Loft Conversion Types in London: Dormer, Hip-to-Gable, Mansard and L-Shaped Explained

A rooflight loft conversion (also called a Velux conversion, after the most well-known manufacturer of roof windows) involves no external alteration to the roof structure — only the installation of roof windows into the existing roof slope, and the conversion of the existing loft space into a habitable room with a new structural floor, staircase, insulation, and internal finishes. Structural changes: the existing rafters are typically retained. The loft floor is upgraded from attic boarding to a structural floor (typically 47×220mm engineered joists spanning between the existing masonry walls at eaves level, with a structural plywood or OSB deck). Staircase access is cut through the floor of the room below and a new staircase installed (the staircase takes up approximately 3m² of first-floor room). Insulation: Building Regulations Part L requires the new loft floor to be insulated to achieve a minimum U-value of 0.18 W/m²K (roof slope) and 0.13 W/m²K (flat ceiling/floor if applicable). This is typically achieved with 100mm rigid PIR insulation between the rafters and a further 50mm PIR board below (warm roof method at the slope), or 150mm+ blown cellulose at the flat ceiling level for a cold roof approach. Planning rules: a rooflight conversion is typically permitted development (no planning permission required) provided the roof windows do not protrude above the existing roof slope and do not face a highway. Best for: Victorian terrace houses with a standard gabled roof (not a hipped roof) and limited pitch — typically where the existing ridge height provides less than 2.2m head height at the ridge, making a dormer undesirable. Or where the property is in a conservation area and a dormer would require planning permission that may be refused. Cost: £25,000-£45,000 for a standard single-room rooflight conversion in a London Victorian terrace, including structural floor, staircase, insulation, roof windows, plastering and decoration. This is the lowest-cost conversion type.

A rear flat dormer is an extension to the rear slope of the roof, with a vertical rear wall, a flat roof, and dormer windows or a glass wall. It is the most common loft conversion type in London because it maximises usable floor area under full head height within the loft space. A standard rear flat dormer provides full standing height across the full width of the rear of the house and the full depth from the party wall to approximately 1m from the ridge. Planning rules: permitted development allows a rear flat dormer on a single-family house (not a flat or converted house) provided: the total volume increase does not exceed 50m³ for a terrace or 40m³ for a flat roof extension (note: there is a 40m³ limit for extensions, not loft conversions — the loft volume rule is 40m³ for semi-detached and terraced and 50m³ for detached); the dormer does not exceed the height of the existing ridge; the dormer roof is set back at least 20cm from the existing eaves; the materials used in the external surfaces are similar in appearance to the existing house (typically grey zinc or lead for the dormer roof, render or brick to match the main house for the dormer cheek walls). In conservation areas: permitted development rights for rear dormers are typically removed by an Article 4 Direction — a planning application is required for any dormer in most London conservation areas. Structural: a rear dormer requires modification of the existing roof structure. The rafters in the dormer opening are cut and the loads redistributed to new trimmer rafters at each side of the dormer opening, supported on a new wall plate at the eaves and ridge beam or on new posts within the dormer structure. Structural engineer's design required. Best for: 3-bed London Victorian terrace or Edwardian terrace — a rear dormer can typically provide 1 double bedroom, 1 single bedroom (or study), and 1 bathroom within the loft. Cost: £40,000-£65,000 for a rear flat dormer loft conversion on a standard 3-bed London Victorian terrace (structural floor, full rear dormer, staircase, insulation, full internal finishes — client-supplied sanitary ware and second-fix materials). Dormer structural glazed wall (Juliet balcony or frameless sliding door): adds £3,000-£8,000.

Hip-to-gable conversion: a hipped roof (a roof with sloping faces on all four sides, including the side elevations of the house) is the standard roof form on most 1920s-1930s London semi-detached and bay-fronted terraced houses. A hip-to-gable conversion rebuilds the hipped end wall (the triangular side elevation under the hip) as a vertical gable wall, extending the ridge of the roof to the new gable end, and typically adds a rear flat dormer as well. This provides significantly more loft floor area than a dormer alone, because the existing hip roof wastes space at the side of the loft. Planning rules: a hip-to-gable conversion requires full planning permission at the side elevation of the house — permitted development does not permit changes to the principal or side elevation in most cases. In practice, most London Boroughs approve hip-to-gable applications for semi-detached and end-of-terrace houses where they are set back from the street and the design reflects the existing house. Hip-to-gable applications on mid-terrace houses are rarely possible because the hipped end is the visible side elevation. Structural: the hip-to-gable conversion is a major structural change to the roof — the existing hip rafters are removed and the new gable wall is built, with a new ridge beam extended to the gable, new gable end rafters, and typically a rear dormer structure added at the same time. Party wall notice required at the side elevation (Section 1 if a new gable wall is built on or near the boundary). Best for: London 1920s-1930s semi-detached houses. Cost for hip-to-gable + rear dormer combined: £55,000-£85,000 on a 3-bed 1930s London semi. L-shaped dormer: an L-shaped dormer combines a rear flat dormer with a side-facing dormer (typically over the rear outrigger or kitchen extension of a Victorian terrace, creating an L-shape in plan). This is the maximum volume that can typically be achieved under permitted development on a Victorian terrace. The L-shaped dormer provides the rear dormer space plus additional rooms over the rear outrigger — typically allowing 3 bedrooms and a bathroom in the loft rather than 2. Best for: Victorian and Edwardian terraces with a rear outrigger (back addition). Cost: £55,000-£80,000 for an L-shaped dormer including full internal finishes.

A mansard conversion is the highest-specification and highest-cost loft conversion type in London. It involves rebuilding the entire rear slope of the roof at a steep angle (typically 72°) with a flat roof section at the top, giving the loft room nearly vertical walls for its full height on the rear elevation, and maximising the usable floor area at full head height. Planning rules: a mansard conversion always requires full planning permission — it is not permitted development because it substantially alters the roof profile and exceeds the volume limits for permitted development. However, mansard conversions are commonly approved in conservation areas and on listed buildings in London where a standard rear dormer would be refused — because the mansard form (particularly a zinc or slate-clad steep rear slope) is considered more sympathetic to the Victorian and Georgian character of many London conservation areas than a flat-roof dormer. Design: the steepness of the rear mansard slope (72°) means the rear elevation of the new loft room is nearly vertical — giving a headroom of approximately 2.4-2.6m across almost the full plan area of the rear. Structurally, a mansard requires rebuilding the full rear roof slope, with new structural steel frame or timber framework, new flat roof structure at the top, and new rear glazing or dormer window positions at the near-vertical rear face. Cost: £65,000-£100,000+ for a full mansard conversion on a standard London Victorian terrace (including full planning process, structural work, full internal fit-out — client-supplied second-fix materials). In a conservation area with design fees, structural engineer, planning fees, and party wall surveyor: total professional and construction cost can reach £95,000-£130,000+. Best for: properties in London conservation areas or on terraces where maximum loft space and a high-specification finish are the priority, and where planning for a standard dormer is likely to be refused.