150 West 87th St

Hello- I understand Legacy, a developer, has built PH North at 150 West 87th St. I wondered if anyone had any experience with them, the process of buying a top floor (typically small apartment) and then adding an extra floor as they have done. I thought it was an interesting way to create value but not sure how that works with air rights etc.. Curious if others have had good experience with other developers in doing that... While the idea made sense, I understand Legacy was terrible in the execution as there has been multiple reported leaks from it... I pity whomever would be buying that apartment as usually things are either done rights from the beginning or problems never end. Thoughts welcome.