Deems a county inclusionary mandate as a form of development exaction and treats the mandate as a housing affordability impact fee, with certain exemptions. Prohibits a county from adopting, amending, or enforcing an inclusionary mandate or inclusionary mandates for residential or mixed-use development, under certain circumstances. Establishes additional requirements for a needs assessment study for a county-imposed inclusionary mandate. Establishes a criterion that allows luxury residential projects or projects that receive certain discretionary value increases to be subject to inclusionary mandates. Effective 7/1/3000. (HD1)