If you are looking to add a new bath or expand an existing bath, you can always steal space from other parts of the house 
Closets: Commandeer a nearby closet (or one already in the room) to enlarge an existing bath. A large closet can be converted into a small powder room.
Bedrooms: Who needs that spare bedroom more – an occasional guest or your busy family? Use the whole bedroom for a spacious bath, or borrow part of it to expand a nearby bath. A large bedroom can be split into a bath and a smaller bedroom.
Stairs: Is there wasted space under your staircase? Put it to work as a small powder room or turn it into storage and convert another closet elsewhere in the house.
Bump-outs: If there’s nowhere to go but out, a bump-out or cantilevered wall projection can provide extra square feet for a tub or shower without the need for a full addition.
Fake It:
Stuck with a small space no matter what you do? You can always make it look roomier. Try the following perception altering tricks –
Windows: Replace small windows with larger ones & eliminate heavy window treatments (glass block or window films can be used if necessary, for privacy). Install a skylight to add a spacious feeling.
Light: Chase away room-shrinking shadows with a good source of overall light. Add supplemental lighting above each vanity sink, tub, shower, and cosmetics table.
Mirrors: Reflective surfaces double the light and create the illusion of more space. Beware, however, of enclosing the room in a sea of mirrors, which can be disorienting.