Types of Loft Conversion Explained: Dormer, Hip-to-Gable, Mansard and Rooflight

If you're considering a loft conversion in London, one of the first questions you'll face is which type is right for your property. Ask three different contractors and you may get three different answers — each shaped by what they build most often rather than what actually suits your home.

Understanding the four main loft conversion types — rooflight, dormer, hip-to-gable, and mansard — means you can approach the process with clarity, make better decisions, and get more accurate information from the professionals you speak to.

Here's what each type involves, who it suits, and what it means for planning, structure, and your project.

Rooflight (Velux-style) Conversion

A rooflight conversion is the most straightforward option. The roof structure stays as it is — no external changes to the roofline. Rooflights or Velux-style windows are installed flush into the existing slope, the floor is structurally upgraded, insulation is added between and over the rafters, and the space is lined out internally.

The result is a bright, usable room — but headroom is limited by whatever the existing roof pitch provides. If your roof pitch is steep enough, this can work well for a bedroom, studio, or office. On shallower pitches, headroom constraints may make it impractical for regular occupation.

Because there are no external changes to the roof shape, rooflight conversions typically fall within permitted development rights in most cases. Building Regulations approval is still required, as it is for every loft conversion type without exception.

This option suits detached houses with a reasonable roof pitch where the client wants to add usable space without altering the external appearance of the building.

Dormer Conversion

The dormer is the most widely built loft conversion in London. It involves projecting a flat-roofed box structure out from the existing roof slope — typically at the rear — creating a vertical wall that dramatically increases usable headroom and floor area.

The structural floor between the loft and the storey below must be upgraded to meet Building Regulations requirements for a habitable room. The dormer structure itself requires joists, a flat roof, and weatherproofing — and the junction between the new dormer and the existing roof is a critical detail that must be properly designed and built.

Rear dormers on residential properties typically fall within permitted development, subject to volume limits and other restrictions. Proposals at the front, or exceeding permitted volumes, will need a planning application. Party wall implications depend on the proximity of the dormer to the boundary.

Dormers suit a wide range of property types, including semi-detached and terraced houses, and are particularly common across London's Victorian and Edwardian stock.

Hip-to-Gable Conversion

Hip-to-gable conversions are specific to properties with a hipped roof — where the roof slopes in on all four sides rather than having a straight gable at each end. Semi-detached and detached houses in London often have this roof form.

The conversion involves constructing a new structural gable wall where the hip previously sloped inward. This creates a flat vertical end to the roof and significantly increases the loft volume at that end of the building. It is often combined with a rear dormer to maximise the floor area across the full loft.

Because a new structural gable wall is being built, this is a more structurally significant project than a basic dormer. Structural engineering input is essential. On semi-detached properties, the new gable wall is likely to be a party wall matter, and the party wall process must be started well in advance.

Hip-to-gable work typically falls within permitted development on residential properties, subject to volume limits. Checking your property's planning history is advisable before assuming PD applies.

Mansard Conversion

The mansard is the most substantial loft conversion type and produces the greatest usable space. The existing rear roof slope is replaced almost entirely with a near-vertical rear face — typically at around 72 degrees — topped with a shallow flat roof. The result is essentially a full additional storey contained within a roof structure.

Because of the degree of change to the roof form, mansard conversions almost always require full planning permission. They are common in London, particularly on Victorian terraces and converted flats, where the building type and street context make them appropriate. In conservation areas where rear dormers are refused, a mansard may sometimes still be approved because of its specific roof profile.

Mansard conversions are structurally complex and require detailed engineering and architectural design before any pricing can be considered reliable.

Choosing the Right Type

The best approach is to work backwards from your property type, roof form, planning constraints, and what the space needs to achieve. A good design-and-build partner will assess all of these before committing to a recommendation — not after.

Building Regulations approval applies to every loft conversion regardless of planning status. That is not optional.

To discuss which loft conversion type is right for your property, contact RCB Design & Build.

Call: +44 7359 872594 Email: contact@rcbgroup.co.uk