What to consider for a rooftop deck with access from an enclosed porch?

[Asim Purses]

Building: 3 flat with an enclosed Chicago porch where the flat (sloped) roof extends over it. Hatch for roof access.

The landing on the third floor could (technically) easily be extended onto the roof - but realize there are code/safety considerations around that.

Does anyone have experience with this? I'm looking across the street right now and see a neighboring three flat with an enclosed porch and door leading to the roof leading me to believe... it's possible. But with how much money/effort?

Could a hatch suffice? can the deck be suspended from the parapet walls or have to be resting on the roof?

Long story short - what are viable paths here without the city saying "you need to bring your porch up to code" to do this? I realize speaking to an architect will probably be another step, but wanted to start here.