Comment by apparent

And that would be totally unfair if the law was kept secret, and then sprung on unsuspecting property owners.

But we all know that's not the case. Prospective purchasers are well aware of zoning laws. Same reason you can't build a fuel refinery on your tidy plot of R1 land. It would put existing owners, who have a reliance interest in existing zoning laws being respected, in an awfully unfair position.