Leasehold improvements have different depreciation rules depending on whether you are working with U.S. tax basis financial reporting or U.S. generally accepted accounting principles (GAAP) financial reporting. For tax purposes, leasehold improvements are eligible to be depreciated over 15 years. The 15-year rule was enacted by the Internal Revenue Service (IRS) in 2004. Prior to that year, the depreciation term was 39 years. The 15-year rule is not permanent and must be reauthorized every year.