It’s not difficult to get a perfect finish in a bath/shower room; it’s a question of budget, getting someone who understands the materials being used & actually knows what they are doing. I do refurbishments & would normally advise a specialist (even that can be no guarantee) but as your work is part of an extension, you really need to keep the Builder in the loop of responsibility if you’re inexperienced. If you start managing your own “trades” & things go wrong, he won’t want to know & you’ll be on your own. If you get a good Builder, it should be fine but many will have only a general knowledge of different trades, sometimes they penny pinch to keep costs down &, unfortunately, others would be more at home on a horse in a remake of the “OK Corral”.
Check out the credentials of your Builder carefully, get a signed contract & make sure it includes at least a minimum specification for what’s being done, what materials will be used & how long it will take. Are you building the extension under a Building Notice or full plans submission? Remember it’s ultimately your responsibility to comply with relevant Building Regulations so make your Builder responsible for this & for arranging the relevant inspections.
Follow PBD’s advice & get details of who’ll be doing the work, check out their credentials & the quality of their previous work. The preparation & tiling of the floors & walls is particularly critical if it’s going to last. It may be of benefit if you read the Tiling Forum sticky & archive posts to get an idea of do’s & don’ts; you will then be able to specify a minimum standard rather than leave it “open”, ask questions, spot corner cutting or if they are a complete cretin.