The cost of building the portable office building must be depreciated and entered as an Asset of your business since it's useful life is greater than one year. However, you may be able to use the Section 179 deduction through the depreciation schedule to write off the full cost in the current year.
Whether or not you qualify to use Section 179 depends on whether you consider the 'portable office building' as a building meant to be in place permanently, or a structure that can be easily removed and relocated. If it is a building, then it must be depreciated because it is not eligible for the Section 179 deduction.
See the information below for the answer to a similar question for some additional guidance.
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Buildings