Converting the loft is often the most cost-effective way to add a bedroom and bathroom to a London house — without losing any garden. Here is what each type of loft conversion involves, what it costs in 2026 and which rules apply. All figures exclude VAT.
Which type of loft conversion?
| Type | Best for | Typical London cost |
|---|---|---|
| Rear dormer | Terraced and semi-detached houses — the most common choice | £55,000–80,000 |
| Hip-to-gable + dormer | Semi-detached and end-of-terrace houses with a sloping side roof | £65,000–95,000 |
| L-shaped dormer | Victorian terraces with a rear outrigger — space for two rooms | £70,000–100,000 |
| Mansard | Maximum space; common in conservation areas where dormers are not allowed | £85,000–130,000 |
Prices include structure, insulation, windows, a new staircase, a shower room and decoration. A larger en-suite, bespoke joinery and high-spec glazing sit at the top of each range.
Planning permission and permitted development
Many dormer and hip-to-gable conversions fall under permitted development: up to 40 m³ of extra roof space for terraced houses and 50 m³ for semi-detached and detached houses, with no extension beyond the front roof slope facing the road. Mansards, flats, houses in conservation areas and properties with an Article 4 direction usually need planning permission. A Lawful Development Certificate is worth getting even when permission isn't needed — buyers' solicitors ask for it.
Building regulations you can't skip
- Fire safety: a protected escape route down through the house, fire doors and mains-wired, interlinked smoke alarms.
- Structure: new floor joists and usually steel beams, designed by a structural engineer.
- Stairs: a proper staircase with adequate headroom — space for it is often the deciding factor.
- Insulation and ventilation: to current Part L standards, so the new room is warm in winter and not too hot in summer.
Party wall notices
In terraced and semi-detached houses, steel beams usually bear into the party wall. The Party Wall etc. Act 1996 requires written notice to neighbours at least two months before that work starts. Build this into the timeline from the beginning.
Is there enough head height?
As a rule of thumb, measure from the top of the ceiling joists to the underside of the ridge: around 2.2 m or more makes a straightforward conversion. Below that, a mansard or lowering the ceilings below may be needed — both add cost.
How long does it take?
Most loft conversions take 8–12 weeks on site. Much of the early work is done from scaffolding, so families usually stay at home until the new staircase is opened up.
See our roofing & loft conversions service, or use the calculator for a quick range.
Frequently asked questions
How much does a loft conversion cost in London?
A rear dormer loft conversion in London typically costs £55,000–80,000 excluding VAT, including a bedroom and shower room. Hip-to-gable and L-shaped dormers cost more, and mansards typically £85,000–130,000.
Does a loft conversion need planning permission?
Many dormer conversions are permitted development: up to 40 m³ of extra space for terraced houses and 50 m³ for semi-detached and detached houses. Mansards, flats and homes in conservation areas usually need planning permission.
How long does a loft conversion take?
Most loft conversions take 8–12 weeks on site, and families can usually stay at home for most of the work.
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