And what are the rules in your city. Generally, something like this will require a building permit which in turn will require plans for the project.
Before you undertake this, go to your building permit department and find out if you can even do this, what documents are required, and what are the minimum requirements to build according to the building code?
Might be a convenient thing to have , but likely not worth it, in my opinion. When you start installing plumbing in a garage, the City building dept believes you are increasing "living space", and then you have to address their protocol and procedures: If the owner installs a half bath without City permits, they are violating the City Building dept regulations and are subject to fine, forced removal of the work etc. Also disclosure of unpermitted work is also necessary in the future sale of the property.
If the owners chooses to do the work legally,which I would advise, they have to apply for a building permit, and go through zoning approval (may be over the counter, but may take longer), and building dept approval, with a drawings. Not necessary to use an architect, but the drawings have to be to scale etc., and instructions, check list etc, from the building dept need to be followed. The City may approve this easily if they are convinced the owner is not trying to make the garage an illegal living unit or bedroom or mother in law. Even if they grant approval, this initial permit process will take a bit of time, and will cost some money. And property taxes will be adjusted slightly upwards, depending upon how the city values the improvement to the property which is passed on to the County for the Assessor's office.
Then there is the cost of the actual work by the licensed contractor. If permits are taken out, the work has to be done by a lic contractor or homeowner who needs to show they are agreeing to the liability involved. Good luck. If this is Oakland: Building Services: 510-238-3443 Best to just go down there: 250 Frank Ogawa Plaza, 2nd Floor
Not obvious it is a good idea (yet). We need more info for that.