California Code, Business and Professions Code - BPC § 5495.5

A city or county with an ordinance or regulation introduced or adopted prior to March 12, 1983, which is applicable to designated areas within the city or county less than the entire city or county is not in violation of Section 5491 for an ordinance or regulation introduced or adopted on or after March 12, 1983, even though it requires removal of on-premises advertising displays in additional portions of the city or county, if the city or county adopts not more than two such ordinances or regulations on or after March 12, 1983, and if the total effect of the ordinance, or regulation is to apply to less than the entire city or county, and such new ordinance or regulation provides reasonable amortization for conformance.  “Reasonable amortization,” for purposes of this section, shall not be less than 15 years from the date each such ordinance or regulation was adopted.  If these conditions are not met, the city or county is subject to Section 5491 with respect to all those ordinances and regulations.