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Loft conversion types explained: dormer, mansard, hip-to-gable and L-shaped

The right loft conversion depends on your roof shape, the type of house you live in and how much headroom you need. Here are the four types we build most often across London and selected Kent and Essex towns, what each one suits, and how long it usually takes.

By Buildspace Projects · October 2026

Rear dormer

A box-shaped extension built out from the back slope of your roof, with a flat roof and vertical walls. It turns a sloping, low loft into a room you can stand up in across most of the floor.

• Suits: most terraced, semi-detached and detached houses.
• Typical build time: 10–12 weeks.
• Planning: usually permitted development, so no planning application is needed if the design stays within the limits (see below).

L-shaped dormer

Two dormers joined together: one over the main roof and one over the back addition (the rear "outrigger" common on Victorian and Edwardian terraces). It creates the most floor space you can get from this type of house, often enough for two bedrooms and a bathroom.

• Suits: Victorian and Edwardian terraced houses with a back addition.
• Typical build time: 12–14 weeks.

Hip-to-gable with rear dormer

A hipped roof slopes in at the side. We build the side wall up to a vertical gable, which makes the loft much bigger, then add a rear dormer for full headroom.

• Suits: semi-detached and end-of-terrace houses with a hipped roof.
• Typical build time: 12–14 weeks.

Mansard

The back slope of the roof is rebuilt almost vertically (about 72 degrees), with windows set into it. It gives the most internal space and a tidy finish that planners often prefer on period streets.

• Suits: terraced houses, especially in London and in conservation areas.
• Typical build time: 12–16 weeks for a rear mansard, 14–18 for an L-shaped mansard and 16–20 for hip-to-gable plus mansard.
• Planning: needs a planning application.

Do I need planning permission?

Many loft conversions are permitted development. In general that means the new roof space adds no more than 40 cubic metres on a terraced house or 50 cubic metres on a semi or detached house, nothing is built higher than the existing roof, nothing faces the road on the front slope, and materials look similar to the existing house. Mansards, front dormers, flats, listed buildings and most conservation areas need planning permission. Every loft conversion needs building regulations approval, which we handle as part of the drawings.

What about the neighbours?

If you share a wall with a neighbour, a loft conversion usually falls under the Party Wall Act, which means serving notice before work starts. We flag this at design stage and arrange it through our partner party wall surveyors so it doesn't hold up your start date.

How to choose

Start with your house type and roof shape, then decide how much space you need. A rear dormer is the quickest and simplest. An L-shaped dormer or a mansard gives the most room. We confirm what is possible for your home at a free site survey.

More about our loft conversions, design and planning and recent projects.