If 80% or more of the rents are from "dwelling units", then the entire building is Residential (27.5 years) for the owner of that building. If it is less than 80%, the entire building is Nonresidential (39 years) for the owner of that building.
And note that is "for the owner". So just because you live in an apartment building (residential) that you DON'T own, does not make your home office residential over 27.5 years.
But you taught me an interesting thing. In the case of a duplex (or more than one unit), YES, the owner can use 27.5 years for the Home Office. As long as part of the building is rental, then that subjects it to the 80% rule which includes the rental value of your own residence (a single residence that does not rent out can not use that rule, even if your personal portion is 80% or more). So if 50% is rented out and the rental value of your personal-use portion is 30% or more of the building, yes you can use 27.5 years.
https://www.irs.gov/pub/irs-wd/0526002.pdf
So in some cases 27.5 years *IS* correct. But only if you rent out part of the building to somebody else for their living quarters.
