A big mirror will not make your bathroom bigger.

Four things genuinely change how large a small bathroom reads, and none of them are the usual advice. Leave the floor visible by putting the basin on slender legs or hanging it off the wall. Carry one colour over the walls, ceiling and trim so there are no bright edges marking where the room ends. Position the mirror to reflect a doorway or window rather than the opposite wall, so the eye is given somewhere to travel. And use a large-scale pattern rather than a small one. Everything else on the standard list — white paint, a wall-filling mirror, large-format tile — flattens the room without adding a perceptible inch.

You mostly can't, and trying is what makes the room look ordinary. A small bathroom is never going to read as large, and the effort spent faking it produces the flat, white, fitting-filled room most people are trying to escape. What you can change is how much floor is visible, whether there are hard bright edges around the ceiling and skirting, and whether the eye has anywhere to travel. Those three genuinely alter the sense of space. Beyond that, stop measuring the room against a larger one and finish it properly instead.

No. This is the most repeated piece of advice in interiors and it does not work. White has no depth, so the walls read as flat planes at a fixed distance, and in a room with little daylight it turns slightly grey. Worse, white walls are usually paired with a white ceiling and white trim, which creates high-contrast lines at every edge and tells the eye exactly where the box ends. A single saturated colour carried over every surface removes those edges and the boundaries of the room become harder to read.

Not by size alone — position matters far more. A wall-filling mirror in a small bathroom mostly reflects the opposite wall, which doubles the pattern and the fittings and reads as busy rather than spacious. What creates depth is a mirror angled or placed to reflect a doorway, a window, or a view out of the room, because the eye is given somewhere to travel rather than a copy of where it already is. A well-placed modest mirror beats a badly placed enormous one.

Marginally, and at a cost. Fewer grout lines do reduce visual noise, but large-format tile in a small room means cut tiles at every edge, which draws attention to how little floor there is. It also reads commercial — it is the flooring of hotel bathrooms and lobbies. A small warm floor in wood, a herringbone, or a patterned stone gives the room character, and character is what makes people describe a room as lovely rather than small.

No — this is backwards, and it is the most common mistake in small rooms. A small busy pattern reads as texture from a few feet away, and texture makes a wall feel close. A large-scale pattern gives the eye a few big shapes to follow, the repeat is not visible within the room, and the wall recedes. A powder room is the one place a paper that would overwhelm a bedroom works perfectly, because you only see a fragment of it at a time.

Yes, and it is the single most effective change available. A vanity cabinet sitting on the floor fills the room visually to its full footprint. A console basin on slender legs, or a basin hung off the wall, leaves the floor visible underneath — and the eye judges the size of a room largely by how much uninterrupted floor it can see. The same room with the same fittings reads noticeably larger with the floor showing.

No. A white ceiling above a coloured wall draws a hard bright line around the top of the room at exactly the height you want the eye to stop noticing. Carry the wall colour or the wallpaper over the ceiling instead. The room loses its visible upper boundary and reads as taller and less contained, which is the opposite of what most people expect.

No, though it is widely believed. Dark colour makes the edges and corners of a room harder to locate, which makes the walls less defined and the room less obviously bounded. Pale colour does the reverse: it shows every junction, every edge and every fitting clearly. A dark small bathroom reads as deep and enveloping. A pale one reads as small and well-lit, which is not the same as large.

Stripping the room out. The instinct is that less in a small room means more space, so everything is removed — pattern, colour, ornament, anything on the walls — leaving a small white room with fittings in it. But an empty small room still reads as small; it simply has nothing to look at while you notice. A small bathroom with a strong paper, a proper mirror and warm lamplight gives the eye somewhere to go, and people remember the room rather than its dimensions.