vacancy begins

Total vacancy of any area or structure begins when no tenant or regular user is any longer making use of it. This does not apply when a place is shut down for the night, weekend, holidays, emergencies, or other reasons which do not interrupt its current level of occupancy. A structure reaches this phase when the last remaining tenant moves out, or when it is completed without any tenants moving in.

This should not be confused with the event vacating begins, which refers to the moment when tenants begin moving out. There is usually a slight gap between these two events, between which occupants are in the process of leaving.

The milestone is complete absence of regular occupancy, not the departure of the first tenant. A newly completed structure can reach total vacancy without having had previous tenants if nobody moves in.