When you need to calculate your property's basis (e.g. for depreciation purposes or when you sell the asset), the basis of your depreciable property must be reduced by the depreciation that was allowed or allowable, whichever is greater. Because of this, it is best to calculate and take the depreciation on any asset from the time you first place it in service. If you have not done this, you may wish to consider amending your prior year tax returns in order to take the benefit for that expense, since upon disposal of the asset you will be forced to adjust the basis of the property as if you had taken that expense, even if you did not.
Per IRS Publication 946 How To Depreciate Property, starting on page 12, the IRS uses the doctrine of allowed or allowable depreciation:
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).
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