The following text can be found in in Annex 2 of the Framework (the NPPF). Planning Practice Guidance is also a useful supporting tool which assists with how to interpret the Framework. Both the NPPF and the Practice Guidance can be found via this link https://www.gov.uk/government/collections/planning-practice-guidance
The definition of “affordable housing” in Annex 2 of the NPPF is:
“Social rented, affordable rented and intermediate housing, provided to eligible households whose needs are not met by the market. Eligibility is determined with regard to local incomes and local house prices. Affordable housing should include provisions to remain at an affordable price for future eligible households or for the subsidy to be recycled for alternative affordable housing provision.
Social rented housing is owned by local authorities and private registered providers (as defined in section 80 of the Housing and Regeneration Act 2008), for which guideline target rents are determined through the national rent regime. It may also be owned by other persons and provided under equivalent rental arrangements to the above, as agreed with the local authority or with the Homes and Communities Agency.
Affordable rented housing is let by local authorities or private registered providers of social housing to households who are eligible for social rented housing. Affordable Rent is subject to rent controls that require a rent of no more than 80% of the local market rent (including service charges, where applicable).
Intermediate housing is homes for sale and rent provided at a cost above social rent, but below market levels subject to the criteria in the Affordable Housing definition above. These can include shared equity (shared ownership and equity loans), other low cost homes for sale and intermediate rent, but not affordable rented housing.
Homes that do not meet the above definition of affordable housing, such as “low cost market” housing, may not be considered as affordable housing for planning purposes.”