What is an L-Shaped Mansard Loft Conversion?
It involves:
- A main rear mansard (altering the back slope of the main roof)
- Connected to a mansard over the rear outrigger (the kitchen/bathroom extension on the ground floor)
From above, the layout forms an L-shape, dramatically increasing floor area and head height across the loft and rear extension.
Best Suited For
- Victorian/Edwardian mid-terrace or end-terrace homes
- Urban areas like London, where space is tight but valuable
- Properties with a rear outrigger or back addition
Key Features & Benefits
- Huge usable space — often enough for 2 bedrooms + 1 bathroom
- Full-height ceilings throughout (unlike dormers)
- Flat rear roof with steep rear slope (approx. 72°)
- Very natural layout for bedrooms, office, or en suite
- Boosts property value significantly
Planning Permission
Usually required, because:
- Mansard conversions alter the roof shape significantly
- They often exceed the volume limits under Permitted Development
You’ll need to submit:
- Architectural plans
- Possibly a heritage statement if in a conservation area
However, many councils are familiar with L-shaped mansards on period homes and are receptive to well-designed proposals.
Building Regulations
Always required for:
- Structural safety
- Insulation, fire regs, staircase design
- Drainage and ventilation if adding a bathroom
Estimated Costs (UK 2025)
£65,000–£95,000
(Depending on size, spec, and whether plumbing or steelwork is complex)
Typical Layout Options
- Option A: Large master suite + luxury en suite bathroom
Option B: Two double bedrooms + shared bathroom
Option C: Office + bedroom or studio apartment setup
Timeline
- 8–12 weeks for construction
- Add time for design, planning permission, and structural calculations
Pro Tip
Consider installing rooflights on the front roof slope to bring in more natural light, and think about underfloor heating in the en suite to avoid bulky radiators.
