How Much Does a Flat Roof Extension Cost in 2026? (UK)
A single-storey flat roof extension costs roughly £2,000–£2,600 per m² in 2026 for a good-quality build. That puts a typical 20m² rear extension at around £40,000–£52,000, rising to £55,000–£65,000+ with premium glazing, a lantern and a full kitchen fit-out. Flat roofs usually come in a little cheaper than pitched-roof extensions, build faster, and pair beautifully with a large rooflight or roof lantern to flood the space with light. The final figure hinges on ground conditions, glazing, internal fit-out and your region. Here's the full breakdown.
Flat roof extension cost by size (2026)
| Size | Typical cost (standard) | With premium glazing & fit-out |
|---|---|---|
| 15m² | £30,000–£39,000 | £42,000–£50,000 |
| 20m² (typical rear) | £40,000–£52,000 | £55,000–£65,000 |
| 25m² | £50,000–£65,000 | £68,000–£80,000 |
| 30m² (large open-plan) | £60,000–£78,000 | £82,000–£95,000 |
Ranges assume a warm flat roof (EPDM/GRP/single-ply), standard foundations and a mid-range specification. London and the South-East typically sit 10–20% above these figures.
Where the money goes
| Element | Share of budget | Notes |
|---|---|---|
| Foundations & groundworks | 15–20% | Rises sharply with poor ground, trees or drainage diversions |
| Structure (walls, steel, flat roof) | 30–35% | Warm-roof build-up and any large steel beam for open-plan |
| Glazing (doors, rooflight/lantern) | 15–25% | Biggest single variable — bi-folds and lanterns add fast |
| Internal fit-out | 20–30% | Kitchen or bathroom fit-out pushes the top of the range |
| Fees & regs | 5–10% | Design, structural engineer, Building Control, party wall |
Flat roof vs pitched roof extension
A flat roof extension is usually £3,000–£8,000 cheaper than an equivalent pitched-roof build of the same footprint, because there's less structure and roof covering. It also builds faster and suits contemporary rear extensions where a lantern or large rooflight does the aesthetic work. Modern warm flat roofs in EPDM, GRP fibreglass or single-ply membrane carry 20–25 year guarantees, so the old reputation for leaks no longer applies when the detailing is done properly.
How to keep the cost down
- Stay within permitted development where possible to avoid a full planning application.
- Choose one statement rooflight rather than wall-to-wall bi-folds if budget is tight — light for less money.
- Get 3 like-for-like fixed quotes on the same drawings and spec so you're comparing the same job.
- Keep the kitchen/bathroom simple at first fix; you can upgrade finishes later.
FAQs
Related guides
- How much does a rear extension cost in 2026?
- Single vs double storey extension cost
- Which extension type adds most value?
- Do I need planning permission for an extension?
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