At what point do they become apartments though? When the Certificate of Occupancy is issued? "No problem; those units aren't finished yet. [They need a smoke alarm to be legal units and somehow we haven't found the time to get around to it yet.]" Or whatever the threshold is between "this is a vacant lot" and "this is a taxable unoccupied unit", you're just providing incentive to stay on one side of that line if there is a glut of builders using this strategy.