Water — the plumbing side
A room, not a set of fittings
A bathroom is the one job where the plumbing is the smallest part of it. What you are actually buying is the order the work happens in and the standard of what goes back on top.
What the job involves
Strip out — the old suite, the tiles, and usually a certain amount of what is behind them. First fix: the hot, cold and waste runs moved to where the new layout wants them rather than where the last one left them, and everything tested before it disappears. Boarding and, on a wet room or a large walk-in, proper tanking. Tiling. Second fix — suite, screen, taps, shower, towel rail. Then sealing, and the clean.
Fitting round what is already there is normal. Plenty of these jobs are a bath swapped for a walk-in shower in a room that otherwise stays exactly as it is, and that is a two or three day job rather than a fortnight.
Why it matters
Falls and waste runs are the two that cause trouble later. A shower tray or a wet room floor that does not fall correctly towards the gully will sit in water no matter how good the tiling looks, and the point at which that gets fixed is the point at which the floor comes up again.
The other is choosing a shower the water supply cannot feed. A big rain head on a low-pressure gravity system will disappoint every single morning, and that is a decision made before anything is bought — which is why it comes up at the quote and not at the end.
Working round you
Most of these are the only bathroom in the house. The programme gets arranged so that the room is out of use for as few days as it can be, and the state the house is left in at the end of each day is part of the job rather than an afterthought.
Other trades
Any electrical work in a bathroom — a shower circuit, extractor, lighting or shaver point — is notifiable work and is carried out by a qualified electrician, not by a plumber. That gets arranged as part of the job so you are not the one chasing it.
Ring it, or write it down
Say what it is doing and roughly where you are. That is usually enough to know whether it is an hour or a morning.