Loft Conversion Costs UK 2026: Types, Prices & Planning Guide

Loft conversion costs in the UK range from £25,000 for a basic Velux conversion to £65,000+ for a full mansard conversion. The average UK homeowner spends around £40,000–£50,000 for a dormer loft conversion with en-suite.

Loft conversions are one of the best value-for-money ways to add living space. A well-executed loft conversion typically adds 15–20% to a property's value — often returning more than it costs to build.

Velux (rooflight) conversion: £25,000–£40,000

The simplest and cheapest type. Velux windows are installed into the existing roof slope without changing the roofline. Best for houses with sufficient head height (at least 2.2m at the ridge). No planning permission usually required.

Dormer loft conversion: £35,000–£55,000

A box-shaped structure is built out from the roof slope, creating vertical walls and a flat or pitched roof. Dramatically increases usable floor space and headroom. Most popular loft conversion type in the UK. Often permitted under PD rights.

Hip-to-gable conversion: £40,000–£65,000

Converts a sloping hip roof end into a vertical gable wall, extending the ridge and increasing floor space. Common on semi-detached and detached houses. Usually requires planning permission.

Mansard conversion: £50,000–£75,000+

Almost always requires planning permission. The entire roof is rebuilt with near-vertical walls and a shallower pitch, maximising internal space. Most popular in London and for period terraced houses. Typically the most expensive option.

• Head height: Lofts need at least 2.2m (ideally 2.4m+) at the ridge after conversion. If your loft is too shallow, a dormer or mansard becomes the only viable option.
• Structural work: Many Victorian and Edwardian houses have "cut" roofs that need structural reinforcement. Modern "trussed" roofs (post-1960s) are more complex to convert and can add £3,000–£8,000.
• En-suite bathroom: Adding a bathroom adds £5,000–£12,000 depending on spec.
• Staircase: A new loft staircase typically costs £1,500–£5,000 supply and fit. Space for the stairs often requires sacrifice of a bedroom cupboard or landing area.
• Location: London conversions regularly cost 20–40% more than the rest of England.

Most loft conversions in England fall under Permitted Development rights and don't need planning permission, provided:

• The conversion adds no more than 40m³ additional roof space (terraced houses) or 50m³ (detached/semi)
• No part is higher than the existing roof
• No extension beyond the plane of the roof slope fronting the highway
• No balcony, verandah, or raised platform
• Materials match the existing house in appearance
• You're not in a Conservation Area, National Park, or AONB (where all roof alterations need permission)

Mansard conversions almost always require planning permission as they fundamentally alter the roof form.

• Velux conversion: 4–6 weeks
• Dormer conversion: 6–10 weeks
• Hip-to-gable or mansard: 8–14 weeks

Most of the structural work is done from outside first (dormer or rooflight installation, structural steelwork) before the internal fit-out begins. This means minimal disruption to daily life for the first 2–3 weeks.

Loft conversions are specialist work — not every builder has the experience to deliver a structurally sound, Building Regulations-compliant result.

• Use a specialist loft conversion company or a builder with a proven loft portfolio (ask for photos of completed projects and Building Regs sign-off certificates).
• Check insurance: Ensure the contractor has public liability insurance (minimum £2 million) and employer's liability if using subcontractors.
• Request Building Regulations approval: Your loft conversion must achieve Building Regs sign-off for fire escape, structural integrity, insulation, and stairs. This is separate from planning permission.
• Get at least 3 quotes: Prices for identical work can vary by 30–40%. Use Job2Build to compare quotes from vetted loft specialists near you.