[tiedieconderoga]

Maybe a property tax adjustment based on how much time the property has spent vacant over the past N years?

Something like, every year the property owner pays:

    property value * (average vacancy % over N years)

If your property is occupied, you pay nothing extra. If your property sits idle for one year out of five, you pay 20% the property's value to the city every year until your rolling average improves. Maybe throw in some incentives like a partial rebate when the owner signs a new tenant after a stretch of vacancy.

[hsjqllzlfkf]

As a property owner I'll just create shell companies to rent all empty lots for $1/yr from myself. Your move.

[tiedieconderoga]

Write it into the law that self-dealing to evade taxes is fraud, if that even needs to be spelled out. Also, vacant properties would count as vacant whenever they are not productively occupied, even if there is an active lease. Have a city inspector perform periodic spot checks, like how health/fire/etc codes are enforced. Fines for violations scale with the property's assessed value, rather than being defined as flat dollar amounts.

Seems pretty simple.