landlord not giving notice before entering building

[Lane Smith]

I have a family member renting an upstairs unit in a 4 plex. The basement is split with each side housing w/d hookups for 2 units. The basements are only accessible from the stairwells of the 2 units on each side of the building. The landlord has entered a locked doorway belonging to my family members apartment and gone into the locked stairwell leading to the basement. He says it is common space and he can enter at will with no notice required. The basement is shared with another tenant but the only entrances to the basement are through the apartments. I am confused as I thought having to go through a tenants locked space(even if it is a rear stairwell) is not considered 'common' space but the individuals rented area and therefore subject to the common 24 hour notice. This is located in Wisconsin if that helps. It seems like he is violating some type of privacy but I am not sure where to look. If you have any information on this please let me know as it is very stressful. Thank You