If the planning board unfavorably reports on such change or if the owners of 20% or more either of the area of the lots or land included in such proposed change, or of the lots or land within 100 feet of the rear thereof, or of the lots or land within 100 feet on either side or directly across the street therefrom within 100 feet, exclusive of street space, protest in writing such amendment shall not become effective except by favorable vote of two-thirds of all members of the board of commissioners.