Dormer, mansard, hip-to-gable or Velux: which suits your roof?

Every conversion type makes a different demand of the existing roof, so the roof decides the shortlist before budget does. The critical measurements are head height at the centre and at the eaves, the pitch, the span, and whether the roof is a simple gable or has hipped ends.

Rafter type matters too. A traditional cut roof can often be reworked, while a trussed roof manufactured from engineered timbers needs a structural design before anything can be planned. Chimney positions, party walls and the route for the staircase constrain the layout from the start.

Once those are known, the discussion becomes straightforward. Before that, comparing types is guesswork.

A rooflight conversion changes nothing about the outside of the house. Windows are set into the existing roof slope, leaving the roofline, ridge and eaves exactly as they were. That makes it the least expensive route and, in a conservation area, usually the easiest to justify.

The catch is head height. The usable floor area is limited to the middle of the room where the ceiling is high enough to stand, and the low edges under the eaves can only realistically be used for storage. If the existing roof is generous, this is often the most efficient answer. If it is not, the room will feel cramped no matter how well it is finished.

It is also the fastest option to build and the least disruptive to live with, because the roof covering is only opened locally rather than cut away.

A dormer builds a box out of the roof slope to create vertical walls and full-height ceiling, which is why it is the most common conversion on British houses. It adds usable floor area rather than simply revealing it, and it usually increases the headroom dramatically compared with the space already there.

On a terraced or semi-detached house it also gives you somewhere to put proper windows and, often, an ensuite. That is what turns a loft into a genuine bedroom rather than a usable attic.

For most houses it falls within permitted development, provided it stays inside the volume allowance and does not project beyond the front roof slope. Conservation areas and Article 4 directions remove that right, so the position has to be checked for the address.

A hipped roof slopes inwards at the end of the house, which wastes the space at the top of the loft. A hip-to-gable conversion replaces that sloping end with a vertical gable wall, opening up the full width of the roof for use.

It is most relevant on semi-detached and detached houses with hipped ends, particularly on 1930s and similar housing stock where this roof type is common. Combined with a rear dormer it produces a large master suite with an ensuite.

Because it changes the external shape of the building, planning permission is often required, and on a semi-detached house the party wall and the relationship with the neighbouring roof need early attention. It is more structural work than a dormer, and the cost reflects that.

A mansard replaces the rear roof slope with a much steeper, near-vertical face. It creates the most usable space of any conversion type and can produce a full-height room across the entire footprint of the house, which is why it suits larger master suites and projects needing two rooms.

The trade-offs are real. A mansard changes the silhouette of the building, so planning permission is almost always required and the design has to hold a sensible conversation with the street and the neighbours. It involves more structure, more roof, more cost and a longer programme than a dormer.

It is the right answer when space is the priority and the design is handled carefully. It is the wrong answer when a dormer would have delivered the room you actually needed.

The table below sets out how the four types compare on the measures that usually decide the choice. Ranges are broad guides for early planning rather than quotations, and every project is priced against a surveyed scope.