The cost of finishing your basement should have been added to/included in the cost basis of the property, which would have been used to calculate depreciation. If you did not use the correct cost basis starting in 2015, you may want to file a Form 3115 - Change in Accounting Method. This form is only available in TurboTax Desktop. If you want to update the cost basis and amount of depreciation taken on your rental property going back to 2015, you may want to consult with a local tax professional.
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