Property improvements completed after the last renter moved out are not entered anywhere on your tax return. While they do add to the cost basis of the property, you can't/don't enter them since the improvement will never be placed into service as a rental asset. It's up to you to keep track of your cost basis outside of taxes, as that will come into play when one of three things happens in your future.
1. You convert the property back to rental.
2. You sell the property.
3. You die.
