2012 International Residential Code requirements for toilets, sinks, and showers in bathrooms. Includes headroom, size, and clearance specifications. The information on this page is taken from the official government code book in sections R305 – R307 beginning on page 54 of the 2012 IRC.
Bathrooms with sloped ceilings must maintain 6′ 8″ at the center of the front of the plumbing fixture from the top of the finished floor vertically to the bottom of the finished ceiling. The reasoning behind this is to avoid accidental collisions with the ceiling.
Every dwelling must have a a room with a toilet, sink, and a shower or tub. This is for obvious sanitation reasons. They must be connected to an approved fresh water supply, and everything besides the toilet must have hot and cold water. PEX tubing is rapidly replacing all forms of water supply. It is flexible, resistant to chlorine and corrosion, has fewer fittings, and is faster to install.
The bathroom fixtures must also connected to a public sewage system, or an approved private sewage disposal system. Since the dawn of mankind, human waste has been the number one pollutant. Its simple, either connect to a sewer, or have a septic tank professionally installed. There should be no substitutes here.
Bathtubs and showers must be finished with a non-absorbent surface to a minimum height of six feet. They must not be less than 30″ x 30″. This is to make them easier to clean, more durable, and harder for molds to develop. Fiberglass units with a smooth gel coat are the best. Ceramic tile can have great looks and durability, but can have some maintenance. Remember these are finished dimensions. If the walls for the shower are being framed, then the wall coverings must be accounted for.
There must be a minimum of 15″ from each side of the toilet to its center. This counts to the edge 
of a wall, sink, or shower. When framing an opening for a toilet, leave a minimum of 31″ to compensate for drywall. If there will be an additional covering on the wall, then it must be allowed for. For example, if there is to be 1/4″ thick ceramic tile glued to 1/2″ drywall on each side, then to make the rough opening big enough, there must be 31 1/2″ in the clear.
There must be a minimum of 21″ clearance from the front edge of any toilet or sink to the front edge of any other fixture. Toilet sizes vary, so be sure to know its dimensions. This will ensure enough room to walk through for most people. These building code do not make bathrooms handicap accessible.