You may only deduct expenses from the date it is placed in service and available to rent. Any assets you purchased prior to that date are added to the building and depreciated as one asset.
Placed in Service
You place property in service in a rental activity when it is ready and available for a specific use in that activity. Even if you aren’t using the property, it is in service when it is ready and available for its specific use.
Example.
On April 6, you purchased a house to use as residential rental property. You made extensive repairs to the house and had it ready for rent on July 5. You began to advertise the house for rent in July and actually rented it beginning September 1. The house is considered placed in service in July when it was ready and available for rent. You can begin to depreciate the house in July.