It should be a zero setback here but probably some traffic ordinances involved with a garage, so you'll need your builder or architect to go sit down with city planning and ask. From what I understand if your alleyway doesn't go all the way through they don't have to give you access because 15ft is only good for one way traffic. So you need to make sure to offer them enough room to turn around like 24ft, so maybe a 9ft setback. You'd need 29ft at that point to get a proper depth garage in. You'll need someone good to go talk to planning so they can go over the rules that work for you. I've built on the alley in the Heights and I'm doing a detached garage in Oak Forest right now. Different rules apply in different areas.