You do it as a 18 month rolling window - if the property has been empty for 9 months out of the 18 month rolling window, the penalty should apply, in addition t

You do it as a 18 month rolling window - if the property has been empty for 9 months out of the 18 month rolling window, the penalty should apply, in addition there could be relief triggers based on both local, citywide and regional occupancy, if local occupancy was below a certain level (business close because of prolonged construction, or a crime issue, of if there was a city wide vacant windows problem the penalty could be waived by the city. I'm not sure if such a policy would help, it could act as something tantamount to rent control - but its not hard to make a policy in such a way that it wouldnt have a negative feedback loop.