I'm just about to buy a house that was built in 2004. On the bottom story, which is built directly on the foundation, there is only one bathroom, which happens to be a half bath as well containing only a sink and a toilet.

I would like to convert this half bath into a full bath by adding a tub. Tearing-up the foundation to access the drain sounds pretty scary to me. I was thinking about installing about a 1 foot raised floor. And underneath the false floor, I could run my PVC and share the existing toilet drain with the bath drain. Does that sound like a feasible idea that would also live up to plumbing codes?

underneath the false floor I could run my PVC and share the existing toilet drain with the bath drain. Does that sound like a feasible idea that would also live up to plumbing codes?
Only if you ran a separate vent for the tub. A better way would be to connect the new tub to the lavatory drain and let the lavatory roof vent wet vent your tub. That way you would be avoiding having to run a vent for the tub. Good luck, Tom

Mark, Tom can show you how to do the job but I'm going to suggest that you NOT do this job. Raising the floor 12 " may give you room for the tub drain but you still need to reach the toilet drain. Were you thinking of tapping into the vent or what? I have seen many baths built as you describe and each time I find myself asking, " What was he thinking."