What the national figures say, what drives the number, and why the roof matters more than the floor area.
Arrange a site visitThese are published UK national averages, not our prices. They are collected from third-party cost guides so you have a sense of scale before you speak to anyone. National figures are weighted by London and the South East, where build costs run well above South Wales, so treat them as an upper reference rather than a quote. We price each job after a site visit, because roof type and staircase position move the number more than floor area does.
Published UK guides put the average loft conversion at somewhere around £50,000 in 2026, with the realistic spread running from roughly £20,000 for a simple rooflight conversion to £100,000 and beyond for a mansard in inner London. The ranges below are the national figures by type.
| Type | UK average range (2026) | Notes |
|---|---|---|
| Velux / rooflight | £15,000 – £30,000 | No change to the roof shape; lowest cost of the four by a wide margin |
| Dormer | £30,000 – £60,000 | The most common choice on Cardiff terraces |
| Hip-to-gable | £45,000 – £65,000 | Usually on 1930s semis; often paired with a dormer |
| Mansard | £45,000 – £60,000 | Most floor space, most structural work, most likely to need planning |
On a per-square-metre basis the guides land at roughly £1,200 to £3,000 all-in once stairs, a bathroom and decoration are included. That spread is so wide because a square metre of trussed roof with a steel and an en-suite is simply not the same product as a square metre of rooflight conversion in a terrace.
Floor area is a weak predictor. These are the things that genuinely change the number, roughly in order of how much they move it:
The build figure is not the project figure. Budget separately for structural calculations and drawings, the Building Control application, a planning application where permitted development does not apply, a Party Wall surveyor if you share a wall, scaffold, waste removal, and the redecoration of the rooms the new staircase passes through. None of these are large individually. Together they are not a rounding error.
A loft conversion is usually cited as one of the more cost-effective ways to add usable space, because you are building inside a footprint you already own and under a roof that already exists. Whether it pays back depends on your street, not on a national percentage — a fourth bedroom adds more in an area where families are competing for three-bed houses than in one where they are not. The honest answer for your address comes from a local estate agent, and we would rather you asked one before committing than take our word for it.
If a conversion is the wrong answer for your house — not enough head height, a roof that would need rebuilding, or a stair that would cost you the room it is meant to serve — we will tell you at the site visit rather than after the drawings are paid for.
A short site visit tells us the roof type, the head height and where the stairs can land — which is most of what decides the design and the price. We will tell you if a conversion is not the right answer for your house.
Arrange a site visitAll loft conversions