A downstairs toilet is one of the most useful upgrades you can make to a family home. In London, adding or renovating one usually costs £3,000 to £7,000, and up to £9,000 or more if new external drainage is needed. The single biggest factor is how far the new toilet is from your existing soil pipe.
Indicative 2026 London prices for a complete job, including labour, materials, tiling and decoration. London rates usually run 20 to 25% above national averages. A fixed price needs a site visit.
Downstairs toilet costs at a glance
| Type of job | Typical London cost | Usual time on site |
|---|---|---|
| Refit an existing cloakroom (same layout) | £2,500 to £4,500 | 3 to 5 days |
| New under-stairs WC, close to the soil stack | £3,500 to £6,000 | 4 to 7 days |
| New WC with a macerator (pumped waste) | £4,000 to £7,000 | 4 to 7 days |
| New WC needing external drainage work | £6,000 to £9,000+ | 1 to 2 weeks |
National 2026 guides put a simple installation at around £1,500 to £3,000, but that usually excludes tiling, decoration and the higher labour rates of London. Our own bathroom renovation cost guide puts a London cloakroom at £3,000 to £6,000, which is where most of our projects land.
What affects the price
1. Drainage
A toilet needs a 100mm (4 inch) waste pipe that falls steadily to a soil stack or drain. If your under-stairs space sits against the wall where the soil stack runs, connecting is simple. If the drain is on the other side of the house, the waste may need to run under the floor or outside, which adds digging, making good and time.
2. Macerator or gravity drainage
Where gravity drainage really is not possible, a macerator (a small pump behind the toilet) can push waste through a narrow pipe. Building regulations only allow this where the house also has a toilet that drains by gravity, as LABC guidance explains. Macerators work well but are noisier, need power and need occasional servicing, so we treat them as a last resort.
3. Size and shape of the space
There is no legal minimum size for a cloakroom, but as a rule of thumb you need around 80cm of width and about 1.4m of depth, with enough headroom to stand at the toilet. Under stairs, the toilet goes at the tall end and the basin tucks under the slope. A corner or slimline basin and an outward or sliding door make a big difference in tight spaces.
4. Ventilation
A toilet with no opening window needs mechanical extraction. Approved Document F sets the extract rate for a WC at 6 litres per second (bathrooms need 15). See the government Approved Documents for the full rules. A quiet fan with an overrun timer, ducted to outside, is the usual answer.
5. Finishes
- Wall-hung toilet with concealed cistern: looks bigger and is easier to clean. Adds £300 to £700 over a close-coupled toilet.
- Tiling: full-height tiles cost more than a tiled splashback with painted walls, but resist splashes and look premium.
- Lighting: an LED strip in a niche or boxing doubles as a night light.
- Flooring: porcelain tiles or a waterproof luxury vinyl.
Do I need planning permission or building control?
Internal work like a downstairs toilet does not normally need planning permission, unless the house is listed. It does need to meet building regulations: new drainage (Part H), ventilation (Part F) and electrics (Part P). Your builder should either notify building control or use registered installers who can self-certify. Keep the certificates, because buyers’ solicitors ask for them.
Does a downstairs toilet add value?
Estate agents regularly list a downstairs WC as one of the features families look for, especially in homes with one upstairs bathroom. Beyond resale, it takes pressure off the main bathroom every morning and is useful for guests and older relatives.
How we approach an under-stairs toilet
- Survey the space, the stairs and the route to the nearest soil stack or drain.
- Agree the layout: toilet position, basin type, door swing and storage.
- Strip out, alter stud walls and run new waste, water and power.
- Fit the concealed cistern frame, extractor fan and lighting.
- Board, waterproof where needed, tile and decorate.
- Fit the toilet, basin and accessories, test, and hand over the certificates.
You can see each stage in our downstairs toilet renovation project.
Design ideas for a small cloakroom
- Corner or slimline basins save 15 to 20cm of depth compared with a standard basin.
- A short-projection toilet helps where the room is shallow.
- Mirror across a whole wall doubles the sense of space.
- Bold tiles or wallpaper work well in a room you only spend a minute in, and it is a fun place to be brave with colour.
- Storage in the boxing: the frame for a concealed cistern can house a shelf or a small cabinet.
- Sensor lighting means no hunting for a switch under the stairs.
Checklist before you ask for quotes
- Measure the width, depth and headroom of the space.
- Find your soil stack: it is the large pipe outside, usually at the back or side of the house.
- Decide whether you want full-height tiles or a painted finish.
- Choose a toilet and basin, or a budget per item.
- Check where you will put what is currently stored under the stairs.
- Ask each builder to price the same specification so quotes are comparable.
Frequently asked questions
How long does it take to fit a downstairs toilet?
Most take 4 to 7 working days. Jobs that need new external drainage can take up to two weeks.
Can I put a toilet under the stairs?
Usually yes, if there is around 80cm of width, enough headroom at the tall end and a practical route for the waste pipe. We check all three on a survey.
Is a macerator toilet allowed?
Yes, as long as the house also has a toilet that drains by gravity. It should only be used where gravity drainage is not practical.
Do I need a window?
No. A mechanical extractor fan that meets building regulations is enough.
Looking for a bathroom team near you? See our bathroom fitters in Harrow, Barnet, Finchley and Willesden, or our bathroom renovation service.
Thinking about a downstairs toilet? Book a free site visit or call 07754 690740.
Omar runs Arkivon Group Ltd, a London building company based in NW10 that handles extensions, loft conversions, roofing and full renovations across North and West London. Ask Omar a question.