In the leniency of an ‘entrance open to the public domain’ it is not necessary for the entrance to be literally open to the public thoroughfare; rather, the rule applies to any setting where the entrance opens to a place where other people commonly pass by, such as a [heavily] used stairwell of an apartment building or the like. (Dvar Halacha 3)
If the entrance of the house is open to the public domain, and it is likely that other people will enter the house, then yichud-seclusion is also allowed in the inner rooms of the house. However, it is not permissible to be alone in the inner rooms that outsiders would not enter [such a storage room, cellar or the like]. (Ibid.)