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

First: can your loft be converted?

The Federation of Master Builders lists the deciding factors: head height, roof pitch, the type of roof structure, any water tanks or chimney stacks in the loft, and room for a staircase. Measure head height from the top of the floor joists to the underside of the ridge beam. The FMB says the building regulations permit 2.2 metres, but 2.4 metres is far more comfortable once the new floor and ceiling are in. Generally, the steeper the roof the easier the conversion, with most pitches above 30 degrees working best.

Rooflight conversion

The cheapest option, according to the FMB. The roof keeps its shape and roof windows are added; the floor still has to be strengthened, and stairs, electrics, plumbing and insulation put in. It adds the least space of the four. The HomeOwners Alliance, using Checkatrade's figures, puts the average at about £27,500; the FMB says from around £30,000. Because the roof's shape doesn't change, it's the type least likely to need planning permission — but check your property's rights.

Dormer conversion

The FMB describes a flat-roof dormer as one of the simpler and more cost-effective types. It projects vertically from the sloping roof and uses standard windows, creating headroom and a box-shaped room with flat walls and ceilings. It can span the full width of the roof or be a couple of smaller dormers. The HomeOwners Alliance gives an average of about £50,000. Rear dormers can fall under permitted development; a dormer that extends beyond the roof slope on the front of a house facing a road can't.

Hip-to-gable conversion

Suited to semi-detached houses and bungalows with a hipped (sloping) end. The FMB explains that the sloping end is taken back and the end wall built up into a vertical gable with a standard pitched roof, which gives full headroom and room for normal windows. It's often combined with a rear dormer. The HomeOwners Alliance puts the average at about £60,000. It can fall within permitted development if it stays within the volume limits.

Mansard conversion

The FMB calls it almost like adding a whole new storey: the roof slope is rebuilt at a near-vertical angle, usually 72 degrees, with a flat roof on top, and it usually spans from one gable wall to the other. It suits period properties and terraced houses, though the party wall usually has to be raised, which means party wall agreements. The HomeOwners Alliance gives an average of about £65,000. Because it changes the slope on the front of the house too, it usually needs planning permission.

Rules every loft conversion has to meet

A habitable loft always needs building regulations approval. A new stair needs 2 metres of headroom; where that can't be achieved, 1.9 metres at the centre and at least 1.8 metres at one edge is acceptable. Escape windows must open at least 450mm by 450mm with an area of at least 0.33 square metres, with the bottom of the opening between 800mm and 1,100mm from the floor, and a loft that adds a third storey needs a protected stair enclosure. Under permitted development, the extra roof space can't exceed 40 cubic metres on a terraced house or 50 cubic metres otherwise.

Frequently Asked Questions