Bathroom Extension Cost Calculator
A bathroom extension is small but expensive per square metre — drainage, waterproofing and fit-out all land in a tiny footprint.
Open the calculator →Price a bathroom extension in 30 seconds
5 steps. Covers size, shell spec, drainage, the suite itself and region.
How this calculator works
Bathroom extensions carry the highest cost per square metre of any extension type, because the expensive parts — drainage, waterproofing, sanitaryware, tiling — do not scale down with the footprint. A 6m² bathroom needs the same soil connection as a 12m² one.
Drainage is the variable that decides the job. If you can reach the existing stack with gravity fall, it is straightforward. If you cannot, a pumped system adds cost and a maintenance liability. Get this surveyed before you commit to a position for the room. It is worth knowing where the existing soil stack runs before you even sketch the layout, because a bathroom placed near it can use a short gravity connection while the same room on the far side of the house may force a pump, and that single decision can move the whole cost more than the choice of tiles or sanitaryware. Two things people underestimate here. The first is the fit-out: once the shell is up, the sanitaryware, tiling and screed can cost as much again as a standard bathroom refit, so it is worth pricing that separately with our bathroom renovation cost guide rather than assuming it is folded into the per-metre rate. The second is the boundary — a side or rear extension built up to a shared wall usually triggers the Party Wall Act, and a party wall surveyor is a cost and a lead time to plan for, not a surprise to meet halfway through.
Bathroom extension costs by region (2026)
| Region | Typical 2026 cost | Notes |
|---|---|---|
| London (inner) | £33,000–£45,000 | |
| London (outer) | £30,000–£40,000 | |
| South East | £28,000–£37,000 | |
| East Anglia | £25,000–£34,000 | |
| Midlands | £24,000–£32,000 | The national baseline |
| South West | £23,000–£31,000 | |
| North West | £22,000–£30,000 | |
| Yorkshire & Humber | £21,000–£29,000 |
Ranges match our bathroom extension cost guide and assume a typical 6–8m² room with a mid-range suite.
Bathroom extension calculator FAQs
Because the cost is in the services, not the floor area. Drainage, hot and cold feeds, waterproofing, ventilation, tiling and sanitaryware all have to fit into a few square metres, and none of them get cheaper because the room is small.
You either run new drainage to the existing stack — practical if you can achieve gravity fall — or fit a pumped system. Pumps work, but they are a moving part in a wall, they can be noisy, and they need occasional maintenance. Survey it before fixing the room position. Where a pump is unavoidable, budget for a quality macerator rather than the cheapest one and site it where it can be reached, because a buried pump that fails behind tiling is a far more disruptive and expensive fix than the small premium a better unit costs up front.
Yes. A bathroom extension is notifiable in its own right, and the drainage, ventilation and electrics within it all fall under the regulations — Part P for the electrical work in particular, given the zones around a bath or shower.
Almost always, by a wide margin — you avoid foundations, roof and external walls entirely. An extension makes sense when you genuinely have no room to convert, or when the layout only works by adding footprint. If space is the only reason, it is worth pricing a stolen corner of an adjacent room or a landing against a full extension first, since a well-planned internal reconfiguration often finds the room you need at a fraction of the cost of building outwards.
Related cost guides
- Bathroom extension guide — full 2026 pricing by region
- Bathroom cost calculator — price a refit without extending
- Extension cost calculator — price a general extension
- Plumbing cost calculator — drainage and pipework costs
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