Loft Conversion Cost Guide UK 2026

A loft conversion adds space and value to your home without extending outwards. The cost depends on the type of conversion (how much you change the roof structure), the floor area you're creating, and your finish specification.

Types of loft conversion

Velux conversion (£20k-£30k)

Velux conversions keep your existing roof line and add roof windows. You're limited by the existing headroom and roof pitch. This works if your loft already has usable space and you just need light and Building Regs compliance. Typical cost: £800-£1,200/sq m.

Dormer conversion (£25k-£40k)

A dormer extends the roof outwards, creating more headroom and floor area. Most UK lofts use a flat-roof dormer (cheaper, faster to build, covered by Permitted Development in many cases). Typical cost: £1,400-£1,800/sq m.

Hip-to-gable (£35k-£45k)

Replaces a sloping hip end with a vertical gable wall, creating more internal space. Often combined with a dormer. More structural work than a simple dormer, so costs rise. Typical cost: £1,600-£2,000/sq m.

Mansard (£40k-£50k+)

Replaces the entire roof slope with a near-vertical wall and flat roof. Maximum space, maximum cost. Often needs planning permission (goes beyond Permitted Development limits). Typical cost: £1,800-£2,400/sq m.

What affects the cost

Structural work

Your existing loft probably has thin ceiling joists (designed to hold a ceiling, not a floor). The builder will add new floor joists or steel beams to carry the load. If you're changing the roof structure (dormer, hip-to-gable), you'll need steelwork to support the new opening. Structural work is 30-40% of total cost.

Stairs

Building Regulations require a proper staircase (not a loft ladder). The stairs need to fit building regs for pitch, headroom, and fire escape. Finding space for stairs often means losing a bedroom cupboard or part of a landing. Budget £2,000-£4,000 for a basic staircase.

Dormer size

Permitted Development allows a dormer up to 50 cubic metres (terraced and semi-detached houses) or 40 cubic metres (detached). Bigger dormers cost more to build but give you more usable space. A small dormer over the stairs (just for headroom) costs £8k-£12k. A full-width rear dormer costs £15k-£25k.

Windows and velux

Velux roof windows cost £500-£1,200 each (supplied and fitted). Fire escape windows (required by Building Regs) cost slightly more. Dormer windows are cheaper per square metre than velux because they're vertical, not roof-mounted.

Bathroom

Adding an en-suite bathroom costs £4,000-£8,000 on top of the basic conversion. You'll need to run water supply and waste pipes from the existing bathroom below. The waste pipe needs a minimum fall (gradient), which sometimes means boxing in pipes downstairs.

Planning and Building Regulations

Most loft conversions fall under Permitted Development and don't need planning permission. Limits: dormer must be on the rear of the house, no higher than the existing ridge, materials similar to existing house. Check your local planning portal or ask a planning consultant (£200-£400 for a written opinion).

You always need Building Regulations approval, even for Permitted Development. Building Control checks structural calculations, fire safety (escape windows, smoke alarms, fire doors), stairs, insulation, and ventilation. Budget £1,000-£2,000 for Building Control fees and structural engineer drawings.

Return on investment

A loft conversion typically adds 10-15% to your property value, according to Nationwide. In London and the South East, the uplift can be 15-20%. You'll usually recover the full cost when you sell, and sometimes make a profit. Compare this to moving house (stamp duty, estate agent fees, removal costs), which can cost £20k-£40k without adding any value.

How long does it take?

Velux conversions: 4-6 weeks. Dormer conversions: 6-8 weeks. Hip-to-gable and mansard: 8-12 weeks. Weather affects the schedule (roof work stops in heavy rain). The builder will make your existing loft weatherproof within 1-2 days of opening the roof.

Getting quotes

Get quotes from at least 3 builders who specialise in loft conversions (not general builders). Ask to see photos of completed projects and talk to recent customers. Check they're registered with a trade body (Federation of Master Builders, Chartered Institute of Architectural Technologists). Expect quotes to vary by 20-30%, which is normal for this type of work.

A proper quote should include: structural engineer drawings, Building Control fees, materials, labour, scaffolding, skips, making good downstairs (plastering, decorating), and contingency. Don't accept a quote that says "subject to survey" without seeing a survey first.

Sources: Royal Institution of Chartered Surveyors (RICS) Building Cost Information Service, Federation of Master Builders, Gov.uk Planning Portal (Permitted Development guidance), Nationwide House Price Index (property value uplift data).