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

Most loft conversions in South West London fall into five types: the rear dormer, the L-shaped dormer, the hip-to-gable, the mansard and the roof-light conversion. Which one suits your house depends on the shape of the roof, the headroom you already have and, just as much, on what your local council is prepared to accept. This guide explains each type in plain English, what it is good for, and where the planning rules tend to push you.

Start with the roof you have

Before choosing a type, it helps to know three things about your loft. First, the roof shape: a terraced house usually has a simple pitched roof between two party walls, while a semi-detached or detached house often has a hipped roof, where the side slopes down as well as the front and back. Second, the headroom: a common rule of thumb is that you want at least about 2.2 metres from the top of the ceiling joists to the underside of the ridge, although a builder or surveyor should measure it properly. Third, where a new staircase can land on the floor below without losing a bedroom.

Those three factors narrow the options quickly. A generous pitched roof may only need roof lights. A low or hipped roof usually needs extra volume, which is where dormers, hip-to-gable conversions and mansards come in.

Rear dormer

A rear dormer is a box-shaped extension built out from the back slope of the roof, with vertical walls and a flat roof. It turns a sloping, low-ceilinged space into a room with full headroom across most of its depth, which is usually enough for a double bedroom and a shower room. It is the most common type on terraced and semi-detached houses because it adds a lot of usable space without changing the front of the house.

Many rear dormers on houses can be built under permitted development, without a full planning application, provided they stay within the national limits. At the time of writing those include volume allowances (40 cubic metres for a terraced house and 50 cubic metres for a semi-detached or detached house), materials similar to the existing roof, and no extension facing the road. The rules are detailed and change from time to time, so check the current position on the Planning Portal. Permitted development does not apply to flats, and it is restricted in conservation areas.

Councils often have their own preferences for how a dormer should look even where permission is needed. Richmond's guidance, for example, asks for dormers set well in from the edges of the roof and prefers several smaller dormers to one large one, which matters in East Sheen and the other Richmond areas.

L-shaped dormer

Many Victorian and Edwardian terraces have a narrower back addition, sometimes called an outrigger, projecting from the rear. An L-shaped dormer combines a dormer on the main roof with a second dormer over the back addition, so the two meet in an L. The result is usually the largest loft possible on a terraced house, often enough for two bedrooms or a bedroom with a generous bathroom.

Because an L-shaped dormer adds a lot of bulk at the back, councils look at it closely, particularly in conservation areas. Some councils resist building up above back additions altogether, so it is worth checking local guidance before settling on this layout.

Hip-to-gable

A hip-to-gable conversion straightens a sloping side roof, the hip, into a vertical gable wall, extending the ridge out to the side. It is designed for semi-detached and detached houses with hipped roofs, where the sloping side otherwise leaves too little room for a staircase. It is often combined with a rear dormer to create a full-width room.

This is the type where local councils differ most. Richmond's house extensions guidance describes hip-to-gable conversions as not desirable, and Kingston's guidance says they are not usually appropriate, especially on semi-detached houses, where converting one half of a pair can unbalance it. Kingston's own character study names side dormers and hip-to-gable conversions as the greatest threat to the look of parts of Kingston. Hammersmith & Fulham resists them in conservation areas where hipped roofs are part of the original pattern. If you have a hipped roof, check what your council says before designing around one.

Mansard

A mansard conversion replaces the rear roof slope with a new, almost vertical slope, usually finished in slate or tiles with traditional dormer windows set into it, and a flat roof on top. It gives the most usable space of any loft conversion, close to a full extra storey, and it can look more traditional than a large box dormer.

Because it changes the roof so much, a mansard usually needs planning permission, and on the whole councils are more selective about where they accept one. Wandsworth's housing guidance is unusually specific: on mid-terrace houses where the party walls rise through the roof, it favours raising the party walls and building a full-width mansard-style rear extension with traditional dormers, which is why mansards are so common in Southfields. In Fulham, the council accepts a standard slate front mansard on the Peterborough Estate, where mansard attic floors are now the norm. Elsewhere, such as Sutton, rear mansards on conventional roofs are treated as unacceptable.

Roof-light conversion

A roof-light conversion, often called a Velux conversion after the best-known brand, keeps the existing roof shape and adds windows set into the slope. It is the simplest and least disruptive option, and it often falls within permitted development, but it only works where there is already enough headroom, which usually means an older house with a steeply pitched roof.

Roof lights are also the natural choice where the front or side of a roof cannot change, such as in a conservation area or under an Article 4 direction, although even roof lights facing the street can need permission in the most protected areas.

Conservation areas and Article 4 directions

If your house is in a conservation area, permitted development rights are reduced, and an Article 4 direction can remove them entirely for specific changes, such as alterations to street-facing roof slopes, dormers, windows or roof materials. In Merton Park, for example, Article 4 directions mean even a street-facing dormer or a change in roof finish needs planning permission on many roads. Each of our area pages sets out the conservation areas and Article 4 directions that apply locally, with links to the council documents.

Building Regulations and fire safety

Whichever type you choose, every loft conversion needs Building Regulations approval. The inspector will look at the strength of the new floor and any steel beams, the staircase and its headroom, insulation, ventilation and, above all, fire safety. Adding a storey changes how you would escape in a fire, so the stairwell usually needs to be protected with fire-resisting construction and doors, and the house needs interconnected, mains-powered smoke alarms.

On terraced and semi-detached houses, new steel beams are often supported on the party wall, which means serving notice on your neighbours under the Party Wall etc. Act 1996 before work starts. The government's Party Wall guidance explains how that works.

Choosing the right type

In practice, most terraced houses end up with a rear or L-shaped dormer, or a mansard where the council prefers one. Most semi-detached houses with a hipped roof need either a hip-to-gable or a carefully designed rear dormer, depending on the council. Roof-light conversions suit houses with generous existing roofs and those where the roof cannot change. A survey of the roof and a look at the local planning guidance usually make the choice clear. Our loft conversions page explains how we run a project from survey to handover.