One way I look at this is as follows:
What is the value of the house per square foot (not including land)? Or what would it cost to rebuild all new? This can be used for a basis for a new purchase. You could then use a reasonable sq ft number for the basis. Land cannot be included. Another item overlooked is the outside or, Hardscape and Landscape improvements. Those are also depreciable and could be included in the basis.
The ADU basis is simply the costs, including all fees, plans, permits and construction costs.
Hope this helps!
