In Carter v. Cohen (2010) 188 Cal.App.4th 1038, a former tenant sued her former landlord for damages for rent overpayments. (Id. at p. 1042.) She had leased a detached guesthouse, which had been constructed without permits, located on a residential property that also contained a house. (Ibid.) She claimed that the rent increases, imposed by her former landlord after he bought the residential property, exceeded the limits set by a municipal rent stabilization ordinance (RSO). (Ibid.) The issue on appeal was whether the former tenant was entitled to recover her excess rent payments even though the guesthouse lacked a certificate of occupancy and was not registered under the ordinance. (Id. at p. 1046.) The former landlord contended that the action to recover excess rent payments failed as a matter of law because the rental agreement was unlawful and outside the scope of the ordinance. (Id. at p. 1043.) He maintained that the rental agreement was "void and unenforceable because the guesthouse had been built without permits, lacked a certificate of occupancy, and was unregistered under the RSO." (Id. at p. 1047.) The appellate court acknowledged that "rental agreements involving units that were constructed without building permits or lack a certificate of occupancy are ordinarily regarded as unlawful and void. " (Carter, supra, 188 Cal.App.4th at p. 1047.) "Generally, 'the courts ... will not enforce an illegal bargain or lend their assistance to a party who seeks compensation for an illegal act.' (Lewis & Queen v. N. M. Ball Sons (1957) 48 Cal.2d 141, 150 308 P.2d 713 (Lewis & Queen).)" (Ibid.) But the appellate court refused to apply the rule in that case because otherwise public policy would be thwarted. (Id. at pp. 1048-1050.) The appellate court in Carter stated that "the rule barring the enforcement of unlawful contracts is not absolute." (Carter, supra, 188 Cal.App.4th at p. 1048.)