Since the improvements are "a material part of" the structure they are classified as residential rental real estate and depreciated over 27.5 years.
If the improvements were less than $2,500 then you can elect to expense them under the safe harbor de-minimus act. For a brief explanation see https://www.landlordstudio.com/blog/3-irs-safe-harbors-landlords-need-to-know-about
