Loft conversion cost per m² in 2026: rates and what drives them
Loft conversions are quoted as a total, but comparing them properly means working back to a rate. In 2026 that rate runs from about £1,300/m² for a straightforward rooflight conversion to £3,200/m² for a mansard. The counterintuitive part — and the thing that catches most homeowners out — is that smaller lofts cost more per square metre, sometimes dramatically. This page explains the rates, the reason for that, and where your project is likely to land.
- Rooflight: £1,300–£1,900/m²
- Dormer: £1,600–£2,300/m²
- Mansard: £2,200–£3,200/m²
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2026 loft conversion rates per square metre
Rates below are typical UK figures for a finished habitable room to normal domestic standard, including structure, staircase, insulation, windows, fire safety works, plastering, electrics and building control. They exclude VAT on any element where it applies, furniture and any en suite.
| Conversion type | 2026 rate per m² | Typical new floor area | Typical total |
|---|---|---|---|
| Rooflight (within existing roof) | £1,300 – £1,900 | 20 – 30m² | £30,000 – £45,000 |
| Rear dormer | £1,600 – £2,300 | 25 – 35m² | £40,000 – £60,000 |
| Hip-to-gable | £1,800 – £2,600 | 28 – 38m² | £45,000 – £70,000 |
| L-shaped dormer | £1,900 – £2,700 | 32 – 45m² | £55,000 – £80,000 |
| Mansard | £2,200 – £3,200 | 35 – 45m² | £60,000 – £90,000+ |
London and the South East typically sit 25 to 35 percent above these rates. Northern England, Wales and much of Scotland sit at or below the lower end.
Why smaller lofts cost more per square metre
This is the most important thing to understand before you compare rates, because it explains most of the apparent inconsistency between quotes.
A large share of a loft conversion is fixed cost that does not scale with floor area:
- The staircase and the opening formed for it
- The steel beams carrying the new floor
- Fire doors to the existing bedrooms and living room
- Interlinked mains-wired smoke alarms throughout
- The structural engineer's calculations
- The building control application and inspections
- Scaffolding and site set-up
Those items cost roughly the same whether you gain 18m² or 38m². Spread across a small loft they push the rate right up. The practical consequence: if you are choosing between a modest conversion and a slightly larger one, the extra square metres are usually far cheaper than the first ones. Going bigger often improves value per square metre considerably.
Worked comparison
Take £26,000 of fixed costs. On a 20m² rooflight conversion that alone is £1,300/m² before a single variable cost. On a 38m² hip-to-gable it is £684/m². Same money, very different rate — and it is why a headline "cost per m²" comparison between two differently sized lofts tells you almost nothing on its own.
Where the money goes
A representative breakdown for a dormer conversion, as a share of total cost:
| Element | Share of cost | Notes |
|---|---|---|
| Structure — steels, new floor, dormer frame | 20 – 25% | Largest single element |
| Roofing, insulation and weatherproofing | 15 – 20% | Dormer cheeks, flat roof, thermal build-up |
| Staircase and opening | 8 – 12% | Plus alterations to the floor below |
| Windows and rooflights | 6 – 10% | Rises fast with size and specification |
| Fire safety works | 5 – 8% | FD30 doors, alarms, protected stairway |
| Electrics, plumbing and heating | 10 – 15% | More if an en suite is included |
| Plastering, decoration and flooring | 12 – 18% | Often excluded from cheap quotes |
| Fees — engineer, drawings, building control | 5 – 8% | Check who is paying these |
What moves your rate within the range
Your existing roof structure
The biggest hidden variable. Older houses generally have a traditional cut roof — individual rafters and purlins — which leaves usable open space and is comparatively straightforward to convert. Houses built from roughly the mid-1960s onward tend to have W-shaped trussed rafters, which form a single engineered structure that cannot simply be cut away. Converting one requires a new structural floor and a redesigned support system installed before the trusses come out, commonly adding £5,000 to £15,000.
Available headroom
You want at least 2.2 to 2.4m from the top of the existing ceiling joists to the underside of the ridge. That accommodates the new floor build-up and insulated ceiling while leaving usable height. Below about 2.2m a rooflight conversion will not work, and you are pushed into a dormer, hip-to-gable or mansard to create height — a step change in cost, not an incremental one.
Whether you add an en suite
Typically £6,000 to £12,000 including drainage, a pump where gravity drainage is not available, tiling and fittings. Because it occupies only a few square metres it lifts the overall rate noticeably. Most people still judge it worthwhile, as a loft bedroom with an en suite adds materially more value than one without.
Planning route
Many rear dormers and hip-to-gables fall under permitted development, subject to volume allowances and other conditions. A mansard, or anything on designated land or a flat, will usually need full planning permission — adding fees, several months, and design constraints that can raise build cost.
Party wall matters
On a terrace or semi, inserting steels into a shared wall triggers the Party Wall etc. Act. Budget for surveyor fees, and allow time for notices — this is a programme risk as much as a cost.
Access
If materials and steels cannot be brought through or around the house easily, everything slows down. Restricted rear access or a need to crane steels in can add several thousand pounds.
Specification
The difference between a builder-standard finish and a high-specification one — bespoke joinery, premium glazing, underfloor heating, feature staircase — can be £300 to £600/m² on its own.
Using the rate sensibly
Per-square-metre rates are a budgeting tool, not a quoting tool. Use them to sanity-check whether a quote is in the right territory and to compare conversion types against each other. Do not use them to pick between two contractors — for that you need itemised quotes covering identical scope, particularly on fire doors, decoration and building control fees, which are the usual omissions.
For total project figures by conversion type, see our wider cost guides, or read our guide to choosing a loft conversion specialist.
FAQs: loft conversion cost per m² (UK, 2026)
What is the loft conversion cost per square metre in 2026?
In 2026 UK loft conversions typically run £1,300–£1,900/m² for a simple rooflight conversion, £1,600–£2,300/m² for a rear dormer, £1,800–£2,600/m² for a hip-to-gable, and £2,200–£3,200/m² for a mansard. London and the South East commonly sit 25–35 percent above these figures.
Why does a small loft conversion cost more per square metre?
Because a large share of the cost is fixed regardless of size. The staircase, steel beams, fire doors to the existing rooms, structural engineer, building control application and scaffolding cost roughly the same whether you gain 18m² or 38m². Spreading those fixed costs over a smaller floor area pushes the rate up sharply.
Does a trussed rafter roof cost more to convert?
Yes, usually significantly. Houses built from roughly the mid-1960s onward often have W-shaped trussed rafters, which form a single engineered structure and cannot simply be cut away. Converting one requires a new structural floor and a redesigned roof support system before any trusses come out, commonly adding £5,000–£15,000 compared with a traditional cut roof of the same size.
How much headroom do I need for a loft conversion to be viable?
As a rule of thumb, at least 2.2–2.4m measured from the top of the existing ceiling joists to the underside of the ridge. That allows for the new floor build-up and insulated ceiling while leaving usable head height. Below about 2.2m a rooflight conversion is unlikely to work and you are looking at a dormer, hip-to-gable or mansard to create the height.
Does an en suite change the cost per square metre?
Yes. An en suite typically costs £6,000–£12,000 including drainage, a pump if gravity drainage is not available, tiling and fittings. Because it occupies only a few square metres it raises the overall rate noticeably. Most people still consider it worthwhile, since a loft bedroom with an en suite adds materially more to property value than one without.
Get a rate for your actual loft
Your roof structure, headroom and access decide where in the range you land, and none of them can be judged from a table. Compare three itemised quotes from specialists who have surveyed your loft.