Adding rear deck to 3-Family CoO

Hi All – We recently purchased a brownstone with a 3-family CoO. We are not planning to change the lay-out or the CoO. However, we would like to add a rear deck with access from the parlor floor. We’ve been told this may not be possible given the 3-family designation and related ADA regulations. Does anyone have experience/advice on this?

***Also, if you have recently gone through Alt-2 renovations on a 3-family CoO, we would LOVE to buy you a drink and talk.***

Although we aren’t doing significant renovations, seems like we might still run into a number of quirks that we’ve not had to consider in our current 1-family home. I would like to be as prepared as possible so I can avoid/navigate any potential pitfalls. We have an incredible architect manning the ship, but it would be great to get a homeowner/DOB survivor’s perspective as well. 🙂