When it comes to entering the information in TurboTax, one option is to treat the rental as a completely separate property and value the rental structure using the percentage of total square feet method. If the rental space is a separate structure for a single family then it would prorated and be treated as such on schedule E.
For those expenses that are shared (property taxes, insurance, mortgage and refinance) you'll split them using the same percentage as you will do for square foot proration.
Note: Your insurance company may have you take separate policy's for the rental area. If they do then you would list only the insurance for the ADU and not prorate that expense.
