Cal. Sts. & High. Code § 5940
(a) It shall be unlawful to commence work under a home improvement contract, or deliver any property or perform any services other than obtaining building permits or other similar services preliminary to the commencement of work, and the home improvement contract shall be unenforceable, if both of the following occur:
(b) If work has commenced in violation of subdivision (a), then:
(c)
(1) If the contractor has delivered any property to the property owner pursuant to a contract that is unenforceable under subdivision (a), the property owner shall make the property available to the contractor for return within 90 days of execution of the contract provided that:
(d) The property owner may waive the requirements in subdivision (a) if all the following are met: