The regulations, restrictions, and boundaries may be amended, supplemented, changed, modified, or repealed. Notwithstanding section 335.4, as a part of an ordinance changing land from one zoning district to another zoning district or an ordinance approving a site development plan, a board of supervisors may impose conditions on a property owner which are in addition to existing regulations if the additional conditions have been agreed to in writing by the property owner before the public hearing required under this section or any adjournment of the hearing. The conditions must be reasonable and imposed to satisfy public needs which are directly caused by the requested change. In case, however, of a protest against the change signed by the owners of twenty percent or more either of the area included in the proposed change, or of the area immediately adjacent to the proposed change and within five hundred feet of the boundaries of the proposed change, the amendment shall not become effective except by the favorable vote of at least sixty percent of all of the members of the board of supervisors. The provisions of section 335.6 relative to public hearings and official notice shall apply equally to all changes or amendments.