(a)
Except as provided in subdivisions (b) and (c), if the lessor brings an unlawful detainer proceeding and possession of the property is no longer in issue because possession of the property has been delivered to the lessor before trial or, if there is no trial, before judgment is entered, the case becomes an ordinary civil action in which:(1)
The lessor may obtain any relief to which he is entitled, including, where applicable, relief authorized by Section 1951.2; but, if the lessor seeks to recover damages described in paragraph (3) of subdivision (a) of Section 1951.2 or any other damages not recoverable in the unlawful detainer proceeding, the lessor shall first amend the complaint pursuant to Section 472 or 473 of the Code of Civil Procedure so that possession of the property is no longer in issue and to state a claim for such damages and shall serve a copy of the amended complaint on the defendant in the same manner as a copy of a summons and original complaint is served.(2)
The defendant may, by appropriate pleadings or amendments to pleadings, seek any affirmative relief, and assert all defenses, to which he is entitled, whether or not the lessor has amended the complaint; but subdivision (a) of Section 426.30 of the Code of Civil Procedure does not apply unless, after delivering possession of the property to the lessor, the defendant (i) files a cross-complaint or (ii) files an answer or an amended answer in response to an amended complaint filed pursuant to paragraph (1).(b)
The defendant’s time to respond to a complaint for unlawful detainer is not affected by the delivery of possession of the property to the lessor; but, if the complaint is amended as provided in paragraph (1) of subdivision (a), the defendant has the same time to respond to the amended complaint as in an ordinary civil action.(c)
The case shall proceed as an unlawful detainer proceeding if the defendant’s default (1) has been entered on the unlawful detainer complaint and (2) has not been opened by an amendment of the complaint or otherwise set aside.(d)
Nothing in this section affects the pleadings that may be filed, relief that may be sought, or defenses that may be asserted in an unlawful detainer proceeding that has not become an ordinary civil action as provided in subdivision (a).
Source:
Section 1952.3, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CIV§ionNum=1952.3..