Cal. Civ. Code § 302-A

  § 302-a. Abatement of rent in the case of serious violations.
  1. The provisions of this section shall apply to all cities with a
population of four hundred thousand or more.
  2. a. A "rent impairing" violation within the meaning of this section
shall designate a condition in a multiple dwelling which, in the opinion
of the department, constitutes, or if not promptly corrected, will
constitute, a fire hazard or a serious threat to the life, health or
safety of occupants thereof.
  b. The determination as to which violations are "rent impairing" shall
be made in the following manner. Within six months after the enactment
of this section, the department shall promulgate a list of conditions
constituting violations of the provisions of this chapter and of any
regulations promulgated pursuant to the provisions of subdivision four
of section three of this chapter. Such list shall contain a brief
description of the condition constituting the violation, the section of
this chapter or regulation violated, and the order number assigned
thereto. The department may from time to time change the number or
description of violations on such list, as may seem appropriate to the
department. Such list shall be available at all times to the public.
  c. At the time of the promulgation of the list of violations, the
department shall also designate, by reference to the order number, those
violations which it proposes to classify as rent impairing as above
defined. Within thirty days thereafter, the department shall hold a
public hearing at which all persons interested may be heard as to the
propriety of the classification of such violations as rent impairing. At
least twenty days' notice of such hearing shall be given by publication
in the city record or other publication in which official notices of the
city are regularly published. Within a reasonable time after the
hearing, the department shall make and publish a list of those
violations which are classified as rent impairing. Any person interested
may, within four months thereafter, seek a review by the supreme court
of the propriety of the classification of any of such violations as
"Rent Impairing" by a special proceeding pursuant to article
seventy-eight of the civil practice law and rules. No other body or
officer shall have the power to review said classification.
  d. The department may at any time change the number or description of
rent impairing violations but no such change shall be made except in the
manner above set forth after notice and public hearing.
  3. a. If (i) the official records of the department shall note that a
rent impairing violation exists in respect to a multiple dwelling and
that notice of such violation has been given by the department, by mail,
to the owner last registered with the department and (ii) such note of
the violation is not cancelled or removed of record within six months
after the date of such notice of such violation, then for the period
that such violation remains uncorrected after the expiration of said six
months, no rent shall be recovered by any owner for any premises in such
multiple dwelling used by a resident thereof for human habitation in
which the condition constituting such rent impairing violation exists,
provided, however, that if the violation is one that requires approval
of plans by the department for the corrective work and if plans for such
corrective work shall have been duly filed within three months from the
date of notice of such violation by the department to the owner last
registered with the department, the six-months period aforementioned
shall not begin to run until the date that plans for the corrective work
are approved by the department; if plans are not filed within said
three-months period or if so filed, they are disapproved and amendments
are not duly filed within thirty days after the date of notification of
the disapproval by the department to the person having filed the plans,
the six-months period shall be computed as if no plans whatever had been
filed under this proviso. If a condition constituting a rent impairing
violation exists in the part of a multiple dwelling used in common by
the residents or in the part under the control of the owner thereof, the
violation shall be deemed to exist in the respective premises of each
resident of the multiple dwelling.
  b. The provisions of subparagraph a shall not apply if (i) the
condition referred to in the department's notice to the owner last
registered with the department did not in fact exist, notwithstanding
the notation thereof in the records of the department; (ii) the
condition which is the subject of the violation has in fact been
corrected, though the note thereof in the department has not been
removed or cancelled; (iii) the violation has been caused by the
resident from whom rent is sought to be collected or by members of his
family or by his guests or by another resident of the multiple dwelling
or the members of the family of such other resident or by his guests, or
(iv) the resident proceeded against for rent has refused entry to the
owner for the purpose of correcting the condition giving rise to the
violation.
  c. To raise a defense under subparagraph a in any action to recover
rent or in any special proceeding for the recovery of possession because
of non-payment of rent, the resident must affirmatively plead and prove
the material facts under subparagraph a, and must also deposit with the
clerk of the court in which the action or proceeding is pending at the
time of filing of the resident's answer the amount of rent sought to be
recovered in the action or upon which the proceeding to recover
possession is based, to be held by the clerk of the court until final
disposition of the action or proceeding at which time the rent deposited
shall be paid to the owner, if the owner prevails, or be returned to the
resident if the resident prevails. Such deposit of rent shall vitiate
any right on the part of the owner to terminate the lease or rental
agreement of the resident because of nonpayment of rent.
  d. If a resident voluntarily pays rent or an installment of rent when
he would be privileged to withhold the same under subparagraph a, he
shall not thereafter have any claim or cause of action to recover back
the rent or installment of rent so paid.  A voluntary payment within the
meaning hereof shall mean payment other than one made pursuant to a
judgment in an action or special proceeding.
  e. If upon the trial of any action to recover rent or any special
proceeding for the recovery of possession because of non-payment of rent
it shall appear that the resident has raised a defense under this
section in bad faith, or has caused the violation or has refused entry
to the owner for the purpose of correcting the condition giving rise to
the violation, the court, in its discretion, may impose upon the
resident the reasonable costs of the owner, including counsel fees, in
maintaining the action or proceeding not to exceed one hundred dollars.