Tangential to this: In Norway and Denmark (as the only countries AFAIK) there are laws stating that a unit needs to have inhabitants. Ie. it can not sit still on the market. If the owner does not live there, he has to rent it out.

It baffles me, that this is not a general rule as it forces fair market prices for a illiquid supply (it takes time to build new housing, if it is even possible).

In this case you'd just complain that a company keeps units vacant to the municipality who would fine the company until it is brought in order.