Im having trouble parsing your first question, but I’ll take a crack anyway.

I’d probably calculate tax based on what a vacant unit is available for, if it isn’t available could do something like last known rent adjusted for market or calculations based on property value.

The idea being they’ll list it for rent at a reasonable rate instead of keeping it off the market or keeping rent high in order to fill units.

Some dudes fourth property or a CEOs second apartment aren’t the problems with housing though. It’s investment capital grabbing properties and doing nothing with them. Buildings standing vacant because they weren’t profitable enough. Empty lots we won’t even let the homeless camp on.

A number of these properties are essentially abandoned, forgotten about by bankrupt companies where ownership is on an unsorted piece of paper in a filing cabinet. We’ve got an “affordable apartment complex” out here the company stopped building on quietly after the publicity ran out and it was no longer cost effective.

Resources hoarded and wasted for the rest of us to fight over scraps.