There shall be a front yard having a minimum depth of thirty feet from the front property line, except:
(a) Where a uniform setback line has been established or observed on one side of a street, between two intersecting streets or for a distance of 600 feet on either side of the lot in question at the time of the effective date of Ordinance 24-66 passed April 5, 1966, no building hereafter erected or structurally altered shall project beyond such setback line.
(b) Where twenty-five percent or more of all the property, according to front feet, abutting one side of a street between two intersecting streets or for a distance of 600 feet on both sides of the lot in question, is built up with buildings having an average setback line of more or less than thirty feet, no building hereafter erected or structurally altered shall project beyond the average setback line so established. (Ord. 24-66. Passed 4-5-66.)