Affordable housing policy - Public forum - Planning Advisory Service (PAS)

My councils preferred approach to securing affordable housing is via a per bedspace approach. This has casued a number of issues and confusion amongst developers because our existing policy sets the threshold on a per unit basis. I am trying to draft a new policy for the Core Strategy but can't see how this council can secure affordable housing through a per bedspace threshold especailly where we're proposing different targets levels. Does anyone have any experience of other councils adopting a similar approach?