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.
IDK, probably have to do some research into how much time it takes to finish a unit to say when the clock starts ticking.
For example, obviously a smoke alarm is something that takes 15 minutes to install, so that wouldn't be when you start the clock.
You might be able to base it on when any single unit is sold in the block. That is "You sell 1 apartment, you've got a year to sell to rest or you are going to start seeing vacancy taxes".
