Building an extension specifically for a new family bathroom, downstairs wet room, or luxury ensuite adds immense utility and comfort to a home. However, because bathrooms require complex water feeds, waste drainage, hot water heating, safety electrics, and waterproof tiling, a bathroom extension involves specialized labor and trades.
In this 2026 cost guide, we break down the average construction costs of a bathroom extension in the UK, costs for suites and tiling, additional plumbing fees, and how to hire vetted contractors using GetBuilder.
Average bathroom extension costs in 2026
On average, building a bathroom extension in the UK costs between £1,500 and £2,800 per square metre for the structural build. Due to the high concentration of plumbing and tiling, smaller extensions often have a higher cost per square metre as the fixed costs (groundworks, structural engineering) are spread over a smaller area.
Estimated project costs by extension size (national averages, excluding VAT):
| Project Type | Size (m²) | Average Build Cost (Shell only) | Average Total Cost (inc. Fit-out) |
|---|---|---|---|
| Small Ensuite Extension | 4 m² | £7,000 - £11,000 | £12,000 - £18,000 |
| Standard Family Bathroom | 8 m² | £12,000 - £20,000 | £18,000 - £30,000 |
| Large Luxury Bathroom/Spa | 15 m² | £22,500 - £38,000 | £32,000 - £55,000+ |
Cost breakdown of bathroom fit-outs
Once the extension shell is built, you must budget for the bathroom suite, tiling, and installation:
- Bathroom Suite Supply: A standard three-piece suite (bath/shower, toilet, basin) ranges from £800 to £2,500. Premium designer suites range from £3,000 to £8,000+.
- Wall and Floor Tiling: Essential for waterproofing. Tiling labor and materials typically cost £50 to £90 per square metre. For a standard bathroom, expect to pay £1,500 to £3,500 for complete tiling.
- Plumbing Labor: Connecting the new waste pipes to the mains sewer and laying hot/cold copper lines typically costs £1,500 to £3,000.
- Electrical & Lighting: Bathroom electrics must comply with strict Zone Safety regulations. Expect to pay £600 to £1,500 for IP-rated LED downlights, extractor fans, and shaver sockets.
Key cost drivers for bathroom extensions
Several structural factors can impact your final project price:
- Distance to the Soil Stack: Your toilet waste must feed into a 110mm soil pipe. If your extension is built far from your property's existing soil stack, you will need to pay an extra £1,000 to £2,500 to install a new stack, run underground drainage, or install a macerator pump.
- Boiler Capacity: A new bathroom increases the demand for hot water. If you are adding a luxury walk-in shower or a large freestanding tub, you must ensure your combi boiler has the required flow rate. Upgrading your boiler can add £2,200 to £3,500 to the project.
- Ventilation (Building Regulations): Bathrooms must have mechanical ventilation to prevent damp and mould. A high-quality, quiet extractor fan must be installed and vented to the outside, which is inspected by Building Control.
Hire vetted bathroom specialists on GetBuilder
Bathroom extensions require seamless coordination between builders, bricklayers, plumbers, tilers, and electricians. To find trusted bathroom extension builders, follow these steps:
- Post your job requirements: Outline your preferred layout, sizes, and fixtures on GetBuilder for free.
- Compare fully itemized quotes: Ensure quotes break down demolition, structural groundworks, drainage connections, tiling, and electrical certificates separately.
- Verify certifications: Ensure your builder holds valid public liability insurance, and that your plumber and electrician are properly registered (e.g. Gas Safe, NICEIC).
