Why? Because apparently they need some more incentive to keep units occupied. Also, even though a property might be vacant, there’s still imputed rental income there. Its owner is just receiving it in the form of enjoying the unit for himself instead of receiving an actual rent check from a tenant. That imputed rent ought to be taxed like any other income.
Easiest solution would be to have 2 property tax brackets and manage it at a city council level.
Occupied/under maintenance or vacant. If you rent a house you show the city council the lease and they give you a form that says you’re entitled to the lower rate for the term of the tenancy,or you go to them and inform them that the property is underrgoing maintenance and is expected to be that way for X months and again you’re entitled to the lower rate. If you cant provide evidence or tenancy or maintenance/repairs for say… 75% of the year, you pay a higher rate. Not extortion levels of higher but a definite incentive.
Oh and absolutely ball breaking fines for anyone found to be doing dodgy shit.