Cal. Civ. Code § 3319

(a) In each written contract for private works of improvement entered into on or after January 1, 1996, the contracting party and the design professional may agree to contractual provisions that include a late payment penalty, in lieu of any interest otherwise due. The terms of the late payment penalty shall be specifically set forth in the written contract.
(b) The penalty authorized pursuant to subdivision (a) shall be separate from, and in addition to, the design professionals liens provided by Chapter 3 (commencing with Section 8300) of Title 2 of Part 6 of Division 4, mechanics liens provided by Chapter 4 (commencing with Section 8400) of Title 2 of Part 6 of Division 4, and stop payment notices provided by Chapter 5 (commencing with Section 8500) of Title 2 of Part 6 of Division 4.
(c) None of the rights or obligations created or permitted by this section between design professionals and contracting parties shall apply to construction loan funds held by a lender pursuant to a construction loan agreement.
(d) For purposes of this section, the following definitions apply:
1. (1) “Contracting party” means any person or entity entering into a written contract with a design professional for professional design services for a private work of improvement.
2. (2) “Design professional” means a person licensed as an architect pursuant to Chapter 3 (commencing with Section 5500) of Division 3 of the Business and Professions Code, registered as a professional engineer pursuant to Chapter 7 (commencing with Section 6700) of Division 3 of the Business and Professions Code, or licensed as a land surveyor pursuant to Chapter 15 (commencing with Section 8700) of Division 3 of the Business and Professions Code.