Every stock bathroom component is manufactured to the assumption that walls are plumb, corners are 90 degrees and dimensions land on round numbers. Plenty of houses on this coast were not built that way, and plenty more have moved since. Where the room disagrees with the catalogue, something has to be made to the room instead — that is the whole of what custom means here.
Out-of-square is measured before it is designed around. Diagonals across the floor, plumb checked on every wall, the taper written down. Three or four millimetres over two metres is invisible. Twenty-five is not: a 1200mm vanity against a tapering wall leaves a wedge, and a scribe filler absorbs perhaps 15 to 20mm before it starts to look like an apology. Beyond that the cabinet is built to the taper. Tile has the same problem in reverse — the cut course at the end of a run tapers, so the layout puts that taper where the eye does not travel and keeps slivers out of the corners.
A leaning shower wall is the one that catches people. Glass is made straight and plumb; a wall that is neither cannot be both tight to the panel and square to the floor. The honest fix is to float the wall plumb before tiling, which costs thickness and a day, rather than discovering at installation that the panel needs a 12mm packer down one edge.
Ceilings add the other common constraint. Under a rake, standing height at the shower head and in front of the toilet decides the plan, not floor area, and glass cut to an angle is a template job. A window in the shower wall is waterproofed as part of the shower rather than as a window: fall formed on the sill, the membrane returned into the reveal and lapped to the frame, and safety glazing where it belongs. A timber-framed unit in that position is the wrong material however well it is detailed.