Unfortunately my understanding of the R&D expensing rule is that it is lifted directly from GAAP, which means private companies have to adhere to those (heavyweight) rules to comply.

Fair point. I changed the question to "according to the tax code" and it told me that

Construction labor is generally not deductible as an expense in the year incurred if it is related to the construction or improvement of a capital asset (like a building). Instead, under the U.S. tax code (IRC §263A), these costs must usually be capitalized and recovered through depreciation over time. Exceptions may apply for certain small taxpayers or repairs.