I'm not sure that's true, and in any case they can't really enforce that - if a building owner wants to sell individual units, they can sell them. The obvious

I'm not sure that's true, and in any case they can't really enforce that - if a building owner wants to sell individual units, they can sell them. The obvious counterexample to your point is the Barbican. Much of the Barbican is owner occupied and the vast majority of the flats there are privately owned (ie not local authority/housing association). I'm not sure but I think the Barbican still houses the majority of residents of the City proper.