Under Kingston Borough's Section 106 Planning Obligation, if you build more than 10 units you are contractually obliged to...
1)....make half the units affordable and pass them over to the council.2)..."pepper pot" these council houses throughout your development.
3)...build the maximum the land will accommodate.4)...in the case of flats, ensure any service charge is also affordable ("full price" residents subsidise their "council house" neighbours).
...and the same applies to smaller developments wherever practicable.