now that would be interesting. if an old flat or house became unrentable or unsellable once the current occupant moves out because it doesn't match current requirements, then landlords would be begging renters to stay.

on the other hand, individual home owners would be stuck unable to sell their property, because it would drop in value. (even if they could sell it, not everyone would want to buy it). i am not sure if that's good or bad.

in any case everyone would be forced to keep their buildings up to standard.