Southwark writes its dormer rules in metres, and they change if the street is in a conservation area. We check the street first, then the dimensions, then draw.
Photo: Bill Boaden, BY-SA 2.0
It depends on where the house stands, and the SPD sets this out in a table. For a house outside a conservation area that is not listed, permitted development applies within set dimensions. The Householder SPD of July 2025 gives them as a dormer of up to 40 cubic metres on a terraced property and 50 cubic metres on a semi-detached or detached one. The dormer must not be on the street-facing slope, must not be taller than the ridge, must sit at least 20 centimetres up the roof plane from the original eaves and must not overhang the outer face of the wall.
Flats and maisonettes have no permitted development, and in a conservation area planning permission is always required. The SPD says dormers in conservation areas should sit within the existing roof and not dominate it. That means they are symmetrically placed, at least 0.5 metres below the ridge, 1 metre above the eaves, and set in 1 metre from the shared boundary and the edge of the roof. Overly large box dormers will not be permitted in conservation areas or on listed buildings.
The statutory test sits behind the council's table. Class B of the permitted development order gives the 40 and 50 cubic metre figures, and it removes the right if the work would go above the highest part of the roof or beyond a roof slope that fronts the highway. A dormer that is 5 centimetres over any of these limits needs consent, which is why the survey and a volume calculation come before the design.
The Householder SPD gives different tests for dormers, side dormers and mansards.
| Roof extension | What Southwark says | Source |
|---|---|---|
| Rear dormer, permitted development | 40 cubic metres terrace, 50 cubic metres other houses; 20 cm up from the eaves | SPD 3.6.1 |
| Rear dormer, conservation area | 0.5 m below ridge, 1 m above eaves, 1 m in from boundary and roof edge | SPD 3.6.2 |
| Side dormer | Not encouraged unless an original or common feature of the street | SPD 3.6.3 |
| Mansard | Always needs permission; behind the parapets at no more than 70 degrees; not on an outrigger | SPD 3.6.4 |
Southwark Council, Householder SPD, July 2025, section 3.6.
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The SPD says all mansard extensions require planning permission. They should sit behind the front and rear parapet at a maximum angle of 70 degrees, return to match the parapet on a corner house, and will not be permitted if they extend onto an outrigger. A terrace with a parapet roof can take one; a terrace with a rear outrigger cannot.
Southwark's Article 4 page says you must apply for planning permission to develop in the Sunray Estate conservation area in north Dulwich, covering Sunray Avenue, Sunray Gardens, Casino Avenue, Nairne Grove, Red Post Hill and part of Herne Hill. For example, a house on Nairne Grove cannot use the 40 or 50 cubic metre right at all.
Southwark aims to decide an application 8, 13 or 16 weeks after registration depending on type. In the year to June 2026 it decided 435 householder applications with prior approvals set aside and 424 inside 8 weeks.
Every figure is linked to the page or document we read.
Southwark's Householder SPD of July 2025 says permitted development allows a rear dormer of up to 40 cubic metres on a terraced house and 50 cubic metres on a semi-detached or detached house.
Source: Southwark Council, Householder SPD, July 2025, section 3.6.1In a conservation area the SPD asks for rear dormers at least 0.5 metres below the ridge, 1 metre above the eaves and set in 1 metre from the shared boundary and the edge of the roof.
Source: Southwark Council, Householder SPD, July 2025, section 3.6.2The SPD says mansard extensions should be no steeper than 70 degrees and will not be permitted if extending onto an outrigger.
Source: Southwark Council, Householder SPD, July 2025, section 3.6.4Southwark's Sunray Estate conservation area in north Dulwich has an Article 4 direction, so planning permission is needed to develop there.
Source: Southwark Council, Article 4 directions: Sunray EstateSouthwark aims to decide an application 8, 13 or 16 weeks after registration, depending on the application type.
Source: Southwark Council, After you submit your planning applicationSouthwark decided 435 householder applications between July 2025 and June 2026, prior approvals excluded, and 424 of them were decided within 8 weeks.
Source: MHCLG, Planning application statistics PS2, July 2025 to June 2026Class B permits a roof extension of up to 40 cubic metres on a terraced house and 50 cubic metres in any other case.
Source: legislation.gov.uk, GPDO 2015, Schedule 2, Part 1, Class BTell us about your project and get a fixed, no-obligation quote back within one working day. If you don't need planning permission at all, we'll tell you that too — for free.
Yes if the house is not a flat, is outside a conservation area and is not listed, and the dormer stays within 40 cubic metres on a terrace or 50 on a semi or detached house, below the ridge, off the street slope and at least 20 centimetres up from the eaves.
Planning permission is required. The SPD asks for the dormer to sit within the roof, symmetrically placed, at least 0.5 metres below the ridge, 1 metre above the eaves and set in 1 metre from the shared boundary and the edge of the roof.
Always with planning permission. The SPD says they should sit behind the front and rear parapet at a maximum of 70 degrees and will not be permitted if they extend onto an outrigger.
Not encouraged, unless an original or common feature of properties in the street. Where they are allowed, they should sit within the slope of the roof and well clear of any hips and verges.
£765 inc survey (£565 if you supply measured drawings) for a rear dormer. That covers the measured survey, the drawings and the submission, with free revisions until the council decides. Southwark's own application fee is separate.
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