Start with headroom and the stair
Two measurements govern feasibility. First, the height from the existing ceiling joists to the underside of the ridge: allow for a new floor structure below and insulation above, and you generally need something approaching 2.6–2.8m of existing clear height to produce comfortable finished headroom. Second, where a compliant staircase can land, with the required 1.9m headroom over it — which on a narrow terrace usually means taking space from the room below.
Rooflight conversions
The simplest and cheapest option: the roof shape is unchanged and rooflights are inserted between existing rafters, with structure altered only as required. Often permitted development, usually acceptable in conservation areas, and the least disruptive to build. The limitation is floor area with usable head height, so it suits generous roofs and fails narrow ones.
Dormers
A rear dormer converts sloping roof into usable room and is the workhorse of the London loft conversion. The construction and the covering define the quality: lead and zinc dormers give a flat, crisp finish and a long life; the cheek and roof detailing, the window proportions and the way the dormer meets the main roof are what separate a good one from the ones you see from the train. We build dormers in lead, zinc and natural slate, and where consent requires it we set out coursing and proportions to match the approved details.
Hip-to-gable and mansards
On a semi-detached or end-of-terrace house with a hipped roof, extending the hip up to a gable creates volume before a dormer is added. A mansard rebuilds the roof entirely to a steep front slope and a shallow top, which produces the most space and, in many conservation areas, is the only form accepted — though some boroughs resist them and some encourage them. Both are substantial structural operations: a mansard is closer to building a storey than converting a loft.
Fire, insulation and building it over an occupied house
Adding a third storey triggers fire safety requirements: protected escape routes, fire doors to habitable rooms off the stair, and often mains-linked detection throughout. Insulation and airtightness at the new roof are worth doing properly, since the loft is where comfort is won or lost. Practically, the roof will be open at some point: we plan the weathering sequence in advance, use temporary roofs where the programme or the season demands it, and never open more than can be closed the same day.