I am remodeling a small (6'x5'), 2nd story bathroom accessed from a landing that is 1 step (~8") below the upstairs floor level, and I want to bring the landing and bathroom floor up to the same level as the rest of the upstairs. I am trying to determine the best way to construct this "double-decker" floor so that it will be solid and still accommodate plumbing (supply lines and drains for a toilet and sink).
Toilet and sink will be relocated in the slightly enlarged space, so I need to pull up the existing plywood subfloor to access the drains. Would it be better to replace the plywood before installing the next layer of joists? Should the new joists run perpendicular to existing joists or is it better to use a parallel arrangement aligned with the existing joists?
Links to examples or best practices would be helpful as my searches are only turning up information about adding an additional subfloor layer (e.g., plywood over plywood) or building a raised floor over concrete.