I note that Turbotax entered the special depreciation on line 14 of Form 4562.
De Minimis Safe Harbor and Special Depreciation Allowance are two completely different things.
De Minimis Safe Harbor is *not* depreciation. DMSH allows you to expense certain items that cost less than $2,500, instead of capitalizing and depreciating them. When you elect this option, you can not add the cost to your cost basis. Do note that just because an item may cost less than $2,500, does not mean it qualifies for DMSH. The program has no way of knowing of the asset/item qualifies or not. It's up to you to determine.
In your case, it appears you did not elect DMSH, but instead elected to depreciate the item by using the Special Depreciation Allowance that allows you to completely depreciate the asset in the year placed in service. Electing this option does add to the cost basis. But by depreciating it fully in the first year that means when you sell the property, that particular asset will have a cost basis of zero basically. That makes the sales price on the sale of that specific asset a fully taxable gain.
