California Code, Civil Code - CIV § 712

(a) Every provision contained in or otherwise affecting a grant of a fee interest in, or purchase money security instrument upon, real property in this state heretofore or hereafter made, which purports to prohibit or restrict the right of the property owner or his or her agent to display or have displayed on the real property, or on real property owned by others with their consent, or both, signs which are reasonably located, in plain view of the public, are of reasonable dimensions and design, and do not adversely affect public safety, including traffic safety, and which advertise the property for sale, lease, or exchange, or advertise directions to the property, by the property owner or his or her agent is void as an unreasonable restraint upon the power of alienation.

(b) This section shall operate retrospectively, as well as prospectively, to the full extent that it may constitutionally operate retrospectively.

(c) A sign that conforms to the ordinance adopted in conformity with Section 713 shall be deemed to be of reasonable dimension and design pursuant to this section.