I have actually done a bunch of homework on this topic.
My dream is to build a large, high quality, garage apartment and I found a gorgeous design plan. Unfortunately, you can build a nice house for what this
"man cave" will cost.
Basically, it is a large 4 car "supersized" garage with roughly 1700+ square foot deluxe apartment / living space above.
Features four overhead insulated doors that are 10'x10' with ample shop space in the rear area of the building for workbench and tools as well as the interior stairs leading to the second floor apartment. Downstairs garage area also features and a small kitchenette with washer and dryer.
The building is 60' wide and 50' in depth with 12' ceiling height on the lower garage level. The apartment area features two bedrooms, 10' ceilings ( 1 for an office space), 1 large bathroom, kitchen, stone fireplace, rear deck area and large living space / T.V. area.
Check with out your building codes and zoning ordinances.
I currently cannot build one in my area due to zoning restrictions.
Cost-- To build this garage apartment properly with a lower level exhaust fan / ventilation system, Lower level Heat and A/C, upper level Apartment air and heat, 6.0" HD concrete floor, "big ***" ceiling fans, good quality lumber (no particle board or pressed wood) and all the plumbing and electrical, fixtures, appliances etc --- $180,000.
This one will be on hold for a few years, because I will not build it unless it is made from quality materials.