If your rear lot line borders a back alley or a road, the setback of your garage must be at least 12 feet. If it's an alley, you measure that setback from the center of the alley, so, in reality, your garage may be very close to the alley. If it's a road, you must set your garage back the full 12 feet.
Garages that are built into your main house are subject to another set of rules. We're just looking at standalone buildings in this article, so we'll move on.