I rent the basement apartment that accounts for 1/3 of my house. My family occupies the other 2/3rds. In 2016, the basement unit was rented for 121 days. Do I claim 1/3 of the mortgage interest I paid for 121 days (or does Turbotax calculate the amount o

[kdurbs99]

Wouldn't it be simpler to just claim all the mortgage interest I paid as a regular deduction and not claim mortgage interest at all under the rental property (because we would have paid the interest regardless of whether we had the unit rented).

[DianeW]

Yes, the simplest way to do it is to add it to your itemized deductions if you know you will use those expenses.  

To answer your other question though, in your situation with only part of the home rented less than the whole year you would use one third of all expenses for the prorated portion of the year (calculated manually) as well as one third of the house cost for depreciation as an asset.

For the future, if you rent the basement unit the entire year then TurboTax would do all the calculations for you without the need for manual calculations.