Loft conversion costs and what's involved

Turning unused roof space into a proper room is one of the more cost-effective ways to add space to a house, but the price and the structural work involved vary a great deal depending on which type of conversion actually suits the roof you've got.

A rooflight, or Velux-style, conversion is the simplest and cheapest option, since it works within the existing roof shape and just adds windows set into the existing roofline, a staircase, insulation and flooring, without altering the roof's structure or profile. It commonly costs in the region of £25,000-£40,000 fully finished, working out to roughly £1,000-£1,500 per square metre of new floor area. It suits roofs with reasonable existing head height and doesn't usually require major structural alteration, which keeps both the cost and the build time down compared with the other options.

A dormer conversion adds a box-shaped structure that projects out from the existing roof slope, typically at the rear, which meaningfully increases both head height and usable floor space compared with a rooflight conversion alone. It's the most common type of loft conversion in the UK and commonly costs £30,000-£65,000 fully finished, around £1,200-£1,900 per square metre. The extra cost over a rooflight conversion reflects the additional structural work, new external walls, roof alterations and often a larger window arrangement that a dormer involves.

A hip-to-gable conversion changes the shape of the roof itself, extending a sloped, "hipped" side of the roof outward into a vertical gable wall, which is common on detached and semi-detached houses with a hipped roof design and significantly increases the usable loft space before a dormer is even added on top. It commonly costs £33,000-£70,000, roughly £1,400-£2,100 per square metre, reflecting the more substantial structural alteration to the roof shape compared with a straightforward dormer.

A mansard conversion is the most extensive and expensive option, replacing most of one or both roof slopes with a steeply angled, near-vertical structure that maximises internal head height and floor space across the width of the house. It commonly costs £38,000-£90,000, around £1,700-£2,500 per square metre, and is often chosen where planning constraints or an unusually low existing roof make the other options impractical. Across all four types, London and the South East commonly add a further 20-30% to these figures, and inner London mansard conversions in particular can run considerably higher.

Building regulations approval is required for every loft conversion type, regardless of whether planning permission is also needed, since converting a loft into habitable space changes how the building performs structurally and in a fire. Key areas Building Control checks include structural support for the new floor and any altered roof, a compliant fire escape route, commonly requiring upgraded fire doors and smoke alarms linked throughout the house, adequate insulation for energy performance, and a staircase that meets minimum headroom and pitch requirements. A loft conversion cannot legally be signed off as habitable space without this approval, regardless of how the planning side of the project is handled.

The Party Wall Act 1996 becomes relevant whenever a loft conversion involves work on a wall shared with a neighbour, which in practice means most semi-detached and terraced properties, since a hip-to-gable or dormer conversion commonly ties new structure into that shared wall. Written notice needs to be served on the affected neighbour at least two months before work starts, and they then have 14 days to consent or formally dissent; if they dissent, a party wall surveyor, often a single "agreed surveyor" acting for both sides, is appointed to produce a formal award setting out how the work can proceed, with the building owner usually covering the surveyor's reasonable costs. Skipping this step where it applies doesn't stop work being carried out, but it does leave the door open to a neighbour dispute or an injunction partway through a project, which is a considerably worse outcome than the two-month lead time the notice period requires.

Which type actually suits a particular house usually comes down to the existing roof pitch and head height as much as to budget. A shallow-pitched roof often doesn't leave enough head height for a straightforward rooflight conversion to meet the minimum standard needed for a proper bedroom, in which case a dormer, which adds height as well as floor space, becomes the practical option rather than simply the more expensive one. Intended use matters too: a home office or occasional guest room can work well as a simpler, cheaper rooflight conversion, while a full-time bedroom with an en-suite bathroom generally needs the extra space a dormer or hip-to-gable conversion provides, plus additional plumbing runs that add their own cost on top of the structural work. A conversation with a builder or architect about existing roof height, before committing to a particular type on cost grounds alone, is worth having early, since it can rule out the cheapest option before much time is spent designing around it.

Frequently asked questions