I do __not__ think you mean that one can treat ADU as separate property and have it so in schedule E __only__ if it is detached ADU.
No, that is not what I mean. Heck, if you're renting a room in your house, then for tax purposes you can just as well manually pro-rate for that too. That means you'll be prorating everything of course, starting from the beginning with the cost basis being pro-rated for the SCH E.
If course, when you do it this way, when you sell your property you "may or may not" be reporting two sales. But it can be done reporting on one sale. The main thing required of course, is depreciation recapture in the tax year of the sale. No big deal really.
