How much does a loft conversion cost in the UK? (2025 guide)
A loft conversion in the UK costs between £15,000 for a basic Velux and £120,000+ for a premium mansard in London. Most homeowners doing a mid-spec dormer conversion — the most popular type — spend between £38,000 and £55,000.
Loft conversions consistently rank as one of the best-value home improvements in the UK, adding between 15% and 25% to a property's value — often more than the conversion costs.
The four main types — costs compared
| Type | Typical cost |
|---|---|
| Velux / roof light | £15,000–£45,000 |
| Rear dormer | £25,000–£80,000 |
| Hip-to-gable | £32,000–£90,000 |
| Mansard | £45,000–£120,000 |
Do I need planning permission?
Most loft conversions fall under Permitted Development (PD) — no planning application needed, provided the extension stays within volume limits (40m³ for terraces, 50m³ for semis and detached) and doesn't extend beyond the highest part of the existing roof. Key exceptions: properties in conservation areas, listed buildings, and most mansard conversions, which almost always need full planning permission.
Building Regulations approval is required for all loft conversions regardless of planning status — covering structural calculations, fire escape, insulation and staircase design. Your builder or architect will handle this application.
Is my loft suitable for conversion?
The two key checks are head height and roof structure. You need a minimum of approximately 2.2m from the floor joists to the ridge for a viable conversion. Older cut-rafter roofs (pre-1960s) are generally easier and cheaper to convert than modern trussed roofs, which require more structural work to create usable space.
A good builder will carry out a free site visit and assessment before quoting. Post your loft job on GetQuickHelp to get free quotes from verified local loft conversion specialists.