Cal. Civ. Code § 55.1-1241

A. In an action for possession based upon nonpayment of rent or in an action for rent by a landlord when the tenant is in possession, the tenant may assert as a defense that there exists upon the leased premises a condition that constitutes, or will constitute, a fire hazard or a serious threat to the life, health, or safety of the occupants of the dwelling unit, including (i) a lack of heat, running water, light, electricity, or adequate sewage disposal facilities; (ii) an infestation of rodents; or (iii) a condition that constitutes material noncompliance on the part of the landlord with the rental agreement or provisions of law. A tenant may assert a defense as provided for in this section if, prior to the commencement of the action for rent or possession, the landlord or his agent refused or, having a reasonable opportunity to do so, failed to remedy the condition for which he was served notice as defined in § 55.1-1200 by the tenant or by a violation or condemnation notice from an appropriate state or local agency. For the purposes of this subsection, the period of time deemed to be unreasonable delay is left to the discretion of the court, except that there shall be a rebuttable presumption that a period in excess of 30 days from receipt of the notification by the landlord is unreasonable.
B. It shall be a sufficient answer to such a defense provided for in this section if the landlord establishes that (i) the conditions alleged in the defense do not in fact exist; (ii) such conditions have been removed or remedied; (iii) such conditions have been caused by the tenant, his guest or invitee, members of the family of such tenant, or a guest or invitee of such family member; or (iv) the tenant has unreasonably refused entry to the landlord to the premises for the purposes of correcting such conditions.
C. The court shall make findings of fact upon any defense raised under this section or the answer to any defense and shall issue any order as may be required, including any one or more of the following:
1. Reducing rent in such amount as the court determines to be equitable to represent the existence of any condition set forth in subsection A;
2. Terminating the rental agreement at the request of the tenant; or
3. Ordering the landlord to remedy the condition that resulted from the landlord's material noncompliance.
D. If it appears that the tenant has raised a defense under this section in bad faith or has caused the violation or has unreasonably refused entry to the landlord for the purpose of correcting the condition giving rise to the violation, the court may impose upon the tenant the reasonable costs of the landlord, including court costs, the costs of repair where the court finds the tenant has caused the violation, and reasonable attorney fees.
E. If the court finds that the tenant has successfully raised a defense under this section and enters judgment for the tenant, the court, in its discretion, may impose upon the landlord the reasonable costs of the tenant, including court costs, and reasonable attorney fees.
F. Nothing in this section alters the right of a landlord to amend the amount requested at trial based on all amounts due and owing on the trial date, including ongoing rent.