"Law that authorizes filing a request for leave to amend in California.
Both Code of Civil Procedure sections 473(a) and 576 state in pertinent part that a court may, in the furtherance of justice, allow a party to amend any pleading on any terms as may be proper. In most cases the pleading would be an answer, complaint or cross-complaint.
However the party requesting leave to amend must comply with the provisions of California Rule of Court 3.1324 or risk having their motion denied. Rule 3.1324 states that
“(a) Contents of motion
A motion to amend a pleading before trial must:
(1) Include a copy of the proposed amendment or amended pleading, which must be serially numbered to differentiate it from previous pleadings or amendments;
(2) State what allegations in the previous pleading are proposed to be deleted, if any, and where, by page, paragraph, and line number, the deleted allegations are located; and
(3) State what allegations are proposed to be added to the previous pleading, if any, and where, by page, paragraph, and line number, the additional allegations are located.
(b) Supporting declaration
A separate declaration must accompany the motion and must specify:
(1) The effect of the amendment;
2) Why the amendment is necessary and proper;
(3) When the facts giving rise to the amended allegations were discovered; and
(4) The reasons why the request for amendment was not made earlier.
(c) Form of amendment
The court may deem a motion to file an amendment to a pleading to be a motion to file an amended pleading and require the filing of the entire previous pleading with the approved amendments incorporated into it.
(d) Requirements for amendment to a pleading
An amendment to a pleading must not be made by alterations on the face of a pleading except by permission of the court. All alterations must be initialed by the court or the clerk..." http://www.legaldocspro.net/blog/leave-to-amend-in-california/