The cost of new construction depends on the quality of materials, the size of the building and the wages of designers and laborers. An average garage can be built for around $40 to $60 per square foot, with an apartment adding anywhere from 20 to 40 percent to that cost. A 20-by-20-foot garage totalling 400 square feet would cost between $16,000 and $24,000 without an apartment, and anywhere from $20,000 to $35,000 with an apartment. Homeowners who are skilled at construction and can do some or all of the work themselves can cut an enormous amount off of those numbers.