my client wants me to include the square footage of the detached garage in the home square footage. I do not think that I can do this

The problem with just adding the square footage to the house is that when the house is appraised, it will not be calculated in the same manor. The additional square footage probably will not be worth the same price per square foot as the main house. Generally if buyers are looking for a 3500 square foot house, they are not usually looking for a 2500 square foot house and 1000 square foot apartment. They are usually looking for a 3500 square foot house. In my opinion, it should just be added in remarks.