Dont know where to start, there are some here on Trulia that took the time to answer your question, start there. Best of luck.
The right way for you to find out it to contact the planning department of the city and tell them the address. They will tell you what you would like to know. Generally the area of garage is not considered as living part of the floor plan since it did not build with proper protection for habitat as required by code. If someone altered the space into the living quarter most of the time is not legal. You can check out if they had pulled the permit to alter it before you buy it . For house to build before 1985, most of the floor plans were archived into micro fiches for you to review. This is a free service. If you would like have a copy of the floor plan you might have to pay a fee to get it from the city. I hope this answer will be adequate for your answer. More details you can email me later. Good luck.
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!
Under normal conditions the garage is not considered part of the inhabitable square footage. If you suspect that this is not the case, we recommend consulting the home's records through your county records department.
By regulation, for a garage to legally be converted into living space a number of a changes would be required that may include the level of the floor, and structural, electrical, HVAC, and plumbing issues. Your code enforcement officer would be able to best advise you on the extent of these requirements.