Loft conversion costs vary more than almost any other home improvement. The answer depends heavily on the type of conversion, not just the size of the roof. Here’s what the current figures actually look like, and why a Chelsea or West London conversion tends to sit at the higher end.
Typical cost ranges by conversion type
A typical UK loft conversion of 20–50 square metres costs somewhere between £55,000 and £120,000. That’s including VAT, before fittings and planning fees, according to Resi’s loft conversion cost guide. London costs sit above that range, at roughly £60,000–£130,000. Conversions outside London average £50,000–£80,000. The type of conversion drives most of that spread:
| Conversion type | Typical cost |
|---|---|
| Hip-to-gable (no dormer) | around £40,900 |
| Main dormer | £49,000–£65,400 |
| Hip-to-gable with dormer | £47,780–£77,630 |
| L-shaped dormer | around £62,535 |
| Roof terrace addition | around £18,165 |
These figures come from Resi’s published cost data. They’ll move with material and labour costs, so treat them as a starting point for budgeting, not a fixed quote. Every roof and every party wall situation is different.
What sits inside those numbers
A big chunk of a loft conversion budget goes on structural work most people never see. That includes strengthening floor joists, fitting steel beams to support the new roof, and building the staircase up to the new floor. So staircases alone range from around £250 for a simple straight flight to £9,000 for a bespoke design, per Resi’s figures. Windows are the other major line item. For instance, Velux-style rooflights run £900–£1,300 each, while a full dormer window costs £4,500–£6,500 once built and glazed. On top of that, a pre-conversion structural survey typically costs £500–£1,500, and most lenders and building control officers will expect to see one.
Why Chelsea and West London conversions cost more
London loft conversions sit above the national average for a few consistent reasons. Scaffolding and access cost more on dense terraced streets with limited parking. Skip and materials delivery is more constrained too. And a lot of Chelsea’s housing stock sits within conservation areas, which can mean a full planning application rather than permitted development — see our guide to loft conversion planning permission for how that works. That adds professional fees and time before the build even starts.
Period roof structures are a factor too. Older Victorian and Edwardian roofs in Chelsea and neighbouring Fulham were rarely built with a habitable loft in mind. So structural strengthening tends to be more involved than on a modern estate roof. That’s not a reason to avoid a conversion. It’s simply a cost a fair quote should itemise, not hide.
Getting an accurate quote
Decide roughly what you want the space for before you ask for quotes. A bedroom with en-suite is a different structural job to a simple home office, and that shapes whether a rooflight conversion is enough or a full dormer is needed. From there, ask each roofer or builder to break the price down: structural work, staircase, windows, insulation, finishing. Don’t accept one lump figure. We use the same principle when quoting new roofs — a number without a breakdown is hard to compare fairly against a competitor’s quote.
Weighing up a loft conversion in Chelsea, Wandsworth, or anywhere across West London? Get in touch for a free site visit and an itemised, no-obligation quote.