Not less than 20 percent of the total number of units of housing for rent which are financed, or for which financing has been extended or committed, pursuant to this chapter from the proceeds of each bond issuance of the city or county, shall be for occupancy on a priority basis by lower income households, as defined in Section 50079.5 of the Health and Safety Code, at affordable rents, as defined in Section 50053 of the Health and Safety Code.