The undepreciated cost basis of your rental property would be what you paid for it plus the cost of improvements you made. You are correct, Form 4562 only reports the current year depreciation deduction. Maybe you can find the closing statement when you purchased the property to determine the initial cost. If the fair market value of the property was less than the cost when you started renting it, that would be used in place of the cost for the cost basis starting point. Your property appraiser may have a website you can go to that would list the purchase price of the property.
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