In the “B” and “C” Districts, there shall be a front yard having a depth of not less than twenty feet (20') measured from the front property line of the lot to the front of the building, covered porch or terrace or attached accessory building; however, if fifty percent (50%) or store of the property on one side of a street, within the block, is improved with building, the required front yard depth need not exceed the average front yard depth of the existing buildings.