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

There are four common types of loft conversion. A rooflight conversion, often called a Velux conversion after the window brand, keeps the shape of the roof and adds windows in the slope. A dormer adds a box-shaped extension with vertical walls to a roof slope. A hip-to-gable conversion replaces a sloping side of the roof with a vertical gable wall. A mansard rebuilds a roof slope at a much steeper angle with a flatter top. The right choice depends on the shape of your roof, the headroom you have, how much space you need and whether the work fits within permitted development.

By Przemyslaw Smolarczyk Published 10 November 2026 7 minutes read

Bright converted attic room with sloping ceilings and new flooring

Key facts

Every loft conversion into a liveable room needs building regulations approval.

Under permitted development, a roof can be enlarged by up to 40 cubic metres on a terraced house and 50 cubic metres on other houses.

Roof extensions under permitted development cannot go beyond the existing roof slope at the front where the house faces a road.

In a conservation area, and for flats, a roof extension needs planning permission.

A new stair is needed; retractable ladders are not normally acceptable.

Bats may use roof spaces: a survey, and sometimes a licence, may be required.

What is a rooflight (Velux) loft conversion?

A rooflight conversion is the simplest type: the roof keeps its shape and windows are fitted into the existing slope. Because the structure of the roof changes least, it usually involves less work than the other types. It only suits lofts that already have enough height under the ridge for a usable room and a proper stair, so it is common in houses with steeper roofs. Under permitted development, rooflights must not stick out more than 150 millimetres from the roof slope or rise above the highest part of the roof, and side-facing windows must be obscure-glazed.

What is a dormer loft conversion?

A dormer is an extension built out from a roof slope, with vertical walls and usually a flat roof, which turns sloping ceiling into full-height floor space. A rear dormer across the width of the house is a familiar sight on London terraces. In Victorian and Edwardian terraces, a second dormer is often built over the rear addition, giving an L-shaped dormer that adds more space again.

Many dormers can be built under permitted development, provided the added volume stays within the allowance, the dormer is not on the roof slope facing the road, the materials look similar to the existing house and the dormer is set back at least 20 centimetres from the eaves where practicable.

What is a hip-to-gable loft conversion?

A hip-to-gable conversion straightens a hipped roof: the sloping side of the roof is replaced by a vertical gable wall, which extends the loft sideways. It suits detached, semi-detached and end-of-terrace houses with a hipped roof, where the loft is otherwise too narrow at the sides. It is often combined with a rear dormer to make the most of the space. The Planning Portal guidance names hip-to-gable enlargements in its permitted development rules, but the added volume still counts towards the allowance.

What is a mansard loft conversion?

A mansard conversion rebuilds a roof slope, usually at the rear, with a very steep, almost vertical face and a flat or shallow top, which gives near full-height rooms across the floor. Mansards are common on London terraces and usually create the most space of the four types. Because they change the shape of the roof substantially, they often go beyond permitted development and need a planning application. Many London councils publish their own design guidance for mansards, so check it before drawings are prepared.

Which type of loft conversion adds the most space?

In most houses a mansard adds the most usable space, followed by a dormer (or a hip-to-gable with a dormer), with a rooflight conversion adding the least. More space usually means more structural work and a longer build, and a higher chance that you need planning permission. The shape of your roof often decides the matter: a hip-to-gable is only possible on a hipped roof, and a rooflight conversion only works where the height is already there.

When does a loft conversion need planning permission?

A loft conversion needs planning permission when it goes beyond permitted development. The main limits are:

the added volume, including any earlier roof extension, must be no more than 40 cubic metres for a terraced house and 50 cubic metres for other houses;
nothing can extend beyond the existing roof slope at the front where the house faces a road, and nothing can be higher than the highest part of the existing roof;
materials must look similar to the existing house, and verandas, balconies and raised platforms are not allowed (a Juliet balcony is normally fine);
side-facing windows must be obscure-glazed, and can only open if the opening part is more than 1.7 metres above the floor;
the house must not be on designated land such as a conservation area, and the building must not contain flats.

If you are unsure, a Lawful Development Certificate from your council confirms in writing that the proposed work is permitted development.

What do building regulations require for a loft conversion?

Building regulations approval is needed for every loft conversion into a liveable space, whatever the type. The checks cover the strength of the new floor, the stability of the structure including the existing roof, safe escape from fire, safely designed stairs and sound insulation between the loft and the rooms below.

Fire safety often has the biggest effect on the rest of the house. Escaping through a window is too dangerous above first floor level, so the escape route down the stairs usually needs extra protection, typically fire-resisting doors on the rooms off the stairway, and mains-powered, interlinked smoke alarms at each level. A new stair to the loft is needed; where there is no room for a full traditional stair, a space-saving stair may be possible, but retractable ladders are not normally acceptable.

Do you need to tell your neighbours about a loft conversion?

Often, yes. In terraced and semi-detached houses a loft conversion usually involves work on the party wall, for example supporting new beams in it, so the Party Wall etc. Act 1996 applies. You must give your neighbours notice between 2 months and a year before the work starts.

Frequently asked questions

Can I convert the loft in a flat?

Not under permitted development. The rights do not apply to buildings containing flats, so a loft conversion in a flat needs a planning application, and usually the agreement of the freeholder as well.

Do I need a bat survey for a loft conversion?

Possibly. Work on a loft or roof may affect bats, which are a protected species. A survey may be required, and if bats are using the building a licence may be needed. Natural England can advise.

Is a loft conversion for storage covered by building regulations?

It can be. Even boarding a loft for storage may need approval if the ceiling joists were not designed to carry the extra load. Ask your building control body before you start.

How can GONT LTD help with your loft conversion?

GONT LTD is a London building company that carries out loft conversions, turning unused roof space into a bedroom, an office or a playroom. We offer a free consultation and a free quote. Call +44 7783 930505 (Monday to Friday, 9am to 5pm), email info@gont.co.uk, visit our showroom at 9 Royal Parade, Dawes Road, Fulham, or tell us about your project on our contact page.

Sources

General guidance for houses in England, based on the sources above. Your council has the final say on planning, and your building control body on building regulations.