Extension vs Loft Conversion: Which Costs Less in 2026?
A loft conversion almost always costs less. Expect £30,000–£70,000 for a loft conversion against £35,000–£90,000 for a single-storey extension in 2026. The loft wins because the roof, walls and foundations already exist — you are fitting out a shell, not building one. Full head-to-head below.
Head-to-Head: Cost, Space and Disruption
Both add a bedroom-sized space to a typical UK home. The gap is in what you have to build from scratch — and in whether you lose garden.
Uses the existing roof structure and footprint. No new foundations, no lost garden. A Velux conversion at the bottom of the range is the cheapest route to an extra room.
Needs foundations, walls, a roof and often drainage diversions. Costs more per square metre but gives ground-floor living space a loft cannot.
Loft conversions run roughly £1,250–£1,800/m²; single-storey extensions £2,100–£3,000/m². That is the clearest way to compare like for like.
The only way to settle it for your house is real quotes. We match you with vetted specialists who can price the option that suits your property.
Get matched →Extension vs Loft Conversion — 2026 Comparison
| Factor | Loft Conversion | Single-Storey Extension |
|---|---|---|
| Typical total cost | £30k–£70k | £35k–£90k |
| Cost per m² | £1,250–£1,800 | £2,100–£3,000 |
| Build time | 6–10 weeks | 10–16 weeks |
| Planning permission | Usually permitted development | Often permitted development, but more likely to need consent |
| Garden lost | None | Yes — the full footprint |
| Value added | 10–20% | 5–15% |
| Best for | Extra bedroom or en-suite | Bigger kitchen or living space |
Figures are UK averages for Q3 2026. London and the South East typically run 20–30% higher on both options.
Why the Loft Usually Wins on Cost
1. No foundations
Foundations and groundworks typically swallow 15–20% of an extension budget — often £6,000–£12,000 before anything is visible above ground. A loft conversion skips this entirely; the existing walls already carry the load, with steels spreading the new floor.
2. The roof and walls already exist
You are converting a shell rather than building one. Even a dormer conversion, which does involve new structure, only rebuilds part of one roof slope.
3. Fewer external finishes
An extension needs brickwork, a new roof covering, guttering, external doors and often a rendered or matched facade. On a loft that list shrinks to dormer cladding and windows.
4. But the loft has hard limits
You need at least 2.2 m of head height across a usable area, and a compliant staircase has to fit without wrecking the floor below. Modern trussed-rafter roofs need the trusses cut and replaced with a new structure, which can add £5,000–£10,000 and narrows the gap considerably.
5. When the extension is the better buy
If you need kitchen or living space rather than a bedroom, a loft cannot deliver it — comparing cost alone misses the point. And on a bungalow or a house with a low-pitched roof, a loft conversion may be impossible or need a full roof rebuild costing more than an extension.
Same House, Both Options Priced
A three-bed 1930s semi in the Midlands, adding roughly 25 m² of space either way.
Option A: dormer loft conversion
- Structural steels, new floor and dormer: £18,500
- Staircase, insulation, plastering and electrics: £14,200
- En-suite, glazing, building control and fees: £9,800
- Total: £42,500 — about £1,700/m²
Option B: single-storey rear extension
- Groundworks and foundations: £11,400
- Superstructure, roof and glazing: £29,600
- Fit-out, drainage diversion, fees and building control: £18,000
- Total: £59,000 — about £2,360/m²
The loft comes in £16,500 cheaper for the same floor area — and keeps the garden. But it delivers a bedroom and en-suite, not the open-plan kitchen-diner the extension would give.
Extension vs Loft Conversion — FAQ
A loft conversion, in most cases. Lofts run £1,250–£1,800/m² against £2,100–£3,000/m² for a single-storey extension, mainly because there are no foundations and the roof and walls already exist.
Yes. If your roof uses modern trussed rafters they must be cut out and replaced with a new structure, adding £5,000–£10,000. Mansard conversions in conservation areas and low-pitch roofs needing a full rebuild can also overtake a modest extension.
A loft conversion typically adds 10–20% because it usually creates an extra bedroom, which is what buyers price on. A single-storey extension adds 5–15%. Both are capped by your street’s ceiling price.
Both often fall under permitted development, but limits apply on volume, height and position. Extensions are more likely to need consent, especially two-storey or side extensions. See our loft conversion planning guide and extension planning guide.
A loft conversion, typically 6–10 weeks against 10–16 for an extension. A loft is also less disruptive day to day because most of the work happens above your living space rather than through it.
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