@notmrscookie : We did something similar to this - though not exactly what you are proposing. We made our downstairs loo and sink room into a wet room by moving the door from a rear hallway and making a new door opening off the dining room. We also made the door open out from the wetroom, rather than opening in, as this also gave us more space. I just drew what I wanted on a bit of graph paper (so I knew we could fit it in) and the plumber obliged. It sounds a bit odd but has been very successful.