The main question is whether you can get a permit for it. Lots of hurdles involved, but if you can, your cost will be reflected in the new value of the property. If not, then you are taking big risks and could be adding zero value, in some cases even negative value.
The plumbing will likely be your biggest expense, but also your biggest contributor to an increase in value and usefulness.
If it is the typical 10′ x 20′ single garage size, you can just squeeze a br and small ba in.