Notwithstanding any other law to the contrary, the counties are authorized to provide flexibility in land use density provisions and public facility requirements to encourage the development of any rental housing project where at least a portion of the rental units are set aside for persons and families with incomes at or below one hundred forty per cent of the area median family income, of which twenty per cent are set aside for persons and families with incomes at or below eighty per cent of the area median family income.