In rental terms, "affordable housing" is defined as 80% of the open market rent.
This quote is from an article in the Independent two years ago. Yes, the figures quoted will have changed in that time, but the general idea remains the same:
The lowest average rents in London have been identified as being in the borough of Bexley, where they have just passed the £1,000 per month mark. In Kensington they are well over £5,000 per month and the all-London average is nearly £2,000 per month. The Government’s definition suggests that the average Londoner can “afford” to pay over £400 a week in rent.
That's why my blood boils every time I hear politicians boasting about providing more of the very badly-named "affordable" housing, as if that will solve the housing problems for low-paid people, and almost as a justification for further penalising those in need of housing benefit in order to keep a roof over their heads!