Either way, you may not deduct any of the construction cost.
The portion of your home office deduction, attributable to the building structure, comes from depreciation. You add the construction cost to your home cost basis ,divide that by 27.5 years, and multiply that by the square foot percentage used for the office. The calculation is the same, whether you you use the new space or old space.
If you constructed a free standing building, as opposed to an addition, the answer would only be slightly different. You depreciate only the new structure.
