1) You start depreciation when it is available for use. 

2) Storage units can qualify as “tangible personal property” under the U.S. tax code because they are considered movable and not permanently attached to the land. This classification allows self-storage facilities to benefit from a shorter depreciation schedule. Instead of depreciating assets over the standard 39-year class life for commercial buildings, storage units can be depreciated over just seven years, resulting in significant tax savings.

https://engineeredtaxservices.com/capitalize-on-tax-planning-opportunities-in-self-storage-industry/