Deadline to amend Answer?

You will need to make a motion to amend your answer, unless the opposing party will agree to a stipulated amendment and order. An exception to this rule is if the plaintiff files a demurrer to your answer. If so, you can amend at any time prior to the hearing on the demurrer. A demurrer to an answer must be filed within 10 days of service.

Ab answer can be amended as a matter of right only during the time that a demurrer could have been interposed; in other words, within 10 days after the answer was filed. (See Code of Civil Procedure § 430.40(b).) If the plaintiff is willing to let you amend your answer, you would prepare and file a Stipulation for Leave to Amend Answer. If the plaintiff is not willing to permit you to amend, you would need to file a Notice of Motion and Motion for Leave to File Amended Answer. Courts usually display great liberality in allowing amendments to answers because “a defendant denied leave to amend is permanently deprived of a defense.” (Hulsey v. Koehler (1990) 218 Cal.App.3d 1150, 1159.)

You can try what the other attys suggested. You might want to consult with an atty to see how critical the thing is you want to add.