rentable vs "nonrentable" parts of apartment building?

[Clyde Benfer]

Hello, I am looking at a large older house that is split into 3 apartments. There is a 1st floor 2 bedroom, 2nd floor 2 bedroom and 3rd floor 1 bedroom (due to 3rd floor being smaller).

On the 3rd floor there is also 1 room that is separate from the 3rd floor apartment. This one room has enough space for 1 bed, 1 dresser, microwave and minifridge. It also has a small closet and a tiny attached restroom with standing shower. The room is probably about 12'x12' and the "Bathroom" about 10'x 3.5". These are very rough estimates.

The overall house is very large. This room/bathroom would make up about 5% of the total square footage of the house I would estimate, probably less.

Ok so here is my question. If I was to buy this property and a few days a week stay in this "room/bathroom" would I have to call it my residence? I would also have a residence at a different location also. Would it affect depreciation and mortgage interest deductions?

The current landlord said she is not legally allowed to rent the room out so it sits dormant due to there not being 2 usable exits and it being on the 3rd floor. The 1 "exit" leads to the door of the 3rd floor apartment. The other leads down stairs to the outside. The room/bathroom could theoretically be added to the 2nd floor apartment she said but they would need a key to get entrance to the 2nd floor apartment. The 2nd floor apartment entrance is on the way down the stairwell to the outside.

Overall, would staying in that room a few days a week make the taxes complicated? or is there a exception for that part of the property because it is so minute?

Thank you for any information you can provide,
Clyde