[bluGill]

That is very easy to cheat though. Two obvious ones: a different unit is held open every month; or these units are closed for remodeling. Until you get to long term 40% vacancy it is really hard to tell - and be careful not to kill rural small towns that no longer have demand and so apartments that will never be full anymore get torn down thus harming the few renters who remain (since it isn't worth replacing the building at current rents and many cannot afford the rent that a new apartment would need just to be worth building)

[anticensor]

Allow up to 1 apartment or 3 separate units of house ownership per person. And only allow 1 unit vacant per apartment after the initial year after the construction, or multiple units up to 3 months per 5 years (must be simultaneous in all affected units with a tolerance of 15 days from either side to the simultaneity, no tolerance to the per-unit duration, or should be considered intentional vacancy) for maintenance and remodelling.