Loft Dormer Types Explained: Which Is Right for Your London Home?

**What a Velux (roof window) conversion is**:

A roof window or Velux conversion (Velux is a brand name now used generically) does not add any new structure to the roof profile. Instead, roof windows are installed flush with the existing roof slope, bringing in light and providing a means of escape. The loft floor structure is strengthened and insulated, the eaves are insulated, and the space is converted within the existing roof volume.

**When Velux conversions are appropriate**:

• •*Properties with steep roof pitches*: A steep roof pitch (40° or above) creates adequate head height within the existing roof volume without needing to raise the ridge or add dormers
• •*Properties where the design brief is a simple, low-cost conversion*: The Velux conversion is the lowest cost loft option — no new brick or render cladding, no masonry dormer cheeks, no additional roof structure
• •*Planning-restricted properties*: Many London Conservation Area properties permit roof windows to a rear slope (non-highway facing) without planning permission — but a full dormer would require planning permission. A Velux-only conversion may be the only achievable option in a strict Conservation Area

**Planning position**:

• Roof windows to the rear slope of a dwelling are Permitted Development (Part 1, Class B, GPDO) provided:
• •They do not protrude more than 150mm above the plane of the roof slope (i.e., flush or minimal projection)
• •The highest part of the window does not exceed the highest part of the roof
• •No roof windows are on the principal elevation (the front, highway-facing slope)
• •Not in a Conservation Area on a slope that fronts a highway

**Limitations**:

• •Head height at the centre of the roof only — the usable floor area is dictated by the existing roof pitch; on a standard London Victorian terrace with a 35°–40° roof pitch, head height (over 1.8m) covers only the central portion of the floor plan
• •No significant increase in head height over the side walls (eaves) — the conversion typically has a narrow usable floor strip
• •For a full bedroom and en-suite, a Velux conversion on a standard London terrace rarely provides sufficient floor area

**Cost**: £18,000–£35,000 for a standard Velux conversion on a London Victorian terrace (loft floor, staircase, two Velux windows, insulation, boarding, electrics, fire door).

**What a box dormer is**:

A box dormer (also called a flat-roof dormer or rear dormer) is a dormer that projects from the rear slope of the roof, replacing a section of the sloping roof with a vertical-faced structure (with a flat roof above). The box dormer typically runs across the full width (or close to the full width) of the rear roof slope, creating a large vertical rear wall with windows — a significant increase in head height and usable floor area compared to a Velux-only conversion.

**Why box dormers are so common in London**:

Box dormers are the dominant loft conversion type in London for a simple reason: London terraces and semis are typically built with relatively shallow-pitched (30°–40°) roofs, and the existing roof volume at the eaves is insufficient for a usable room without adding structure. A box dormer raises the roof line at the rear, creating full head height across most of the floor plan.

**What a box dormer can create**:

• On a standard London Victorian 2-bedroom terrace (footprint approximately 5m × 8m), a rear box dormer typically creates:
• •One double bedroom with head height across most of the room
• •One bathroom (or en-suite) — either within the dormer or utilising existing head height near the ridge
• •A new staircase from the first floor landing

**Planning position for box dormers**:

• Box dormers on the rear slope of a dwelling are Permitted Development provided:
• •Volume added does not exceed 40m³ for a terrace (50m³ for a detached or semi-detached) — the permitted development allowance includes the dormer volume and any previous additions
• •The dormer does not exceed the height of the existing roof ridge
• •The dormer is set back at least 200mm from the eaves
• •No materials on the dormer that are not similar to the existing house materials
• •Not a principal elevation or side elevation facing a highway
• •Not in a Conservation Area (PD rights for dormers are removed in Conservation Areas)

In Conservation Areas, a planning application is required for any dormer — local planning policy on loft conversion design varies significantly by borough (Wandsworth has well-established supplementary design guidance for loft dormers, for example).

**Cost**: £35,000–£65,000 for a standard single-storey rear box dormer conversion on a London Victorian terrace (dormer structure, staircase, one bedroom, one bathroom, insulation, boarding, electrics, plumbing, fire compliance, redecoration).

**Hip-to-gable conversion**:

A hip-to-gable conversion changes the roof form of a semi-detached or end-of-terrace house — the sloping 'hip' at the end of the house is converted to a vertical 'gable' wall, allowing the internal floor area of the loft to extend to the full width of the house.

*Who can have a hip-to-gable*: Only semi-detached houses, detached houses, and end-of-terrace houses have a hip end — mid-terrace houses have party walls on both sides and do not have a hip.

*The benefit of hip-to-gable*: The conversion significantly increases floor area — on a semi-detached property, converting the hip to a gable adds typically 25–40% more floor area compared to a box dormer alone. Combined with a rear box dormer, a hip-to-gable conversion creates one of the larger loft conversion options for London semis.

*Planning position*: Hip-to-gable conversions are Permitted Development (same conditions as box dormers, plus the total volume must remain within the 50m³ limit for semis and detacheds). In Conservation Areas, planning permission is typically required.

*Party Wall Act*: A hip-to-gable conversion on a semi-detached house involves extending the party wall upward to form the new gable. This falls under the Party Wall etc. Act 1996 (Section 2 — alterations to a party wall). A Party Wall Notice must be served on the neighbour and a Party Wall Award obtained if the neighbour does not consent.

*Cost*: £50,000–£85,000 for a hip-to-gable + rear dormer combination on a London semi-detached house.

**L-shaped dormer**:

An L-shaped dormer is a combination dormer — a rear dormer running across the main rear roof slope combined with a second dormer on the outrigger (the rear extension that is common on Victorian London terraces and creates the 'L' shape when viewed from above). The two dormers together create an 'L-shaped' roof addition.

*Who can have an L-shaped dormer*: Properties with an outrigger (rear addition) that has its own roof — very common in Victorian and Edwardian London terraces and semis. The outrigger is typically a single-storey or two-storey rear addition built originally for the kitchen, scullery, or WC.

*The benefit of an L-shaped dormer*: The outrigger dormer adds floor area over the outrigger roof — typically a second bedroom or bathroom. On some properties, the outrigger dormer creates enough additional space for a second bedroom, making the combined L-shaped conversion a true 2-bedroom loft addition (turning a 2-bedroom Victorian terrace into a 4-bedroom house).

*Planning position*: An L-shaped dormer is PD if both individual dormers meet the PD conditions (volume, height, set-back). In practice, the outrigger dormer is often on the rear slope visible from the neighbouring garden — the 50mm set-back and similar materials conditions must be satisfied. In Conservation Areas, planning is typically required.

*Cost*: £55,000–£85,000 for a full L-shaped dormer conversion on a standard London Victorian terrace (main rear dormer + outrigger dormer, staircase, two bedrooms, bathroom).

**Mansard conversion**:

A mansard is a fundamental change to the roof form — the existing sloping roof is replaced with a near-vertical rear face (typically at 70°–72.5° to the horizontal) and a flat top. The mansard is recessed at the front (if the property is a terrace, the mansard is typically built at the rear only — leaving the original front roof slope in place). Mansard conversions create the maximum possible floor area and head height within the loft — the near-vertical rear wall gives full-height rooms across the full width of the rear roof.

*Who needs a mansard*: Properties with very shallow roof pitches (under 30°) where a box dormer would still leave insufficient head height; properties in Conservation Areas where a full mansard (set back from the parapet) is the locally preferred design solution; clients wanting maximum floor area from a loft conversion.

*Planning*: Mansard conversions almost always require planning permission — the change in roof form is a material change to the building's appearance. Planning policy varies significantly by borough — some London boroughs actively support mansard conversions as a design solution in certain streets; others resist them. Pre-application discussion with the planning department is strongly recommended.

*Cost*: £70,000–£120,000 for a full mansard conversion on a London terrace.