It depends upon what you consider a bathroom. I think it would be ok to have a bucket and some toilet paper. Anything beyond that you need to discuss with an attorney and/or architect to find out if it is possible. I have been in real estate since 1993 and I have not seen many garages with bathrooms, that is for sure. Good luck!
Mitchell S. Feldman
Associate Broker/ Director of Sales
Madison Estates & Properties, Inc.
Office: (718) 645-1665/ Cell: (917) 805-0783