A great place to steal space for a bathroom expansion is from the linen closet. You can make up some of the lost storage by finding small spaces in between wall studs for small niches or built-in shelves. Try stealing space with optical illusions. There are many ways to make a small room appear larger. To transform a small bath, install a bow window or a skylight.
Vaulted ceilings are a nice touch too.
If you are going to expand outside the existing home, consider a small bump out of two to four feet. This may allow you to cantilever the floor joists and eliminate the need for excavation and foundation. If possible, be careful not to extend beyond the roof line which would add a new roof to your job. Whenever you are adding on new space to a home, have a heating contractor determine whether your existing heating system can accommodate and heat the extra space. You don't want to damage your existing heating system and be forced to replace the entire unit.