get the apartment appraised separately and then use that as my current value for it.
That's not how it works when selling the property. The current appraised value is not used to determine cost basis for the purpose of determining gain or loss on the sale. (unless the property was inherited. Then your cost basis is the FMV of the property at the time the person you inherited it from, passed away.)
For determining the gain/loss on the sale, your cost basis is what you originally paid for it, plus the cost of any property improvements, minus the total depreciation taken during the time you owned it.
