Is this the master bethroom or a bathroom used by everyone. The question IMMO is confusing, What you have is a walkout basement and yes you can put a master suite in a walkout as for the actual plumbing you will need to have a pro check out your situation

I screwed up - it's a master bedroom and bath in the basement with walk out. You are 100% correct with the confusion. What I've decided to do is to have this be more of a "guest master bedroom" or future inlaw place. Every home is initially designed for the current occupant but also with future occupants in mind. Therefore what I'll do is focus on the upstairs primary bedroom and then if I do decide to stay, I'll give this to my soon to be teen-age daughter and I'll build my primary bedroom in the basement. When someone comes to look at the home to buy, they can look at the downstairs as a large "room" which could be a bedroom (staged as such) and an awesome bathroom ideally suited for a bedroom or just an awesome entertainment room.

If you want a bathroom there, now or in the future, that needs to be planned before you start construction.