Co-Op Roof Deck, is it Legal?

I was recently looking at a co-op for sale that had a roof deck built on top, it is landmarked but supposedly the offering plan for the building has the right built in for the owner of the top unit to have a roof deck built as long as it was in the back portion of the roof. Now when i look online at the buildings department, there is no c/o for the deck, should there be one? Also, the co-op plans on redoing the roof completely because they had problems and they will be tearing down the current deck. I just want to be able to build a new deck. What gives me the right to do so, the buildings offering plan or a c/o for it in place already? how does this all work? I love the place but want this figured out before I make an offer.