Average Loft Conversion Cost UK 2026
The average loft conversion in the UK costs £20,000–£60,000 depending on the type, size, and location. London and the South East command a 20–40% premium over the national average. A basic Velux conversion starts from £15,000 while a full mansard conversion can exceed £80,000.
Velux (Rooflight) Loft Conversion — £15,000–£25,000
The simplest and cheapest type. Velux windows are fitted into the existing roof slope without changing the roofline. No planning permission usually required. Suitable where the existing roof pitch provides adequate headroom (minimum 2.2m at the ridge). Takes 4–6 weeks.
Dormer Loft Conversion — £25,000–£45,000
The most popular type in the UK. A dormer extends the roof vertically to create a flat-roofed box structure, significantly increasing headroom and floor space. Rear dormers usually don't require planning permission under permitted development. Takes 6–10 weeks.
Hip-to-Gable Loft Conversion — £30,000–£50,000
Suitable for detached and semi-detached houses with hipped roofs. The sloping hip end is replaced with a vertical gable, creating more internal space. Usually combined with a rear dormer. Takes 6–10 weeks. May require planning permission.
Mansard Loft Conversion — £40,000–£80,000+
The most expensive but most spacious type. The entire roof is replaced with near-vertical sides and a flat top. Maximises space but almost always requires planning permission and is more complex structurally. Common in London terraced houses. Takes 10–16 weeks.
What's Included in the Cost
A full loft conversion price should include: structural engineer calculations, building regulations approval, all structural work (steel beams, joists), insulation, roof windows or dormer structure, staircase, electrics and lighting, plastering, and decoration. Always get a detailed itemised quote.
Do You Need Planning Permission?
Most loft conversions fall under permitted development rights and don't need planning permission. Exceptions include: mansard conversions, properties in conservation areas, listed buildings, and conversions that exceed 40 cubic metres (terrace) or 50 cubic metres (detached/semi). Always check with your local planning authority.
Building Regulations
All loft conversions require building regulations approval regardless of planning permission. This covers structural safety, fire escape, insulation, headroom, and stair specification. Your builder should handle the application.
Return on Investment
A loft conversion typically adds 15–24% to property value — approximately £40,000–£65,000 on the average UK home. It's consistently rated as the single best-value home improvement for ROI, ahead of extensions, new kitchens, and conservatories.
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