Agreed with others that while you don't need to have your fixtures picked out, you do need to have a pretty good design sense of what you want. Do you want a curbless shower, do you want a frameless glass enclosure, do you want a freestanding or built-in tub, do you want wall-mounted or deck-mounted faucets? Those play a pretty big part in a contractor quote, and even just their feedback on what is possible, what they are personally capable of (because one contractors "it can't be done" is another's "I do it all the time"), and what will be a huge expense.
I would also suggest researching questions to be prepared to ask the contractors. A BIG one for me is what waterproofing they use in the shower. So many (so so so many) contractors say they "of course" waterproof the shower, but what they mean is they use cement board, and they are still clinging to outdated ideas about that being waterproof. It's not, and code does not accept it was such. Get specifics from them on what they do to waterproof, and be prepared to google afterwards to see if it's acceptable. Mold growing behind the walls is the number one issue in bathrooms, and the most important thing to separate the wheat from the chaff, so to speak, amongst contractors. My number two question would be do they use the same tile person on each job, and do they have examples of their work. There is a huge range of quality with tile work, and this is something that can really taint your appreciation of the finished product forever if it is not done right. Every crooked grout line will be all you can see.
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