The Sanzenin Mansion is where Nagi lives, though it is actually owned by her grandfather Mikado. Also living in the mansion are Hayate, Maria, Klaus, Tama and Shiranui. Hayate serves as Nagi's butler and follows her orders, he also goes to school with Nagi. Maria is Nagi's maid and has been taking care of her for a long time. Klaus is the head butler who is in charge of hiring and firing Nagi's personal butlers. Tama is Nagi's white tiger pet and Shiranui is a stray black cat whom Nagi accepted into the mansion when Hayate brought him in. The mansion is guarded by the Sanzenin SP.
The entire mansion is very huge, supposedly occupying 65% of Nerima. The mansion grounds include an amusement park,a lake,beach,and many more. The mansion interior is also very big much to Nagi's woes. Though the entire mansion is very big, Nagi only uses a small portion of the place and only employs a few people to maintain the household.
In Hayate no Gotoku! CAN'T TAKE MY EYES OFF YOU, The entire mansion appears to have gotten a whole new interior design. Such as indoor fountains, Nagi's room, Main Hall, etc. Though Nagi, Maria and Hayate live here like used to before they moved to the Violet Mansion, There is no mention on how Nagi is able to live here again after she lost her right to the Sanzenin inheritance.