Designing Kitchens and Bathrooms - BUILDING NEW BATHROOM

WE ARE WANTING TO MAKE A MOTHERINLAW APARTMENT IN THE DOUBLE GARAGE. THE GARAGE IS ALREADY ENCLOSED. WE WANT TO MAKE A SMALL KITCHEN AND NICE BATH ROOM, IN ADDITION TO A SMALL SITTING AREA AND BEDROOM. IS IT POSSIBLE TO ADD PLUMBING TO AN EXISTING SLAB? HOW EXPENSIVE? WE HAVE A WATER LINE TO TAP INTO ON ONE WALL OF THE HOUSE. WE DO PLAN TO HIRE A GENERAL CONTRACTOR TO DO THE JOB. JUST WANT TO KNOW WHAT I AM LOOKING AT. ANY IDEAS ON A BALLPARK FIGURE TO DO JOB. I REALIZE THAT IT DEPENDS ON TYPE OF MATERIALS CHOSEN FOR THE JOB,BUT JUST MEDIAN PRICE.

Yes, you can retrofit pipes in an existing slab. It basically involves having a "channel" cut through the (+ or - 4" thick) slab with a jackhammer, wide enough for a plumber to work the pipes in the channel to your existing sewage line. This is a common practice and shouldn't be cost-preventative, however, regional price differences, the actual distance of the pipes and "site specific" variables make "ball-parking" the price, a bad idea.
Most legitimate contractors give free estimates. Good luck!