The Illinois code is a combination of codes from what I have noticed. It is pretty strict in lots of areas. Thing is the Illinois code it the bare minimum allowed, if a city, county or town wants to make it stricter they can. Here is a great reference to the Illinois Plumbing code. http://www.ilga.gov/commission/jcar/...0sections.html
So one thing you might want to do is talk to the local inspector in the area you are working in and see if they have anamendmentsts to the code.