My understanding (though I'm new to this, and can be corrected if I'm off-base) is that expenses shared by the building must be apportioned to the rental unit and the owner's unit as a %, by floor space.
It's a 3-story building; I live on the 2nd and 3rd floors w/ the rental on the 1st floor. 1 roof/1 chimney to rule us all. Floor space split is 62%/38% (square footage on 3rd floor isn't as big as the other floors since it's a finished attic). If I fix something in just the rental unit (eg, replace an electrical outlet), I call that a rental expense at 100%. But the chimney (like the basement, the garage, the front porch...) is used by both the rental unit and my unit. If it were a SFH that I owned and occupied, I could not deduct the cost of a repair... so the portion of the chimney that I allocate toward me is not tax-deductible; the portion that I allocate toward the rental is deductible.
Eh?