There shall be a setback of not less than twenty feet (20'); provided, however, that where lots comprising twenty five percent (25%) or more of the frontage of any block are developed with buildings having a predominant setback, no buildings hereafter erected or structurally altered shall project beyond the predominant setback line so established; provided further, that this regulation shall not be interpreted so as to require a setback of more than forty feet (40'). (1997 Code §17.20.060)