How do you make the Mii apartments expand in Tomodachi life?
According to Tomodachi Life Wiki
Mii Apartments has 3 upgrades not including the original building. Each upgrade changes the number of available apartments and the appearance of the building.
The first upgrade will have a brownish, brick-like appearance. It will expand to have 48 apartments.
The second upgrade will have a grayish, stony-like material. It looks very similar to how it looks in the beginning. It will expand to have 72 apartments.
The final upgrade will have a elegant golden color. It will have a gate with two trees in front of the office. It will expand to hold 100 Miis. The top four apartments will be in the center of the top floor.