Lambeth's Design Guide grades each kind of dormer, from the linked pair to the box. We pick the type your house can carry, then draw it with the heights the guide asks for.
Photo: Bill Boaden, BY-SA 2.0
Lambeth's Design Guide Part 4 of August 2023 sorts dormers by type, and each type has its own test. In general, Policy Q11 I wants dormers appropriately sited and subordinate to the host building, generally supported at the rear and resisted on front roof pitches where they are not a characteristic feature. Dormers are called the most appropriate way to provide roof accommodation in conservation areas, but need great care.
For a conventional pitched roof the guide gives five design tests. The dormer should be lower than the windows on the elevation below and set in from the sides of the roof, so the roof stays the dominant element. Its window cill should rest on the roof slope, around 1 metre above the attic floor, and its head should be at least 2.1 metres above floor level. Window type should match the main building, construction should be simple and robust, and the dormer should be modest and aligned with the openings below. The front face should contain only window, with no wall, and fascias and bargeboards are generally avoided.
Other forms are graded more strictly. Linking two small dormers may be acceptable on small two-storey cottage-type houses at the rear, but not on heritage assets. Inset dormers lose floor area and are inappropriate in conservation areas. A box dormer must be set well in from the roof edges, aligned with the outer edge of the windows below; anything larger is unlikely to be subordinate. Large or irregular dormers that wrap around hips or rise above the ridge are rarely acceptable.
The guide's dormer paragraphs give measurable tests for each type.
| Dormer type | What Lambeth says | Where it is acceptable |
|---|---|---|
| Conventional pitched | Cill about 1 m above attic floor; head at least 2.1 m; modest and aligned below | Rear generally supported |
| Linked pair | Link recessed one third of the roof depth; no wider than 1.5 times each dormer | Small cottages, rear only, not heritage assets |
| Inset | Cuts into the roof and loses floor area | Not in conservation areas |
| Box | Set well in, aligned with windows below | Not in conservation areas |
| Wrap-around or above ridge | Rarely acceptable | Rare |
Lambeth Council, Design Guide Part 4, August 2023, paragraphs 4.90 to 4.98.
The fee includes our measured visit. The bracketed figure applies if you already have measured drawings.
On heritage assets and buildings in conservation areas, the guide says roof lights should be small, and those that open to form roof terraces will not be acceptable. Traditional-style roof lights set flush into the slope, slim-framed and black, are the most appropriate. Traditional mansards typically rise behind parapets and drain to the rear through concealed gutters; that is a form for terraces with parapet roofs rather than standard pitched ones.
Lambeth lists nine conservation areas with Article 4 directions on external alterations: Albert Square, Hyde Farm, Lansdowne Gardens, Leigham Court Estate, Park Hall Road, St Marks, Stockwell Park, Streatham Lodge and Vauxhall. For example, a house in Leigham Court Estate in Streatham needs the direction read before anyone relies on the 40 or 50 cubic metre right.
Lambeth says a householder project should get a response in 8 weeks, and decided 354 householder applications in the year to June 2026 with prior approvals set aside, 333 of them inside 8 weeks. Class B permitted development gives the figures of 40 and 50 cubic metres where it applies.
Every figure is linked to the page or document we read.
Lambeth's Design Guide Part 4 of August 2023 says a dormer cill should rest on the roof slope around 1 metre above the attic floor, with the dormer head at minimum 2.1 metres above floor level.
Source: Lambeth Council, Design Guide Part 4, para 4.91The guide says a link between two dormers should be recessed by one third of the dormer roof depth and be no wider than 1.5 times each dormer.
Source: Lambeth Council, Design Guide Part 4, para 4.94Lambeth lists 9 conservation areas with Article 4 directions on external alterations, including Hyde Farm, Leigham Court Estate and Stockwell Park.
Source: Lambeth Council, Article 4 DirectionsLambeth says a householder project on an existing home should get a response in 8 weeks.
Source: Lambeth Council, Apply for householder planning permissionLambeth decided 354 householder applications between July 2025 and June 2026, prior approvals excluded, and 333 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.
Lambeth sets no cubic-metre figure for full applications. The Design Guide asks for a modest dormer, subordinate to the roof, with the cill about 1 metre above the attic floor, the head at least 2.1 metres above it, and aligned with the openings below.
Only where they sit well in from the roof edges, aligned with the windows below. Anything larger is unlikely to be subordinate, and the guide calls the box dormer inappropriate in conservation areas and for heritage assets.
Lambeth resists dormers on front roof pitches where they are not characteristic of the building type or group, and generally supports them at the rear. Check the street's existing roofline before designing.
Albert Square, Hyde Farm, Lansdowne Gardens, Leigham Court Estate, Park Hall Road, St Marks, Stockwell Park, Streatham Lodge and Vauxhall, in all or part. Planning permission is needed for the works each direction lists.
£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. Lambeth's own application fee is separate.
Answer five quick questions and get a free planning score for your exact address — instantly. Then, if you want the full picture, unlock the detailed analysis: your council's live approval rates, local precedents, constraints on your street and a step-by-step route to permission.
Same fixed quotes, same fast drawings — wherever your project is.