Unless the garage has been turned into habitable living space, it is NOT counted in the square footage of the home. In order to be habitable, certain inspections, permits, etc., must be obtained, and a certificate of occupancy must be received. In addition, in many communities the lack of on-street parking must be considered. In my town, one cannot convert a garage unless you provide the equivalent of 1.5 off-street parking spots. Rules like this can mean a garage stays a garage...period.
If you think the seller or his/her agent is mistaken about the square footage of the house, or inadvertently including the garage (if it is finished off), ask to see the permit[s], C of O, or other evidence that the room is properly built and permitted. If it isn't, decide whether or not you want to deal with the possible consequences if you buy the house. You could be required to remove the improvements, or make repairs or upgrades to meet code requirements, or you could be fined. And, if you decide to rent out that garage room, your tenant has a built-in reason to break his or her lease. Good luck with this!