https://innago.com/how-to-calculate-depreciation/
To depreciate a building that you use for rental activities, begin depreciation when the building is placed in service. A property “placed in service” is ready and available to rent (it doesn’t matter whether it’s occupied or not). You will want documentation that proves when your property was placed in service. In other words, keep copies of rental ads or listings advertising that the property is available to rent.