The article mentions new build developments. A property developer must have approval to build, there is no legal right to build on acquired land. Approval comes

The article mentions new build developments. A property developer must have approval to build, there is no legal right to build on acquired land. Approval comes from the local council, according to various local criteria (like, are local schools at capacity?). Therefore, they could conceivably require that property developers must obey these new rules to be granted permission ("do you agree to only sell to people who don't already own a home?").