Cal. Bus. & Prof. Code § 6749
(a) A professional engineer shall use a written contract when contracting to provide professional engineering services to a client pursuant to this chapter. The written contract shall be executed by the professional engineer and the client or the client’s representative prior to the professional engineer commencing work, unless the client knowingly states in writing that work may be commenced before the contract is executed. The written contract shall include, but not be limited to, all of the following:
(b) This section shall not apply to any of the following:
(4) Professional engineering services rendered by a professional engineer to any of the following: