Roughly, your cost basis is what you paid for the property, plus the cost of any property improvements. Period. (I'm not mentioning the finer details such as your sales expenses incurred at the time of purchase, etc.)
For the purposes of depreciation, you will depreciate based on either the cost basis, or the FMV at the time the property is placed in service..... whichever is *lower*. Most likely, the FMV is higher now. So FMV value would not be used for depreciation.
Plan to rent both the ADU and a part of the addition.
I take "part of the addition" to be referring to a part of the main property.
I would suggest you treat the ADU as a physically separate rental property from the main structure and enter it as such on the SCH E. Then assuming the main structure is your primary residence and will remain as such, treat that one as renting a part of your primary residence. This will most likely give you the greatest ease of flexibility in the future if (and when) things change years down the road.
How you treat the cost of utilities can vary, and can depend on how the utilities for the main house and the ADU are metered and billed.
