Real estate signs advertising the sale, rental or lease of the premises on which they are maintained shall set back from the right-of-way line of any street at least one-half the depth of the required front yard in the districts where located; a sign shall not exceed six square feet in area. If a sign is attached flat against the building to which it pertains, it shall be permitted on any portion of the building below the roof-line. Such real estate signs on any one lot shall not exceed, in the aggregate, fifteen square feet in area.