Dormer, hip-to-gable or mansard? Choosing a loft conversion type in London

Most London loft conversions fall into one of four types: a rooflight conversion within the existing roof, a rear dormer, a hip-to-gable conversion (often with a rear dormer as well), or a mansard. They differ in floor space, structural work and, crucially, planning treatment. This guide sets out the rules for each, using the government's permitted development technical guidance, the Planning Portal loft conversion pages and the National Planning Policy Framework published in August 2026. For the wider regulatory picture, see our guide to loft conversion regulations in London.

Start with the roof you have

London's housing stock gives a rough guide to what is likely to work. Victorian and Edwardian terraces in inner boroughs such as Islington, Lambeth and Hammersmith & Fulham often have pitched roofs suited to a rear dormer, while many have the "London" or butterfly roof hidden behind a front parapet, where a mansard is the usual way to add a storey. Interwar semi-detached houses, common in outer boroughs such as Barnet, Ealing and Croydon, frequently have hipped roofs, which is where hip-to-gable conversions come in. Treat this as a starting point, not a rule.

The permitted development baseline (Class B)

Roof enlargements on houses can be permitted development under Class B of Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015. In summary, the work must:

• add no more than 40 cubic metres of roof space on a terraced house, or 50 cubic metres on other houses, counting any earlier roof enlargements;
• not rise above the highest part of the existing roof;
• not extend beyond the plane of the roof slope forming the principal elevation where it fronts a highway, so front dormers need planning permission;
• use materials of similar appearance to the existing house, with side-facing windows obscure-glazed and non-opening below 1.7 metres from the floor;
• include no balconies, verandas or raised platforms.

Class B does not apply to flats, or on article 2(3) land, which includes conservation areas. That matters in London, where large parts of boroughs such as Camden and Kensington & Chelsea are designated. Article 4 directions or conditions on earlier permissions can also remove these rights, so check with your council first.

Rooflight conversions

A rooflight conversion keeps the existing roof shape and adds windows set into the slope. Rooflights fall under Class C (other alterations to the roof), which allows them provided they protrude no more than 0.15 metres beyond the plane of the original roof slope, are no higher than the highest part of the original roof, and are obscure-glazed and fixed below 1.7 metres if they are on a side-facing slope. Because Class C does not enlarge the house, rooflights on the front slope can be permitted development where a front dormer would not. The trade-off is space, which is limited by the existing pitch and height of the roof.

Rear dormers

A rear dormer is the most common way to gain full-height space in a terraced London house. Under Class B it must, as far as practicable, be set back at least 0.2 metres from the eaves, measured along the roof slope, and the original eaves must be kept or reinstated. The dormer must not overhang the outer face of the original walls. An "L-shaped" dormer that joins the main roof to the roof of a rear outrigger is specifically recognised in the technical guidance, and the eaves and overhang rules are relaxed where the enlargement joins the original roof to the roof of a rear or side extension. The volume cap still applies, so ask your designer to calculate it.

Hip-to-gable conversions

On a house with a hipped (sloping) side roof, a hip-to-gable conversion builds the side wall up to a vertical gable, extending the ridge. This is capable of being permitted development under Class B, and the eaves set-back rules do not apply to the relevant parts of a hip-to-gable enlargement. The other limits do: the 50 cubic metre allowance on a semi-detached or detached house covers the new gable and any rear dormer together, and any new window in the side gable must be obscure-glazed and non-opening below 1.7 metres.

Mansards: usually a planning application

A mansard replaces the roof with a steep lower slope, typically with dormer windows, and a shallow upper slope. Because it normally changes the roof profile and often rises above the existing roof, a mansard usually falls outside Class B and needs householder planning permission. From 1 April 2026 the householder application fee for a single house in England is £548, according to the government's planning fees schedule.

National policy has become more supportive. The new National Planning Policy Framework, published on 17 August 2026, says in policy L2 that substantial weight should be given to the benefits of creating additional homes or floorspace by using the airspace above existing residential premises, including the addition of mansard roofs, provided proposals are consistent with the overall street scene (with stated exceptions) and maintain safe access and acceptable living standards for residents and neighbours. Its glossary describes a mansard as generally suitable for buildings in a terrace of at least three, at least two storeys tall, with a parapet along the whole front façade.

Local design guidance still governs the detail. In Tower Hamlets, for example, the council's June 2025 Tredegar Square Conservation Area mansard guidelines apply only to listed addresses and set a 70-degree lower slope and 30-degree upper slope, a set-back of at least 900mm from the front façade, natural Welsh slate, retention of the rear V-shaped parapet, façade enhancement works and a £2,000 contribution to a streetscape fund at the time of publication. Other boroughs take their own approach, so read your council's design guidance first.

Certainty for permitted development schemes

If you are relying on Class B or C, a Lawful Development Certificate for a proposed use or development confirms the position in writing. The fee is half the equivalent planning application fee, which is £274 for a single house from 1 April 2026.

Building regulations apply to every type

Whichever route you take, converting a loft into a habitable room needs building regulations approval. Two rules often shape the design:

• Fire escape: Approved Document B, Volume 1 says that where a conversion creates a storey above 4.5 metres, the full extent of the escape route should be addressed, with fire-resisting doors (minimum E 20) and partitions (minimum REI 30), including upgrading existing doors where necessary. An alternative approach for open-plan layouts involves sprinklers and fire-resisting separation.
• Stair headroom: Approved Document K normally requires 2 metres of headroom over a stair, but for loft conversions where that cannot be achieved it accepts 1.9 metres at the centre of the stair, reducing to 1.8 metres at the side. The position of a dormer often determines whether this is possible.

Mansards and hip-to-gable conversions commonly involve raising or building on a party wall, and inserting steels into one is notifiable under the Party Wall etc. Act 1996. See the Party Wall Act explained and our guide to building regulations sign-off.

Cost and next steps

As a general rule, the more a conversion changes the roof structure, the more it is likely to cost: a rooflight conversion usually involves the least structural work and a mansard the most. Treat any price range as indicative until a builder has surveyed the property. For context on 2026 London build costs, see our London house extension cost guide. When you have drawings, you can request quotes from local builders. Home Builders does not carry out building work: we ask advertisers to provide proof of insurance, and any contract is between you and the builder you choose.

Sources: GOV.UK: Permitted development rights for householders, technical guidance (Classes B and C); Planning Portal loft conversion; MHCLG: Creating a clear, rules-based planning system and the National Planning Policy Framework (August 2026), policy L2 and glossary; MHCLG planning fees from 1 April 2026; Tower Hamlets: Tredegar Square mansard guidelines (June 2025); Approved Document B; Approved Document K. This guide is a summary, not legal advice; confirm details with your council or a planning professional.

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