Of course you see lots of roof top decks around. Whether all of them are compliant or not is another matter. Unless your neighbors call on you, the City probably won't notice. Nonetheless there are serious considerations.
If this deck is for you and a friend or two to sit around drink coffee and yak about life, no big deal and nothing will probably ever happen. Do what you want. However, reality usually doesn't work like that. More and more friends show up, you have a party, put kegs and large plants on the roof, next thing you know there is some real weight up there. Since this is a NC building you'll need to establish what type of trusses were used for the roof and how much top load they can handle. The type of existing roofing material also needs to be figured out. Typically people need to either install additional roofing or suspend the deck over the roof, not on it.
Going the legitimate route will require plans and permits by the City. Plans will need to be done by an architect. Depending on existing building conditions its sometimes easier and cheaper to build a new structure over the existing roofing (I beams run into the parapets).
Typical requirements include egress path, guardrails and load rating. Depending on proximity to adjacent building fire separation may also come into play. Without a Spec sheet or Plans for contractors to bid off of you will continue to get varied answers.
A note of caution, the cheaper bids usually includes the contractor nailing everything down. You should NOT consider this acceptable. Everything should be screwed or bolted. Any perceived upfront savings will disappear if the roof needs to be serviced and nailed components need to come up.
This is why I wrote the informative article "Chicago roof Decks - What you Need to Know." The link is below. It explains alot of the common problems I see as a roofer fixing roofs beneath decks in the Chicago area.
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Roof Decks on Primary Buildings
Building a roof deck requires a Building Permit. A roof deck must be designed to meet the structural and architectural requirements of the Chicago Building Code and plans are required. These plans must be prepared by a Licensed in Illinois Architect or Structural Engineer.
Although you may see many roof decks in the city that do not fulfill this requirement, bottom line, they are not built to code.
I would start by talking to an Architect. They're the ones responsible for certifying any drawings which the city will use to issue permits. As previously stated, you must have 2 exits and meet strict weight limits and have a building structure that can support a roof deck .
Often, if the top floor unit owns the Roof Rights, they can build a deck. Doing so would likely make any current roof warranty void in the event of damage caused by the deck to the roof membrane. If the association is responsible for the roof then it would be the unit owners (two in your situation) that would vote on such an addition based on the Condominium documents and pay according to % of ownership.
If it can increase the marketability of your home by offering stunning views it may be worth doing. But if you can't do it right and comply with city code, you may undertake expensive work that ultimately could be shut down during construction or be requested by the city to be removed after it's already built.
Best of Luck, Ken.