Average Cost of a Loft Conversion in 2026 (UK)
The average UK loft conversion costs around £45,000 in 2026 — but that single figure hides a range from £25,000 to over £100,000. This page explains why the average is the wrong number to budget with, and which figure to use instead.
Why the “Average” Loft Conversion Cost Misleads
Search for the average cost of a loft conversion and you will get a number somewhere around £45,000. It is a real figure — it is roughly where UK loft conversion spending clusters in 2026 — but it is close to useless for planning, because almost nobody actually spends it.
The reason is that a loft conversion is not one product. A Velux (rooflight) conversion, where you insulate the existing roof space, add rooflights and put in a staircase, is a fundamentally different job from a mansard, which rebuilds the entire roof structure. They share a name and nothing else. Averaging them produces a number that sits in the gap between two very different projects.
The gap is large. A Velux conversion at the bottom of its range and an L-shaped dormer at the top are separated by more than £75,000. If you budget £45,000 because that is the average, you have either massively over-budgeted for a rooflight conversion or fallen £50,000 short of a mansard.
What to Use Instead
Two numbers are more useful than the average. First, the range for your conversion type — which is determined largely by your roof shape and head height, not by preference. Second, the per-m² rate, which runs £1,000–£2,500 per m² in 2026 and lets you scale to your actual floor area. Multiply your usable loft area by the mid-point of that range and you will land closer than any national average will take you.
Average Loft Conversion Cost by Type
| Conversion Type | Cost Range (2026) | Typical Spend | Notes |
|---|---|---|---|
| Velux / rooflight | £25,000–£40,000 | £31,000 | No roof structure change; needs existing head height |
| Rear dormer | £30,000–£65,000 | £44,000 | The most common choice; adds real floor area |
| Hip-to-gable | £33,000–£70,000 | £49,000 | For hipped roofs; often paired with a rear dormer |
| Mansard | £38,000–£90,000 | £61,000 | Rebuilds the roof; maximum space, maximum cost |
| L-shaped dormer | £55,000–£100,000 | £73,000 | Victorian terraces with a rear outrigger |
Typical spend is the mid-market figure within each range. Ranges include VAT and building control but exclude furnishing. London and the South East typically run 20–30% above these figures.
What Moves You Within the Range
Once your type is fixed, four things decide where in the range you land. Floor area is the largest — at £1,000–£2,500 per m², an extra 10 m² is £10,000–£25,000. Bathroom inclusion adds £6,000–£12,000 once you account for soil-pipe routing as well as the fit-out. Staircase position matters more than people expect: a straightforward run over the existing stairs is cheap, while reconfiguring a landing or losing a bedroom to fit the stairs pulls in structural and making-good costs. And region shifts everything by 20–30% between the North and London.
Structural surprises are the wildcard. Undersized existing joists are near-universal and are priced in by any competent quote, but inadequate foundations for a mansard, or a party wall agreement that turns adversarial on a terrace, can add five figures. Build in 10–15% contingency on any loft project.
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Around £45,000, but the range runs from £25,000 for a simple Velux conversion to over £100,000 for an L-shaped dormer on a Victorian terrace. Because the types differ so much, the average is not a useful planning figure — identify which type your roof allows, then budget from that type's range.
Between £1,000 and £2,500 per m² in 2026. Rooflight conversions sit at the bottom of that band because there is no structural work; mansards and L-shaped dormers sit at the top because the roof is being rebuilt. Multiplying your usable loft area by the mid-point gives a better starting estimate than any national average.
A Velux or rooflight conversion, at £25,000–£40,000. It is cheapest because it leaves the roof structure alone — you insulate, add rooflights, lay a floor and fit a staircase. The catch is that it only works if you already have enough head height (a usable 2.2 m or more at the ridge). If you do not, you are into dormer or mansard territory regardless of budget.
Often, but not reliably, and it depends heavily on local ceiling prices. Adding a genuine double bedroom and en-suite to a three-bed house in an area where four-beds command a clear premium tends to perform well. In areas where prices are capped by street or property type, you can easily spend more than you recover. Check what four-bed properties on your street actually sell for before committing.
Allow 10–15% on top of the quoted price. Lofts hide problems — undersized joists, unexpected wiring, chimney breasts that need supporting, party wall matters on terraces. A competent surveyor and a detailed quote reduce the risk but do not remove it, and the contingency is what stops a surprise becoming a stalled project.