In any residential or business district, where twenty- five percent or more of the street frontage between intersecting streets is occupied by a building plus required minimum side yards, the front yard requirements for the frontage shall be the average yard determined by all existing buildings if such yard dimensions are less than required by this Zoning Ordinance.
In no case, however, shall such yard requirements plus the distance to the center line of the fronting street be less than thirty feet in a business district or twenty-five feet in a residential district, unless determined otherwise by the Board of Zoning Appeals.
(a) Steep Slopes; Front Yard Garage. In any R District where the natural grade of lot within the required front yard has an average slope, normal to the front line at every point along such line, of such a degree of slope that is not practicable to provide a driveway with a grade of twelve percent or less to a private garage conforming to the requirements of this Ordinance, such garage may be located within such front yard, but not in any case closer than six feet to the street line
(b) Double Frontage Lots. Buildings on lots having frontage on two nonintersecting streets need not have a rear yard if an equivalent open space is provided on the lot in lieu of such required rear yard; applicable front yards must be provided, however, on both streets.