officer will look at the cheek treatment first, because it is the largest new surface. Standing seam zinc or lead effect panels in a dark grey recede. The same cheek in render or plastic trim advances. Rooflights sit flush if they are conservation pattern with a central glazing bar, and the same reasoning applies to the frames you choose elsewhere on the house, as with replacement windows in an unlisted period house.

Side facing windows are worth planning early. Under Class B they must be obscure glazed and fixed shut below 1.7 metres, which rules out using a side window as the bathroom's only opening light.

Reading the neighbouring roofscape

Before drawing anything, walk the rear of the terrace or the pair from the garden and from any road or footpath behind. Photograph every roof you can see. If three neighbours already have set back dormers of a similar size, the argument writes itself and belongs in your Planning Statement as a short paragraph with the application references for each one.

If yours would be the first, expect the officer to weigh character harm more heavily, and expect the design to have to work harder. Set the dormer further down the slope, reduce its width, and match the flat roof upstand to the depth of the neighbour's fascia.

Rear roofs facing a railway, a service yard or dense tree cover are a different conversation from rear roofs facing a public park. Say which you have.

Escape from the new storey

Planning and Building Regulations run in parallel, and the loft conversion is usually where they collide. Approved Document B treats a new storey more than 4.5 metres above ground level differently from one below it: an openable escape window is no longer enough on its own, and the staircase has to become a protected route to a final exit, with fire doors to the rooms off it and mains powered interlinked alarms on every storey.

On a standard two storey semi the new loft floor almost always sits above 4.5 metres. That means fire doors on the first floor bedrooms, as well as at the top of the stairs, and it often means the existing stair is too open at ground floor level to be enclosed without moving a wall. Find this out before the planning drawings are fixed, because the enclosure changes the landing layout and sometimes the dormer position.

Drawings that show the set back

An officer refusing a dormer for want of information is a wasted eight weeks. The set that avoids it is small.

1. Existing and proposed rear elevations at 1:50, dimensioned from eaves to dormer cheek and from ridge to dormer roof.
2. A section through the dormer at 1:20 showing the 0.2 metre dimension measured along the roof slope.
3. A roof plan showing the dormer width against the full width of the slope, with the set in from each end dimensioned.
4. A materials note on the drawing itself, naming the cheek material and the window frame colour.
5. Street and rear context photographs, keyed to a location plan.

Where the application is a householder one, the narrative work sits in the design and access statement or, for a small scheme, a short covering statement. A Design and Access Statement is only formally required in limited cases, but a page explaining the set back, the width and the material choice is worth writing whether or not it is required.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. If the works have already been built, or you want certainty that a proposed dormer falls within Class B, a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm it, and an informal email from the planning department is not.

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