Except as provided in Section 17.120.100, where three (3) or more parcels comprising more than fifty (50) percent of a single street frontage of a block are improved with buildings at the time of passage of this resolution, every building hereafter erected shall provide a front yard of not less than the average depth of the front yards of existing buildings, or the required front yards of existing buildings, or the required front setback, whichever is less.