No,em10, your council won't be building affordable housing itself but it should be requiring other developers to build some alongside the commercial housing in the same development. What happens too is that Housing Associations build affordable housing.
Just to show how daft our planning law and practice has been, a Housing Association came to me and offered to buy some land on the edge of this village. Their bargaining point was that it was outside the "village envelope", that is it could not be built on at present by me, but they, being a Housing Association, would get permission. They would build 25 units on an acre. They offered me £65,000 for an acre. Now, it would ordinarily have been worth £1 million with planning permission. I said "No" unless they took another site inside the village too and built commercial housing there, so I'd end up as if I had developed both sites (affordable plus commercial, as per the rules). They refused that offer.
Result? No houses or flats were built and the village is still short of about 60 homes on present estimates by the villages themselves. And the land? I gave the acre plus another to the village because the village wanted allotments. And people wonder why there's a housing shortage here.