If you doubled your store size, you would enter a new asset under the Assets section in the Schedule C interview. It sounds like an asset you would depreciate, not amortize, and the annual expense would show under depreciation, not other expenses. The new asset could be called something like 2018 improvements. You can enter and begin depreciation this as soon as the business is open.
Why are you expecting to amortize it? "Expenditures that would have otherwise been capitalized, such as the costs associated with the construction of a capital asset, are not startup costs" (Rev. Rul. 81-150). IRS
