You can elect, for any class of property, not to deduct any special allowances for all property in such class placed in service during the tax year.
To make an election, attach a statement to your return indicating what election you are making and the class of property for which you are making the election.taxmap/pubs/p946-017.htm#en_us_publink1000107501
Generally, you must make the election on a timely filed tax return (including extensions) for the year in which you place the property in service.
However, if you timely filed your return for the year without making the election, you can still make the election by filing an amended return within 6 months of the due date of the original return (not including extensions). Attach the election statement to the amended return. On the amended return, write "Filed pursuant to section 301.9100-2."taxmap/pubs/p946-017.htm#en_us_publink1000107502
Once you elect not to deduct a special depreciation allowance for a class of property, you cannot revoke the election without IRS consent. A request to revoke the election is a request for a letter ruling.
If you elect not to have any special allowance apply, the property may be subject to an alternative minimum tax adjustment for depreciation.