Hidden water damage should be dealt with before the new bathroom is installed. Once old tiles, trays or baths are removed, rotten timber, damaged plasterboard, loose surfaces or damp subfloors may become visible even if the old room looked acceptable from the outside.
Covering damaged material with new finishes risks movement, failed seals and recurring damp. The affected area should be assessed, the source of moisture identified and the substrate repaired or replaced as needed before waterproofing and tiling continue.
Doherty's Glasgow bathroom page specifically notes that hidden water damage and unsuitable wall or floor surfaces can affect cost and timescale. That is why a realistic renovation quote should allow for the possibility of remedial work once the old room is stripped out.
