Cal. Civ. Proc. Code § 1161.4

(a) A landlord shall not cause a tenant or occupant to quit involuntarily or bring an action to recover possession because of the immigration or citizenship status of a tenant, occupant, or other person known to the landlord to be associated with a tenant or occupant, unless the landlord is complying with any legal obligation under any federal government program that provides for rent limitations or rental assistance to a qualified tenant.
(b) In an unlawful detainer action, a tenant or occupant may raise, as an affirmative defense, that the landlord violated subdivision (a).
(c) It is a rebuttable presumption that a tenant or occupant has established an affirmative defense under this section in an unlawful detainer action if the landlord did both of the following:
1. (1) Approved the tenant or occupant to take possession of the unit before filing the unlawful detainer action.
2. (2) Included in the unlawful detainer action a claim based on one of the following:
  1. (A) The failure at any time of a previously approved tenant or occupant to provide a valid social security number.
  2. (B) The failure at any time of a previously approved tenant or occupant to provide information required to obtain a consumer credit report under Section 1785.11 of the Civil Code.
  3. (C) The failure at any time of a previously approved tenant or occupant to provide a form of identification deemed acceptable by the landlord.
(d) This section does not create a rebuttable presumption that a tenant or occupant has established an affirmative defense under this section if a landlord has requested the information described in paragraph (2) of subdivision (c) for the purpose of complying with any legal obligation under any federal government program that provides for rent limitations or rental assistance to a qualified tenant, or any other federal law, or a subpoena, warrant, or other order issued by a court.
(e) The rebuttable presumption in this section does not limit paragraph (2) of subdivision (c) of Section 1940.3 of the Civil Code.
(f) No affirmative defense is established under subdivision (b) if a landlord files an unlawful detainer action for the purpose of complying with any legal obligation under any federal government program that provides for rent limitations or rental assistance to a qualified tenant.
(g) For purposes of this section, “immigration or citizenship status” includes a perception that the person has a particular immigration status or citizenship status, or that the person is associated with a person who has, or is perceived to have, a particular immigration status or citizenship status.