Yes, you can use a detached building as a home office. To determine your square footage, you will add the total square footage of the building to the total square footage of your home to determine the overall sq footage for the two buildings.
Then you will report the total square footage of the building and home and then your building space is a ratio of building sq footage/overall sq footage of garage and home.
Turbo Tax will determine the ratio after you enter your detached building square footage and then the overall square footage.