Loft Conversion Costs in London: Full Price Guide for 2025
A loft conversion is one of the most cost-effective ways to add space and value to a London home. But costs vary significantly depending on the type of conversion, the size of the roof space, the specification, and the condition of the existing structure. Here is a realistic guide to what to budget for in 2025.
Types of Loft Conversion and Their Costs
Roof Light (Velux) Conversion
The simplest and most affordable type. Rooflights (Velux-style windows) are installed in the existing roof slope — no structural change to the roof profile. The floor is strengthened, insulation installed, a staircase created, and the space is plastered and fitted out.
Best for: Spaces with adequate headroom (minimum 2.2m ridge height above the new floor level) and where the conversion is for a single room without a dormer.
Typical cost in London: £30,000–£55,000
Rear Dormer Conversion
A rear dormer extends the rear roof slope vertically, creating a box-shaped addition at the back of the house. This significantly increases usable floor area and headroom. The most popular type of loft conversion in London.
Best for: Victorian and Edwardian terraced and semi-detached houses; conversions intended to provide a full bedroom and bathroom at loft level.
Typical cost in London: £50,000–£90,000
Hip-to-Gable Conversion
On a hipped roof (where the roof slopes on all four sides), the sloping hip end is replaced with a vertical gable wall, extending the ridgeline and significantly increasing internal space. Often combined with a rear dormer for maximum space.
Best for: Semi-detached and detached houses with hipped roofs (common in interwar London properties).
Typical cost in London: £65,000–£110,000 (including rear dormer where combined)
Mansard Conversion
The most extensive type — both rear and front roof slopes are rebuilt at a nearly vertical angle, with flat tops. Creates the maximum possible floor area at loft level. Most common in London's terraced housing where the shared party wall allows the mansard to follow the full roof width.
Best for: London terraces where maximum space and headroom are required; conservation areas where mansard form is the accepted precedent.
Typical cost in London: £90,000–£150,000+
What's Included in These Costs?
The costs above include:
- Structural works (floor strengthening, new steel beams, dormer structure)
- Insulation to new roof and walls
- Plasterboard and skim
- Staircase (standard design — see separate guidance on staircase types)
- One bedroom fit-out
- One bathroom (standard specification — see fit-out costs below)
- Rooflights or dormer windows
- Decoration (primer only in most cases)
- Building Control fees
Costs above typically exclude:
- Architect / planning fees
- Structural engineer fees
- Party wall surveyor fees (where required)
- High-specification bathroom or premium specification items
- Bespoke staircase designs
What Affects the Final Price?
- Headroom: Low-ridge properties may require more structural intervention to achieve adequate headroom
- Structural condition of existing roof: Older timbers in poor condition add cost
- Bathroom specification: A simple shower room versus a full master en-suite with freestanding bath varies by £5,000–£15,000
- Staircase design: A bespoke joinery staircase adds £5,000–£15,000 over a standard flight
- Conservation area: Conservation area or planning permission may add architect fees and pre-application time
RCB Design & Build
RCB Design & Build delivers all loft conversion types across London as principal contractor — managing structural engineering, Building Control, and all construction trades under a single point of accountability.