My fiancé and I are moving into my mothers garage and we need a bathroom a full bathroom.
I need to know about how much it would cost us.

my fiance and i are moving into my mothers garage and we need a bathroom a full bathroom.
i need to know about how much it would cost us.
Sorry, It's difficult to estimate material and labor on your job without a wholesalers price list and the labor conditions, (union or nonunion) in your area so we don't attempt to do so.
While we're not there to estimate your job , if you wish to do the work yourself we can help you with that. Regards, Tom

Biggest problem will be drain. You will have to get somebody who understands consruction to come over and give you idea what is involved and what you can do there.

First tell us: is the main house on slab or raised foundation ? Any plumbing inside the garage ?

Back in the late 80's and 90's, my company used to specialize in installing full and complete bath inside garage. That time we were overloaded with work. We used to install a 5'x8' full bath - A to Z complete project ( toilet, sink, tub/shower combo) in 2 weeks for $9,999.00. That included framing, drywall, insulation, plumbing fixtures, cabinet, floor tile, paint, medicine cabinet, one light, one switch, one GFCI receptacle,door and trim. There wasn't whole lot of profit in it but I was able to keep my guys working continuously for few years w/o worries - and still make a few bucks. Now, that was some 20 years ago - so adjust for inflation to arrive on approximate cost of your project.