ownership rights to the roof should are set out in the condo declaration. if this is a new condo project, you should try to negotiate roof rights with the developer BEFORE the developer files the declaration with the county so that those rights can be spelled out from the get-go. if the condominium already exists as a legal entity, the process is much more complication. you will need to amend the declaration to document this change of ownership. to do that, you will need approval from the condo association and from the several mortgage companies that hold liens on the property.
typically, the roof is a "common element." this is significant because it means that (a) all the other unit owners share ownership of the roof and (b) each of their mortgage lenders has a lien interest in it as well.
An architect is the best person to ask about how to execute the roof deck so that it is in compliance with Chicago's stringent codes.
As far as roof rights go. You should check with your real estate attorney to make sure that the association or developer can sell you the rights to the space. This should be accompanied by a satisfactory deed or other instrument to guarantee that you are able to develop or use the space. It further should happen with title insurance so it is a well protected right.
As far as the construction related issues go. There are a couple of obstacles to overcome, the first of which is to find out if a roofdeck can be built. I am a Realtor and General Contractor, so hopefully I can shed some light on this subject. The following is how I would approach the issue:
Before you purchase (if purchasing is possible), engage a lawyer to draft the purchase contract. The contract should have contingencies for Dept of Zoning, Dept of Building's and City of Chicago Approval. Once you and the seller have reached terms on the contract, engage Architect or Porch Builder to consult with you and draft plans for the city, one plans are completed, you'll have an understanding of exits, layouts, construction cost, etc. You will also need a survey for the city. If the city approves the plans, move forward. If the city does not approve the plans, you can exit the purchase contract if it was properly prepared, with only legal and architect's fees as a loss. In some cases, you may end up in the Dept of Zoning for a variance. This process can be costly, so watch your budget if you move forward.
Because the City Code, Roof decks can be costly in Chicago, which is one of the reasons they are scarce. Because they are scarce, they add tremendously to the value of you property. A new Porch, which has been the sore spot in City building court, has to hit 100 lbs/sq.ft.! A roofdeck even higher because of snow load (dead load). Be prepared to spend as much as $30-$40/sq.ft. of deck area, plus plans and permits if you need to modify structure, stairs, and porch assemblies. The price can climb from there. Your average 500 sq.ft. roofdeck should cost about 20-30K.
Hope this helps. Email me if you need resources for builders or architects.