Sidewalks with a depth of at least 12 feet, measured perpendicular to the curb of a street, shall be provided along the entire street frontage of a zoning lot. In locations where the width of the sidewalk within the street is less than 12 feet, a sidewalk widening shall be provided on the zoning lot so that the combined width of the sidewalk within the street and the sidewalk widening equals 12 feet. However, existing buildings to remain on the zoning lot need not be removed in order to comply with this requirement. All sidewalk widenings shall be improved to Department of Transportation standards for sidewalks, shall be at the same level as the adjoining public sidewalks, and shall be accessible to the public at all times.