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if they move, they can take it. It’s not completely permanent

and a 39-year value doesn’t match the likely economic use.

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You know much more about what the structure than I do, but generally if it is on a slab and has electricity, that is fairly permanent.  As I said in my original comment, if it is not easily removable, it is 39 years. 

The Recovery Period for tax depreciation doesn't necessarily align with economic use.