All of the above suggestions are good, including the Ibuprofen!
I was a building inspector for a year and this came up all the time. People would call or stop in and ask if they could build this or that here or there. The city of Chicago directs you here for Zoning and Land Use, and the Building Code.
What I can tell you is this:
1. Carefully review the zoning requirements to confirm that you can build what you want. According to title 17, it looks like you are wanting to be in one of the following districts:
- RT, Residential Two-Flat, Townhouse and Multi-Unit Districts.
RM, Residential Multi-Unit Districts.
These are further classified by lot area, and floor area ratio (floor area of the building divided by the total gross area of the zoning lot upon which the building is located). There are set back requirements and other things to consider. Read the whole ordinance.
2. Hire an architect or consultant to perform a code review to determine the specific requirements for your intended use group classification, which is Class A-2. This is where the rubber meets the road. There are specific requirements dealing with fire protection, fire resistant construction (fire walls, etc), egress, and other things. You need to determine what you are going to be required to build to assess costs.
3. If you are going to proceed after doing the above, either find a design and have it modified for your requirements, or hire an architect to design your building. Either way you probably will need an architect to get permits. Check with the building department to see if they require an architect to pull the permit and oversee construction or if a general contractor can do it. The GC will build it.
Good luck! I hope you can do it.