Except for accessory structures existing adjacent to any mobile home park space as of the effective date of this section, no new accessory structures shall be permitted adjacent to any mobile home park space (other than for a stairway and landing to gain access to said mobile home). No accessory structure existing as of the effective date of this section (other than for stairways and landings) may be replaced or expanded. No such landing shall be larger than four feet by eight (4 x 8) feet in dimension.