The rehabilitation must be substantial. That is, during a 24-month period selected by the taxpayer, rehabilitation expenditures must exceed the greater of $5,000 or the adjusted basis of the building and its structural components. The adjusted basis is generally the purchase price, minus the cost of land, plus improvements already made, minus depreciation already taken. Once the substantial rehabilitation test is met, all qualified expenditures, including those incurred outside of the measuring period, qualify for the credit.
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Definition taken from Preservation Tax Incentives for Historic Buildings by the National Park Service.