There is no minimum bathroom size in the UK building regulations. No square metre figure, no minimum length, nothing you can point a builder at and say the room is too small to sign off. What the regulations do fix, and fix quite precisely, is the clear space that has to survive around the fittings once they are in. Get those clearances right and the room size looks after itself. Get them wrong and you have a bathroom that technically contains everything and is unpleasant to use, which is the far more common outcome in a London retrofit.
What the regulations actually require
Two Approved Documents do the work. Part G sets what must be there: requirement G5 says a dwelling must have a bathroom containing a wash basin and either a fixed bath or a shower, with hot water to it. In a new dwelling or one formed by a material change of use, the water delivered to a bath must not exceed 48 degrees Celsius, which is why new baths get a thermostatic mixing valve.
Part M sets how much room you need around the WC. That is requirement M4(1), the visitable dwellings standard, which applies to newly erected dwellings and dwellings undergoing material alteration. It does not apply to extensions, and the higher M4(2) and M4(3) standards only bite where a planning condition imposes them.
The clearances that decide everything
Approved Document M requires a clear space of 750mm in front of the WC pan, and clear width either side of the WC centre line. For frontal access that is a minimum of 450mm each side, with 500mm preferred. For oblique access it is 450mm on the approach side and 400mm on the far side. Any basin has to be positioned so it does not impede that access, and the door to the room has to open outwards.
The door width matters too, and it is the detail most often missed on a small cloakroom. The clear opening width has to meet Approved Document M's Table 1.1, which pairs door and corridor: a door with a clear opening of 750mm or more needs 900mm of corridor when it is approached head on, and a 750mm door needs 1200mm of corridor where the approach is not head on. A 750mm clear opening is not a 750mm door leaf; the leaf has to be wider to allow for the frame and the door thickness when open.
Working out a real minimum from the official fitting sizes
Approved Document M publishes a furniture schedule that gives the sizes designers should assume: a WC with cistern at 500 by 700mm, a bath at 700 by 1700mm, a wash hand basin at 600 by 450mm, and a hand rinse basin at 350 by 200mm. Put those together with the clearances and you get arithmetic, not regulation, but it is arithmetic worth having.
- Downstairs cloakroom. A WC 700mm deep plus 750mm of clear space in front is 1450mm of depth. The 450mm either side of the centre line is 900mm of width. So 900 by 1450mm is the absolute floor before you have fitted a basin. It is not a coincidence that the example accessible cloakroom layout in Approved Document M is 1450mm deep.
- Shower room. Swap the bath for a shower and the room shortens, but the WC clearance does not move. A workable shower room usually lands around 1700 by 1700mm once the shower tray, WC and basin each have room in front of them.
- Family bathroom with a bath. Set the 1700mm bath along one wall and the room is 1700mm wide before anything else. Put the WC and a wash hand basin on the opposite wall, 500mm and 600mm of wall respectively, and depth becomes 700mm for the bath, 700mm for the WC, and 750mm of clear space between them. That is 2150mm. So roughly 1700 by 2150mm, a little under 3.7 square metres, is the point below which something has to give.
Those figures assume standard fittings and nothing awkward in the way. A soil stack in a corner, a sloping ceiling under a loft conversion or a chimney breast will each cost you 200mm somewhere, and it is always the WC clearance that quietly disappears first.
The space standard nobody mentions
If your project is a new dwelling or a conversion into flats, the nationally described space standard may be attached as a planning condition, and it makes one bathroom decision for you. The minimum gross internal area for a one-bedroom, one-person flat is 39 square metres with a bathroom, but 37 square metres if it has a shower room instead. The two square metres you save by dropping the bath is the standard's own arithmetic, and it is why so many small new flats have no bath at all.
Ventilation is the other thing that gets forgotten
Approved Document F requires an intermittent extract rate of 15 litres per second in a bathroom. That is a fan specification, not a room size, but it interacts with layout: a windowless internal bathroom needs the ducting run and the fan positioned where it will actually pull air across the room rather than straight back out of the door gap. Plan the duct route before you plan the tiling, because retro-fitting one through a joist run is a genuinely disruptive job.
Where the space usually comes from
In a period London house the honest options are limited and worth knowing before you draw anything. Stealing 300mm from a landing or a bedroom cupboard is often the cheapest square metre you will ever buy. Moving the WC to the wall where the soil stack already runs saves both money and depth. A wall-hung WC on a concealed frame gives back around 150mm of projection compared with a close-coupled pan, which can be the difference between meeting the 750mm and not. And a wet room removes the tray upstand and the shower enclosure footprint, though it brings tanking and falls with it.
What almost never works is squeezing the door swing. It has to open outwards on a room containing a WC, and a door that fouls the landing is a problem you cannot design away later.
If you are planning the room rather than sizing it, our guide to how to design a bathroom covers layout, materials and lighting, and designing an ensuite and designing a cloakroom deal with the two hardest small rooms. For the consent side, see building regulations for a London renovation, and start from the Vertigo Interiors homepage for the rest.
Frequently Asked Questions
Is there a legal minimum bathroom size in the UK?
No. The building regulations set no minimum floor area for a bathroom. What they set is what must be in the room, under Part G, and how much clear space has to be kept around the WC, under Part M. In practice those clearances produce a working minimum of roughly 900 by 1450mm for a cloakroom.
How much space do you need in front of a toilet?
750mm of clear space in front of the pan, under Approved Document M. Alongside that you need clear width either side of the WC centre line: at least 450mm on each side for frontal access, with 500mm preferred, or 450mm and 400mm where the approach is oblique. A basin must not project into those zones in a way that impedes access.
What is the smallest a downstairs cloakroom can be?
Around 900mm wide by 1450mm deep, working from the official fitting sizes and clearances: a WC with cistern is 700mm deep, the clear space in front is 750mm, and the side clearances give 900mm of width. That is before a basin, so most usable cloakrooms end up wider. Approved Document M's own example accessible cloakroom is 1450mm deep for the same reason.
Does a bathroom door have to open outwards?
For a room containing a WC in a new or materially altered dwelling, yes. Approved Document M requires the door to open outwards, and it also sets the clear opening width, which has to be read together with the corridor width outside: 900mm of corridor for a head-on approach, or 1200mm where the approach is not head on.
What size bath and basin should I plan around?
Approved Document M's furniture schedule uses a bath of 700 by 1700mm, a wash hand basin of 600 by 450mm, a hand rinse basin of 350 by 200mm and a WC with cistern of 500 by 700mm. Those are the sizes a designer is expected to demonstrate will fit, so they are the sensible ones to plan against even where the standard does not strictly apply.
Can a new flat have a shower room instead of a bathroom?
Yes, and it changes the floor area required. Where the nationally described space standard applies, a one-bedroom, one-person dwelling needs 39 square metres with a bathroom but only 37 square metres with a shower room. Part G is satisfied by a wash basin plus either a fixed bath or a shower, so a bath is not compulsory.
Sources
- Approved Document M Volume 1, access to and use of dwellings, paragraph 1.17, Table 1.1, Diagram 1.3 and Appendix D: gov.uk
- Approved Document G, sanitation, hot water safety and water efficiency, requirement G5: gov.uk
- Technical housing standards, nationally described space standard: gov.uk
Checked on 25 August 2026. Approved Document M4(2) and M4(3) apply only where a planning condition imposes them, and the nationally described space standard applies only where a local plan adopts it. Confirm which standards your project is subject to with your building control body before you finalise a layout.