In Edmonds v. County of Los Angeles (1953) 40 Cal.2d 642, plaintiffs operated a trailer court as a lawful nonconforming use in a residential zone. The board of supervisors then granted an exception permitting plaintiffs to add 30 trailers on condition that they abandon the entire nonconforming use of their property for trailer court purposes at the end of three years. (Id. at pp. 644-645.) Having taken the benefits of the conditional exception, the plaintiffs were estopped to challenge the effectiveness of the exception. (Id. at p. 653.)