We built an ADU above our attached garage with the sole purpose of renting it. Do we do an Adj. Cost Basis for the whole house or is the Basis separate from the house?

Hired an architect in Aug 2016 with the intent to build an ADU above our garage and rent it for additional income.   Our first tenant moved in on 3/8/2018 after Final Inspection.  Can we deduct construction costs? Do we treat the ADU as a separate entity from the house or is it seen as an improvement to the house?