(a) In Residential and Office-Residential Districts on streets where a front yard more than that required by this Zoning Code has been maintained for existing structures on lots having a frontage of fifty (50) percent or more of the total frontage on one (1) side of that portion of the street lying between two (2) intersecting streets, there shall be maintained a front yard of not less than the average front yard of the existing structures, provided that this regulation shall not be interpreted to require a front yard of more than fifty (50) feet. In Residence and Office-Residence Districts on streets where a front yard less than that required by this Zoning Code has been maintained for existing structures on lots having a frontage of fifty (50) percent or more of the total frontage on one side of that portion of the street lying between two (2) intersecting streets, the front yard need not be greater than the average front yard of the existing structures, provided that this regulation shall not be interpreted to permit a front yard less than the block face average between the two (2) intersecting streets.
(b) In any zoning district where the natural grade of a lot within the required front yard has an average slope, such slope being normal to the front lot line at every point along the line, of such a severity that it is not practicable to provide a driveway with a grade of twelve (12) percent or less to a private garage conforming to the requirements of this Zoning Code, the garage may be located within the front yard, but not in any case closer than twenty (20) feet to the street lines.