Loft Conversion Cost London 2026: Full Breakdown by Type

There are five main types of loft conversion available to London homeowners. Each has a different cost profile, planning requirement, and space gain.

**Velux (Rooflight) Loft Conversion: From £18,000–£32,000** The simplest and cheapest loft conversion. Roof windows are installed in the existing slope — no structural change to the roof shape. The ceiling height and footprint of the loft remain unchanged.

Best for: lofts with good existing head height (2.2m+ at the ridge); properties in Conservation Areas where a dormer would require planning permission; clients who want to maximise cost efficiency.

Limitations: the floor area and head height are fixed by the existing roof. If the loft is not large enough with the existing pitch, a Velux conversion will not be viable as a comfortable habitable room.

Includes: new structural floor (replacing ceiling joists with structural floor joists), roof windows (Velux FK06 or equivalent size), insulation (100mm+ Kingspan between and below rafters), staircase, plastering, decoration, skirting, landing modification. Excludes: bathroom, flooring, second-fix materials (client-supplied).

**Rear Dormer Loft Conversion: From £35,000–£65,000** The most popular loft conversion type in London. A dormer extension is added to the rear slope of the roof, creating a vertical-walled box that provides full head height across a large portion of the loft floor. Dramatically increases usable space compared with a Velux conversion.

Best for: Victorian and Edwardian terraces with a standard pitched roof; maximum habitable space; adding bedroom + en-suite.

Planning: most rear dormers on non-listed terrace houses can be done under Permitted Development. Conservation Areas and some boroughs restrict dormers.

Includes (at mid-specification): structural floor, rear dormer structure (timber frame, EPDM flat roof, vertical window), insulation, staircase, plastering, decoration, skirtings. Excludes: bathroom, flooring, second-fix materials (client-supplied).

**L-Shaped Dormer Loft Conversion: From £48,000–£80,000** Combines a rear dormer with an additional dormer over the rear addition of the house — creating an L-shaped plan. More common in semi-detached and larger terraced houses with a rear addition. Provides the largest loft footprint of any PD-eligible conversion type.

Planning: usually Permitted Development on both the main roof and the rear addition roof, subject to limits.

Includes: all rear dormer elements plus additional dormer over rear addition, with a connecting junction between the two dormers. Additional floor area and complexity increases cost vs. standard rear dormer.

**Hip-to-Gable Loft Conversion: From £45,000–£75,000** For semi-detached or end-of-terrace houses with a hipped roof (where the roof slopes down on the side elevation as well as the rear). Converting the hip to a gable (a flat-faced end wall) provides significantly more loft floor area than a Velux conversion on the same property.

Planning: hip-to-gable conversions require planning permission for semi-detached houses in most London boroughs (they affect the principal side elevation). Permitted Development for detached houses.

**Mansard Loft Conversion: From £60,000–£95,000** The most extensive and most expensive loft conversion type. The entire roof structure is rebuilt — replacing the existing pitched roof with a near-vertical (72°) rear slope and a shallow pitch at the top. Creates maximum head height and floor area, right to the perimeter walls.

Planning: always requires planning permission. The most appropriate type for Conservation Areas in central London where dormers are restricted. Mansard conversions are common in Georgian and Victorian Belgravia, Kensington, Marylebone, and other central London conservation areas.

Includes: full structural rebuild of the roof (structural engineer required), masonry parapet walls or timber frame to mansard profile, new roof covering, dormer windows. Most expensive because the roof is essentially a new build element.

When comparing loft conversion quotes in London, the scope of what is included varies enormously. A fair comparison requires checking the following:

• **Structural works** (should always be included):
• •Structural engineer's design and calculations
• •New structural floor (replacing ceiling joists with structural LVL or engineered floor joists designed to carry imposed load)
• •Structural trimming around the staircase opening
• •Steel beams where required (hip-to-gable steel, ridge steel where ridge beam needed)
• •Temporary works during construction

• **Roof and weatherproofing** (should be included):
• •Dormer structure (timber frame, sheathing board, membrane, cladding, EPDM flat roof to dormer)
• •Roof window installation (for Velux)
• •Insulation to rafters and floor (Part L compliant — minimum 100mm PIR insulation between/below rafters)
• •Lead or EPDM flashings at all junctions

• **Staircase** (should be included — check specification):
• •New staircase from the floor below to the loft (often the tightest constraint in a small terrace)
• •Staircase balustrade and guarding (Part K)
• •New opening in existing ceiling/floor (structural trimmer)
• •Landing modification on the floor below

• **Fire safety** (must be included — Building Regulations Part B):
• •Mains-powered smoke alarms in all storeys
• •Fire doors to all rooms on all storeys (where a new storey is added)
• •30-minute fire-resistant door to the staircase enclosure (FD30S)

• **Plastering and decoration** (usually included):
• •Plasterboard and skim to all new surfaces
• •Decoration (usually one coat or base coat — verify what is included)
• •Skirtings and window boards

• **Not usually included in the build quote** (separate budget needed):
• •Bathroom or en-suite (£6,000–£18,000 for building works; tiles and suite client-supplied)
• •Flooring (client-supplied and installed, or separate quote)
• •Fitted wardrobes (client-supplied)
• •Second-fix materials (tiles, sanitaryware, flooring, light fittings)

A useful benchmark for London loft conversions:

• •Velux conversion: £700–£1,100/m² of new loft floor area
• •Rear dormer: £900–£1,500/m²
• •L-shaped dormer: £1,000–£1,600/m²
• •Hip-to-gable: £1,000–£1,700/m²
• •Mansard: £1,400–£2,200/m²

Note: the per-m² cost is higher for loft conversions than for rear extensions because loft conversions involve more complex structural work (the roof structure), higher labour cost at height, and more specialist elements (dormer frame, Velux installation).

• **Typical sizes for London loft conversions**:
• •Small Victorian terrace (2.5m wide rear return): 18–25m² of new floor area
• •Standard Victorian terrace: 25–35m² new floor area
• •Larger Victorian semi-detached: 35–55m² new floor area

• At these sizes and the per-m² rates above:
• •Velux on small terrace: £13,000–£27,000
• •Rear dormer on standard terrace (30m²): £27,000–£45,000
• •Mansard on larger semi (45m²): £63,000–£99,000

(Figures above are indicative construction costs only, 2026 London, exc. VAT. Actual costs depend on specific property, specification, and site conditions.)

Beyond the main construction cost, a loft conversion involves several additional items that must be budgeted:

**Party wall**: loft conversions almost always require party wall notices on both immediate neighbours (one on each side), because the structural floor must bear on the party walls or requires insulation to be inserted into the party wall. Budget £1,500–£4,000 in party wall surveyor fees if neighbours dissent.

**Bathroom/en-suite**: a loft bedroom without an en-suite is less valuable than one with. Budget £6,000–£18,000 for building works for a new en-suite (plumbing, electrical, waterproofing, tiling labour, suite installation). Tiles and sanitaryware are client-supplied.

**Staircase quality**: the structural staircase in a loft conversion is typically a basic softwood construction included in the quote. A quality hardwood or glass staircase is a significant upgrade — budget £3,000–£18,000 for a premium staircase over the included basic version.

• **Dormer cladding**: the external cladding of the dormer box affects both appearance and durability. Common options:
• •EPDM/GRP (included in most quotes): functional, long-lasting, plain
• •Zinc cladding: premium appearance, excellent durability, adds £3,000–£8,000 over EPDM/GRP
• •Brick-effect slip tiles: blends with existing brickwork, adds £2,000–£5,000
• •Fibre cement cladding (Hardie/Cedral): functional mid-spec upgrade

**Planning fees**: if planning permission is required (dormer on Conservation Area, mansard, hip-to-gable on semi), budget £258 application fee plus £1,500–£4,000 for architect drawings.

**Building Regulations**: £400–£900 for Building Control fees on a loft conversion.