The provisions of the San Francisco Municipal Code, Section 87 and Sections 678 to 681, inclusive, of Chapter VIII, and Sections 186 to 191, inclusive, and Sections 192 to 195, inclusive, of this Article do not apply to signs not exceeding 20 square feet in size, familiarly known as "real estate signs," advertising for sale or rent the property upon which they stand, but all such signs shall be securely fastened to the ground or to the structures to which they are attached.