The new windows become "a physical part of" the structure. No question on that. They are classified as residential rental real estate and depreciated over 27.5 years. You will enter the windows as another asset in the Assets/Depreciation date, and depreciation will start on the first day the windows were placed "in service", or on the date the property was place "in service".... whichever date is later.
Depreciation is never retroactive. Depreciation starts on the first day an asset is "available" for the production of income.
