Expensive, although obviously it depends on how big the room is. The walls of a wet room have to be "tanked" which is like a waterproof membrane put behind the tiles. Think of a sheet of polystyrene insulation like 5 ml thick but waterproof. The existing room will have to be taken right back to brick work, tanked, sealed, and tiled on top - or if not tiles then similar, you can't have plaster board and painted. The existing fittings may need replaced if they cant accomodate the tanking. Floor has to be angled to ensure water runs to the drain, and tiled over. (tilers hate this on a not-level floor!) It's worth getting a really good extractor fan factored in as well, as wet rooms have more steam circulating than in a cubicle iyswim.
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