A new bathroom often requires an extra-wide "wet wall" built of 2x6s or two side-by-side 2x4 walls. Because only 2-inch pipe runs through the walls, codes may permit a standard 2x4 wall. Unless you are in a warm climate, avoid running pipes in an exterior wall.
To convert existing space to a bathroom, you may need to move a wall. Before removing any wall, make sure it's not load-bearing; check with a carpenter or structural engineer if you are not sure.
Whether you are framing a new space or remodeling an existing space, make sure the framing accommodates the tub. Remove drywall or plaster from the areas where you will run plumbing. Clear out all cabinets, fixtures, and other obstructions.
If wiring is in the path of plumbing, shut off power to the circuit and test to make sure it's off. You may want to remove cables and reinstall them after the plumbing is in place.
A day or more to remove wallcoverings; about half a day to mark for positions of fixtures
Framing square, tape measure, hammer, pry bar, level, drill, hole saw or jigsaw, circular saw, reciprocating saw
Basic carpentry skills, planning for plumbing runs, careful measuring
Have your plumbing plans approved by your building department.
Lumber for any framing that's needed
Determine the exact center of the toilet drain. For most toilets, this is 12 inches from the finished wall surface. So if you plan to install 1/2-inch drywall, measure 12-1/2 inches out from the framing. (Toilets with drains set 10 or 14 inches from the wall are available, though usually only in white.)
Mark the bottom plate with the location of the vent and drainpipes. Position the tub/shower vent so it will not interfere with the installation of the faucet. You may need to move a stud or two to make room for the tub/shower faucet. Also mark where the supply lines will enter the room. Check all your measurements twice.
Use a hole saw or saber saw to cut a hole in the floor for the toilet drain. Use a circular saw and a reciprocating saw to cut out a section of flooring large enough to allow you to run the plumbing. Wherever you need to run pipes through joists, give yourself plenty of room to work.