How Much Does a Flat Roof Bathroom Extension Cost in 2026? (UK)
A flat-roof bathroom extension in the UK costs £15,000–£30,000 all-in in 2026. The shell runs £1,800–£2,600 per m²; the bathroom fit-out adds £4,000–£9,000 on top. A 4m² downstairs WC sits at the bottom of that range; a 6m² family bathroom or wet room at the top.
What you are actually paying for
A flat-roof bathroom extension is a small single-storey addition — usually a rear or side infill of 3–7m² — built specifically to house a downstairs WC, family bathroom, en-suite or wet room. Two cost blocks combine: the building shell (foundations, walls, warm flat roof, glazing, plaster) and the bathroom fit-out (first and second-fix plumbing, tiling, sanitaryware, extraction and electrics).
Flat roof bathroom extension cost table 2026
| Element | 2026 UK cost |
|---|---|
| Shell, per m² | £1,800–£2,600 |
| 4m² downstairs WC (all-in) | £15,000–£22,000 |
| 6m² family bathroom (all-in) | £20,000–£30,000 |
| Wet room upgrade (tanking + gradient) | +£1,500–£3,500 |
| Warm flat roof (EPDM / GRP) | £90–£140 per m² |
The five factors that move the price
- Drainage runs: tying into an existing soil stack is cheap; a new run to the main sewer adds £1,500–£4,000.
- Roof spec: a warm-deck EPDM or GRP roof meets Building Regs Part L and outlasts felt by decades — do not accept a cold-deck felt quote.
- Glazing: a rooflight adds £600–£1,400 but is often essential for ventilation and daylight in an internal bathroom.
- Sanitaryware tier: a budget suite is £600; a mid-range walk-in shower and vanity setup is £2,500–£4,500.
- Underfloor heating: electric UFH under tile adds £700–£1,400 and is the standard upgrade for a wet room.
Do you need planning permission?
Most rear flat-roof extensions fall under Permitted Development if they extend no more than 3m (attached) or 4m (detached) from the original rear wall and stay under 4m in height. Conservation areas and Article 4 zones need full planning. You will still need Building Regulations approval for structure, drainage, ventilation and electrics regardless of the planning route. See our extension planning permission guide.
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