There is not and never could be a Legal definition of Affordable Housing. Built examples range from those out of reach of all the local young people to those that are only affordable by huge public subsidies, by local government housing associations and local make work bodies.
Affordable is site specific let alone Parish or County criteria. I am reminded of the early debates about how many angels could dance on the head of a pin. A futile exercise.
This is not the answer to local housing problems. Wales has always exported its young,more survive localy than at any time in the past. Does it make for happy successful communities, young people existing on State benefits and make work low paid non jobs.