This is the apartment that Ryo Saeba owns. He, Hideyuki Makimura (until Season 1 Episode 5), and Kaori Makimura lives in the apartment.
It consists of 6 floors, rooftop, a garage, an armory, shooting range, and possibly 9 rooms. Although the 4th and 5th floor are connected since in some of the scenes show that there's a stairs located in the living room. There is no information if Ryo actually owns or has rented out the ground to the third floor of his apartment to other people as it may have been used as storage or extra room for guests (although it the entirety of the show/manga, it was shown that most of his guests sleeps nearby where Ryo can access the rooms.
In Season 1, it is shown that Ryo may have own a few vehicles other than the three cars shown most of the time in the series from the underground garage. However, it is speculated that the apartment may have a shared garage that's within the area of where he lives.
It was only the first quarter of Season 1 where Kaori becomes a resident of the apartment with her very own room. Two episodes after the death of her brother.
The apartment is located somewhere in Shinjuku.
It may not look like it but sometimes, the design of the apartment changes as the Seasons of the show go by.