That's partly because developers of large estates often have conditions on their planning permission such as including a certain proportion of "affordable" home

That's partly because developers of large estates often have conditions on their planning permission such as including a certain proportion of "affordable" homes within their mix and funding various community facilities. So on paper they have approval to build all of that. The reality is that they can often make far more profit by building the expensive houses first, drip feeding them onto the market, and possibly never finishing the whole estate as originally approved. It's no accident that many homes are approved but never built. It's just a small group of wily giant building firms playing the system and getting away with it. In theory local authorities who administer the planning system should call them on it. In reality it's a David and Goliath situation. Attempting enforcement actions can result in years of expensive legal battles ultimately funded by local taxpayers with no guarantee of success even in what appear to be cut-and-dried cases.