Dont know what the codes are, but since its inside work,,99.9 % chance they will never know. I am gonna add a master bath and I sure aint getting a permit, since the city has eleminated the home owner permits, allowing the homeowner to do the elect and plumbing. I think the unions musta paid them off, so now I am supposed to pay an electritan $4000 to do a job that uses $300 worth of materials. And pay a plumber $1800 to move a sink.
I would consider circular stairs since they dont take much space.
If you are concerened about the floor, I used tounge and grove 2x6's and spanned 48 inches and its very solid.
Pop in a couple of skylights too, much easier than building dormers.