Just tax corporate owned vacancy. In a slump there will be apartment buildings that are mostly empty because they refuse to lower the rent as lowering rent triggers property re-valuation.

Office buildings sit mostly empty for the same reason.

Tax the owners to punish the bad bets and eternal growth expectations of banks to force them to use the space to the benefit of the community or be forced to sell when they run out of money. Use zoning laws to prevent the destruction of units to avoid taxes.