I would also be interested to see what would happen if the rental and sales markets were forced to only supply houses which matched the current building codes - it would make building much more competitive.
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.
It's good because it would motivate owners to own only as few houses as possible (to minimize work on updates), and many more people could own a home then.
That would either destroy vast numbers of perfectly usable and often interesting or historic buildings or require the creation of a system for giving exemptions in order to preserve them. And of course this gives an opportunity for corruption and cronyism.