How Much Does a Bathroom Extension Cost in 2026? (UK)
A new UK bathroom extension in 2026 typically costs £15,000–£45,000, with a national average of £26,500 for a 5–6m² mid-range extension. Price is driven by three things — size, plumbing complexity, and spec. A small 3m² cloakroom extension runs £15k–£22k; a family bathroom extension with vanity, bath, shower and WC can reach £42k+. This guide shows what your budget actually buys you in 2026, with worked examples, a cost-per-m² ladder, and the hidden extras (macerators, drainage, waste runs) that catch homeowners out.
How much is a bathroom extension in 2026?
Typical 2026 UK bathroom extension costs by size:
- Small cloakroom extension (2–3m²) — £15,000–£22,000
- En-suite extension (3–5m²) — £18,000–£28,000
- Family bathroom extension (5–7m²) — £24,000–£35,000
- Large family bathroom (7–10m²) — £30,000–£42,000
- Luxury wet room / spa bathroom (10m²+) — £38,000–£55,000+
Add 15–30% for London and the South East. Upper-floor extensions run 15–25% above ground-floor equivalents due to structural work and scaffolding.
Bathroom Extension Cost by Size
The footprint you choose drives everything else — bigger extensions need deeper foundations, more scaffolding, longer plumbing runs and more finishes. Here's what each size realistically gets you in 2026.
2–3m² — Cloakroom / WC extension
Just enough for a WC, small basin and a narrow storage unit. Typically a single-storey lean-to off the side or rear. Minimum viable bathroom addition — useful for families with only one bathroom who want a downstairs WC.
3–5m² — En-suite extension
Adds an en-suite bedroom extension or an en-suite off an existing master — shower, WC, basin, sometimes a compact vanity. The most common bathroom extension we see, typically added to 3-bed semis and detached homes upgrading to 4-bed layouts.
5–7m² — Family bathroom extension
Full family bathroom: bath, separate shower, basin, WC, space for a towel rail and light storage. Suits homes where the existing bathroom is tight (3–4m²) and the owners want a proper premium bathroom to match a 4–5 bedroom plan.
7–10m² — Large family bathroom or master en-suite
Premium layout: double vanity, freestanding bath, walk-in shower enclosure, separate WC. Typical in 5-bed extensions or master en-suites aimed at long-term owners who want a boutique-hotel feel.
10m²+ — Luxury wet room or spa bathroom
Full spa: steam shower, freestanding bath with view, underfloor heating across the whole slab, feature tile or stone walls, bespoke joinery. You're paying more for fittings and finishes than for the shell at this spec.
What Actually Drives Your Final Bill
Two 6m² bathroom extensions can land £15,000 apart — same footprint, very different price. Here's what explains the spread.
1. Plumbing run distance
Every extra metre of new soil stack, hot/cold feed and waste adds cost. Extending off the kitchen wall (where plumbing already exists) is cheapest. Extending off the opposite end of the house can add £2,000–£5,000 just in pipe runs and trenching. If you need a macerator (no gravity fall available), add £600–£1,200.
2. Foundations & ground conditions
Standard strip foundations on firm clay add £2,500–£4,500. If you hit trees, shrinkable clay, a high water table, or underground obstructions, you'll need piled or raft foundations — £6,000–£12,000. A ground survey during design (£400–£900) is cheap insurance against a nasty discovery on day 3 of a build.
3. Sanitaryware & fixtures spec
Budget suite (high-street brand): £900–£1,800. Mid-range (Burlington, Crosswater): £2,500–£4,500. Luxury (freestanding cast-iron bath, designer brass ware, marble-look vanity): £6,000–£14,000+. This single decision can add or remove £8,000 from your total.
4. Tile & finish choices
Large-format porcelain with simple grid patterns is cheapest to fit. Mosaics, book-matched stone slabs, and patterned micro-cement all push labour up. Budget tile package: £1,200–£2,000. Premium (book-matched porcelain, feature wall): £3,500–£5,500. Stone slabs or Venetian plaster: £5,500–£9,000+.
5. Roof type
A simple pitched tile roof matching the main house is cheapest (£3,500–£5,500). A flat roof with EPDM or warm-roof build-up costs similar (£3,000–£5,000). A glass lantern adds £2,500–£5,500. A double-pitched slate roof can be £6,000–£9,000.
6. Location / region
London labour rates run 25–40% above national average. South East adds 15–25%. Midlands and North West sit near the national average. Scotland and Wales are 5–15% below. Check our town-level data on the builder directory.
Bathroom Extension Cost by UK Region
Based on real project data from 519 UK towns — typical mid-range 5–6m² family bathroom extension.
Real Project: Family Bathroom Extension in Reading
Semi-detached 3-bed home, single-storey rear extension adding a 6m² family bathroom. Completed February 2026, Berkshire.
| Architect drawings, Building Regs application | £2,200 |
| Foundations & ground slab (clay soil, 900mm depth) | £3,400 |
| Walls (brick + block cavity, matching mortar) | £4,900 |
| Pitched tile roof + rainwater goods | £3,800 |
| First-fix plumbing (new soil stack, hot/cold) | £2,600 |
| First-fix electrics (lighting, extraction, UFH) | £1,700 |
| Plastering & tanking | £1,800 |
| Sanitaryware (mid-range suite + brass fittings) | £3,200 |
| Tiles (large-format porcelain, feature wall) | £2,100 |
| Walk-in shower glazing & tray | £1,400 |
| Tiling labour, second-fix, commissioning | £2,800 |
| VAT (on £27k ex-VAT subtotal) | £5,400 |
| Contingency used (3%) | £850 |
| Total (11 weeks on site) | £31,450 |
Common Questions
Related Guides
More cost, planning and comparison guides to help you make the right call for your project.
How much does a rear extension cost?
Rear extensions are the most popular type — 2026 cost breakdown by size, spec and region.
Read Guide →Do I need planning permission for an extension?
Understand when Permitted Development covers your project.
Read Guide →Bathroom renovation cost UK 2026
Cheaper alternative to extending — refurbish for £3.5k–£15k.
Read Guide →