This is the current home base of the Hisakawa family: Noriko, Ichirou, Kaito, Yuki, Miku and Minoru. Noriko lives here, but is constantly going back to Japan to see her parents and make sure all is well with them. Ichirou is here half the time, the other half he's in Japan on business. Kaito lives here, as a 9th year he is allowed to. Yuki and Miku live in the school dorms during the school year, sometimes coming over on the weekends. And Minoru lives here — there's always someone here to look after him.
The move was very unexpected and hasty, making the apartment rather messy and random, added by the fact that most of them aren't there much.