Basis adjustment for depreciation allowed or allowable. You must reduce the basis of property by the depreciation allowed or allowable, whichever is greater. Depreciation allowed is depreciation you actually deducted (from which you received a tax benefit). Depreciation allowable is depreciation you are entitled to deduct.
If you do not claim depreciation you are entitled to deduct, you must still reduce the basis of the property by the full amount of depreciation allowable. If you deduct more depreciation than you should, you must reduce your basis by any amount deducted from which you received a tax benefit (the depreciation allowed).
IRS pub 946
