In any R district or A district, where the slope of the natural ground in the required front yard of the lot exceeds a rate of one foot rise or fall for each four feet from the established street grade at the front lot line, or where the ground elevation at the front lot line is five feet or more above or below the established street grade, a private garage or required parking space may be located in a required front yard; provided, however, that no such garage or required parking space shall occupy an area between the front lot line and any special building line, future width line or official right-of-way line established by ordinance. (Prior gen. code § 8-60.31)