Loft conversions in London: dormer, hip-to-gable, mansard and L-shaped

The type sets the bracket. A Velux conversion is the least expensive because the roof line does not move; a mansard is the most, because the roof is rebuilt. Between them, dormer size, an en-suite, the staircase position and the fire-door work on the floors below account for most of the variation. Our cost guides explain the variables, and a firm price follows a site visit and the survey.

Usually not. Most conversions sit inside permitted development, with volume allowances of 40 cubic metres for a terraced house and 50 for a semi-detached or detached one, provided nothing breaks the front roof plane and the materials match. Mansards almost always need permission. Permitted development is removed in conservation areas and by Article 4 directions, which vary street by street, so confirm your own address.

On site, six to eight weeks for a Velux conversion, eight to twelve for a rear dormer with an en-suite, and twelve to sixteen for a mansard or full-width dormer. Approvals and party wall notices run before that. The two-month party wall period is the single most common reason a start date moves, which is why we serve notices at design stage.

A terrace normally takes a rear dormer, because the back slope is the only face that can grow without breaking the front roof plane. A semi or end-of-terrace often suits a hip-to-gable, which squares off the sloping side and is usually combined with a rear dormer. Where the house has a back addition, an L-shaped dormer picks that up as well and gives a second room.

That figure is closer to a simple Velux conversion in a roof that already has the height, no dormer and no en-suite, than to anything structural. It rarely survives contact with the fire-door and alarm work that adding a storey requires on the floors below. We would rather tell you at the first visit which version of the job the budget reaches than agree a number that only works if nothing is found.

A Velux, or rooflight, conversion. Nothing is added to the outside of the roof, so there is no dormer structure, no cladding and, in most cases, no planning application. The trade-off is that it needs the most existing head height and gives the least usable floor area, because the space is limited by the slope on both sides.

Yes, on every loft conversion, whether or not planning permission is needed. Structure, fire safety, the staircase, insulation and ventilation are all controlled, and the work is inspected as it goes. The completion certificate at the end is the document a buyer's solicitor asks for, and a conversion without one is a problem at sale rather than at handover.

Usually not. The work happens above you, and the roof is opened and made weathertight in one push rather than left open. The disruptive stretch is the stairwell: fire doors, the new stair opening and the making good on the landing. We sequence that as late as the programme allows and keep the house secure and clean at the end of each day.

Sometimes, and it depends on the roof rather than the wish. Two rooms need enough width for a landing between them, head height across both, and a window in each that satisfies escape and daylight requirements. An L-shaped dormer on a house with a back addition is the usual way it works. On a standard terrace, one good room and an en-suite is normally the better outcome.