"Home office depreciation is almost always calculated using a 39 year recovery period. A 27.5 year recovery period can be used for home office depreciation however by an on-site landlord of a building in which at least one dwelling unit is rented out and 80% or more of the gross renal income is rental income from dwelling units within the building."
This is a complicated subject, more at: Expenses Related to Your Home Office Are Deductible (bizfilings.com)
