When I do my taxes I'm going to need to pay capital gains tax on the portion I rented out... but how do I calculate what the gain was?
The portion rented out should already have a value that was established by you in the first year you rented it out. (I am assuming 2020 was not the first year you rented it out.) That value is on the IRS Form 4562 which is tied to the SCH E you used to report the rental income & Expenses. You can not depreciate the rental portion as required by law, without that portion having a value.
