Housing and second floor
Each House has a wing of the building. 1-4 years all live together in one big dorm room. 5th years get to move to a smaller dorm room for just them.
Sixth and Seventh years get individual rooms, but they all share a bathroom. The rooms with the little towers on the edges are for the Head Boy and Head Girl of each house.
If someone is wildly out of whack, year-wise, or they have a magical issue that means they need to sleep in security of some flavor, they'll get to move to an individual room early.
Note: favor and individual rooms can always be purchased.