A straightforward loft conversion runs between £1,500 and £2,500 per square metre. On a typical 30 square metre conversion that's £45,000 to £75,000.
Those figures are for handing the whole job to a builder and letting them run it. If you manage the project yourself and bring in your own trades, you'd expect to pay around 20% less. That's the basis Avalor uses, so the figures it gives you will sit below the ranges here.
The type of conversion matters more than anything else. A rooflight conversion, where you keep the existing roof and just add windows and a staircase, sits at the bottom of the range. A dormer, where you build out to create headroom, adds £10,000 to £20,000. A hip to gable or mansard, where you rebuild a section of roof entirely, pushes you toward the top end and beyond.
Location moves the number a lot. Outer London runs about 25% above the national average and inner London more than that, mostly down to labour rates and access. Getting materials into a terraced street in Zone 3 costs more than a driveway in Derby. The South East is nearer 5 to 10% above, and the North East and Wales sit around 8 to 10% below.
The build is only part of it. You'll also need a structural engineer, architect drawings, building control and possibly party wall agreements if you're attached to a neighbour. Allow around 12% on top of the build for fees and at least 10% for contingency, though both depend on the job and where it is.
Most loft conversions fall under permitted development, so you won't need planning permission. The exceptions are conservation areas, flats, and anything that goes beyond the permitted volume, which is 40 cubic metres for a terrace and 50 for a semi or detached.
Avalor prices this using BCIS benchmark rates, adjusted for your postcode and set to self delivery, so the figures assume you're running the job with your own trades rather than handing it to a builder. If you've got a real quote you can put that in instead.