For any residence for one to four families, the total area of the required front yard covered with pavement, sidewalk, stepping stones or other impervious material or gravel shall not exceed 50% of the area of such required front yard, and the remainder of such required front yard shall be landscaped with grass, ground-cover or other plant material. For any residence for one to four families, the total area of the required rear yard, exclusive of accessory buildings, covered with pavement, sidewalks, stepping stones or other impervious material or gravel shall not exceed 50% of the area of such required rear yard exclusive of accessory buildings, and the remainder of the required rear yard shall be landscaped with grass, ground cover or other plant material.