My friend looked into this last year, although she didn't go ahead due to the cost involved. But what I can tell you is that she asked an architect to draw up plans for her, very impressive they were too! It may sound a bit extreme getting architect plans, but as Wildwood said, it is a pretty major job and involves raising your bathroom floor to accommodate the under-floor drainage and ensuring it is fully water-proofed. The architect also advised her about building regulations and planning permission generally, as, with flats, you need to obtain planning permission before carrying out work of this kind as flats do not have any 'permitted development rights' unlike houses etc. It may be worth you getting advice from an architect. The cost could be around �100-�150, but well worth it in this situation. Wet rooms certainly do look amazing, so good luck.