The rental is a business expense. It was purchased for a business. So, since it is not being rented now, you would enter it as a business asset and depreciate it. Since you may want to rent it you could create two assets, one for each half, and depreciate it as a business asset for the business that is using it for storage and the business that may one day rent it out. Then it depreciates as commercial property for 39 years.
@glodowa