L shaped dormers on a terrace: what Class B allows
An L shaped dormer is one enlargement across two roof planes. Whether the 20 centimetre set back applies to the lower limb depends on whether the back addition is original.
An L shaped dormer in planning terms is one roof enlargement across two roof planes, and Class B of Schedule 2 Part 1 to the General Permitted Development Order 2015 tests it as a single operation. That means one volume allowance covering both limbs, one height limit taken from the highest part of the existing roof, and a set back condition that may apply to one limb and not the other depending on whether your rear back addition was part of the original house. Those three points decide whether the work needs permission, and they are the three most often got wrong on site.
How an L shaped dormer is formed
On a Victorian or Edwardian terrace, the rear of the house usually steps down into a narrower back addition, sometimes called an outrigger, with its own lower roof. An L shaped dormer continues the main rear dormer around the internal corner and out over that lower roof, giving a usable room plus a bathroom or stair at the back.
The construction sequence matters to the planning analysis. The dormer face on the main roof, the dormer face on the outrigger roof and the infill at the internal corner are all part of one enlargement of the roof space. You do not get two allowances because there are two limbs.
The corner infill is the piece people forget to count. Where the two dormers meet, the space between the old main roof slope and the old outrigger roof is filled in, and that wedge is usually the largest single volume in the scheme.
Counting volume across both limbs
The allowance is 40 cubic metres for a terrace house, including an end of terrace, and 50 cubic metres for anything else. It is measured externally, against the original roof space as the house was built or as it stood on 1 July 1948, and every earlier roof enlargement counts against it.
Take an illustrative calculation on a mid-terrace house. A main rear dormer 4.5 metres wide across a 40 degree roof slope, rising about 2.1 metres from its base, adds a triangular wedge of roughly 2.6 square metres in section, so about 12 cubic metres. A dormer over a 2.5 metre wide outrigger, rising about 1.8 metres, adds roughly 6 cubic metres. The corner infill on a typical plan adds several more. These figures are illustrative only, derived from the roof geometry, and your own roof needs measuring from the drawings.
The useful conclusion is that a straightforward L shaped dormer on a terrace often sits inside the 40 cubic metre allowance with margin to spare. What pushes it over is history: an earlier side dormer, a previous rear box, or a roof alteration carried out before the current owner bought the house.
| Element | What drives the volume | Easiest way to reduce it |
|---|---|---|
| Main rear dormer | Width, and height above its base | Pull in from the party walls |
| Outrigger dormer | Width of the back addition, and height | Keep the face in line with the outrigger wall |
| Corner infill | Plan area of the internal corner | Set the junction back from the corner |
| Earlier enlargements | What previous owners built | Nothing, but it must be found and counted |
The set back, and whether your back addition is original
Class B requires the edge of the enlargement closest to the eaves of the original roof to sit at least 20 centimetres back from those eaves, measured along the roof slope, so far as practicable. There is an express exception for an enlargement joining the original roof to the roof of a rear or side extension.
That exception is where the terrace case turns, and it depends on a question of fact about your house.
If the back addition was built as part of the original house, which is the usual position on a Victorian terrace, then its roof is part of the original roof. The enlargement is not joining the original roof to an extension roof, so the exception does not apply, and the 20 centimetre set back is required at the eaves of the outrigger roof as well as at the eaves of the main roof.
If the back addition is a later extension, the exception does apply to the part of the enlargement that joins the two roofs together, and the set back is not required at that junction.
Establish which you have before the drawings are finalised. Historic Ordnance Survey mapping, the original terrace layout repeated along the street, and the construction of the party wall all help. Where it is uncertain, draw in the set back on both limbs. The cost is a slightly smaller room and the benefit is a scheme that is lawful on either reading.
Height: the limit is the main ridge, not the outrigger
No part of the enlargement may exceed the height of the highest part of the existing roof. On a terrace, the highest part is the main ridge, not the outrigger ridge.
So a dormer over the back addition may rise above the outrigger's own ridge line, as long as it stays below the main ridge. That is counter-intuitive and it is what makes the outrigger limb usable: without it there would be no headroom over a narrow back addition at all.
Two practical constraints still apply. The dormer roof has to drain somewhere, and a flat roof tucked beside the main roof slope creates a valley that needs designing rather than improvising. And structurally the outrigger walls are often a single brick thick, so the new roof load needs a beam rather than a bearing.
The party wall, and the two regimes that apply to it
Class B imposes no set back from a party wall. A full width box dormer running right up to the party wall at both sides is permitted development, provided the volume, height and other conditions are met.
That is the planning position. The Party Wall etc. Act 1996 is a separate regime, and work that cuts into, raises or otherwise affects a party wall requires notice to the adjoining owner, with a notice period set by the Act. The two have nothing to do with each other: a lawful permitted development dormer still needs the party wall procedure followed, and a party wall award does not grant any planning right.
Where two outriggers back onto each other along a party wall, the shared valley gutter becomes the awkward detail. Agree access and the gutter arrangement with the neighbour early, because the alternative is a dispute conducted from scaffolding.
Why neighbours object, and what reduces it
L shaped dormers are among the most commonly objected-to householder works, and the objections are predictable.
Bulk seen from the neighbouring garden and from their upper windows, because the outrigger limb sits close to the boundary and is seen obliquely. Overlooking from the new openings, particularly where the outrigger dormer faces along the rear of the terrace rather than straight out. The effect on a consistent terrace roofscape where no other house has been altered. And disruption during the works.
Four design moves answer most of this.
- Set the dormer in from the party wall by half a metre or so, even though Class B does not require it. It reads far better from below and removes the continuous wall effect.
- Keep a clear margin below the main ridge rather than running up to the limit.
- Match the roof and wall materials to the existing house, which is a Class B condition in any event.
- Obscure glaze and restrict the openings in any side-facing cheek, which is also a Class B condition, and consider the position of rear openings relative to the neighbour's windows. The same overlooking logic applies to Juliet balconies, terraces and overlooking.
Where the scheme needs an application rather than falling within Class B, the local design guide takes over, and many authorities resist full width flat-roofed boxes on terraces. In a conservation area Class B does not apply at all, the work needs permission, and a Heritage Statement assessing the effect on the character and appearance of the area will be expected. Rear roofscape is frequently identified in conservation area appraisals, so do not assume the rear is invisible. The same reasoning that makes front extensions harder than rear ones applies to roofs facing a public vantage point.
The calculation to submit
Whether you are applying for a lawful development certificate or a householder permission, the volume calculation is the document that settles the argument.
- A method note stating that volumes are measured externally and that the comparison is with the original roof space.
- Sections through both limbs, dimensioned, with the 20 centimetre set back shown where it applies and the main ridge marked as a datum.
- A roof plan showing the corner infill area.
- A schedule of earlier roof alterations with planning references, or a statement that none exist and the evidence for that.
- A short Planning Statement explaining the original footprint and the set back analysis where the back addition's status is arguable.
A certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that the works would be lawful without permission, and on an L shaped dormer, where the analysis depends on a factual question about the original house, it is worth having.
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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.
Frequently asked questions
Does an L shaped dormer get two volume allowances?
No. It is one enlargement of the roof space, so a single allowance applies: 40 cubic metres for a terrace house including an end of terrace, and 50 cubic metres otherwise. The corner infill between the two limbs counts too, and it is usually the largest single volume in the scheme.
Does the 20 centimetre set back apply over the back addition?
It depends on whether the back addition is original. If it was built with the house, its roof is part of the original roof and the set back applies at its eaves. If it is a later extension, the exception for an enlargement joining the original roof to an extension roof applies at that junction.
Can a dormer over the outrigger rise above the outrigger ridge?
Yes, provided it stays below the highest part of the existing roof, which on a terrace is the main ridge rather than the outrigger ridge. That reading is what makes the lower limb usable, because a narrow back addition would otherwise offer no headroom at all.
Can the dormer go right up to the party wall?
Class B imposes no set back from a party wall, so a full width dormer is permitted development if the volume, height and other conditions are met. The Party Wall etc. Act 1996 is a separate regime, and work affecting the party wall still requires notice to the adjoining owner under that Act.
What if the house is in a conservation area?
Class B does not apply on designated land, so the dormer needs planning permission and usually a heritage statement. Rear roofscape is often identified as a defining feature in conservation area appraisals, so do not assume a rear dormer is invisible or uncontroversial because it faces away from the street.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.