Decisions to lock in
Layout: WC, basin, bath/shower positions confirmed against the soil-pipe location. Suite: every item, toilet, basin, taps, bath, shower valve, shower head, chosen by exact model number. Tiles: walls, floor, and any feature panel decided with quantities calculated to include 10% wastage.
Electrics: extractor type (in-line vs ceiling), shaver socket position, lighting plan (downlights, mirror lights, IP rating zones), mirror demister and any shaver socket position.
Things to order in advance
All tiles should arrive 5 working days before the start date so the fitter can sort batches and check for damage. The suite, brassware, and shower screen should be on site before day 1. Lighting and the extractor too, the electrician needs them on day 2, not whenever they happen to arrive.
Get the boxes inside the house, not in the garage or shed. Damp boxes mean warped boards in adhesive backing.
Practical prep
Clear the loft hatch, fitters often need access for cable runs or pipework. Identify the stop-cock and make sure it actually closes (test it before day 1, not after a leak starts). Move car off the drive on strip-out day if there'll be a skip. Tell neighbours about deliveries.
If you have pets, plan where they'll be during the noisier days. If you work from home, expect drilling, hammering, and shouting between trades, book meetings off-site for week one.