Loft conversion costs in London: Velux, dormer, hip-to-gable and mansard

Which loft conversion suits your house?

The right type depends on your roof shape and how much space you need. Victorian and Edwardian terraces with a pitched roof and a rear outrigger suit a rear dormer, which creates a full-height room across the back of the house. 1930s semis with a hipped roof usually need a hip-to-gable conversion to make the stair landing and a dormer work. Georgian and Victorian terraces in Islington, Camden and Westminster are often limited to a mansard, which is more expensive but adds the most floor area.

A rooflight conversion keeps the existing roof shape and only adds Velux windows. It is the cheapest option but only works where you already have 2.4 m or more at the ridge.

Cost breakdown by type

These are ICV Buildings' typical 2026 prices for Greater London, including structural steels, a new staircase, insulation to Building Regulations, electrics, plastering and decoration, and a shower room where the layout allows. They exclude VAT and bespoke fittings.

• Rooflight (Velux) conversion: £35,000–£50,000
• Rear dormer (flat roof, full width): £50,000–£85,000
• L-shaped dormer over the outrigger: £65,000–£95,000
• Hip-to-gable with rear dormer: £65,000–£95,000
• Mansard: £80,000–£120,000
• Add an en-suite bathroom: £8,000–£15,000

Do you need planning permission?

Most London lofts are built under permitted development: you can add up to 40 m³ of roof space on a terrace and 50 m³ on a semi or detached house, provided the dormer sits below the ridge, is set back 20 cm from the eaves and uses matching materials. Front dormers and anything in a conservation area, on a flat or on a listed building need full planning permission.

Several boroughs, including Islington and parts of Camden and Hackney, have removed permitted development rights for roof extensions through Article 4 directions. We check this before you commit and apply for a Lawful Development Certificate so you have proof when you sell.

Head height, stairs and Building Regulations

You need at least 2.2 m from the top of the ceiling joists to the underside of the ridge for a conversion to be viable, and the finished room should have 2 m of headroom over the stairs. The staircase usually lands over the existing stairs to save space, and the new floor is strengthened with steel beams and new joists so the old ceiling joists are not loaded.

Building Regulations also require a protected escape route: fire doors to every room off the hall and stairs, interlinked smoke alarms on every floor and, in most cases, an escape window in the new room. Insulation must meet Part L, which is why modern lofts use high-performance PIR boards between and under the rafters.

How long does it take?

A dormer loft takes 8–10 weeks on site; a hip-to-gable or mansard takes 10–12 weeks. Scaffolding goes up first, the roof is opened and the dormer built and made watertight within three weeks, then the internal work follows. Because most of the work is done from outside, families usually stay in the house throughout.