I have what they call a "granny flat" which shares a common wall to the main house but has an outside entrance. It also shares a common celing that is the garage floor. What does it take to include the sq footage of that room to the sqft of the main house? I was told all I need to do is make sure it has central heating from the main house. Is that correct? Or does it depend on other variables or state laws? The room has: Bathroom Closet Window Exhuast fan In-Wall gas heater Outside entrance