Sometimes, when you follow the rules regarding the expert witness demand and the initial expert witness disclosure and declaration and are now in receipt of opposing party’s expert disclosure, you find that the opposing party plans to call experts at trial in a subject area which you assumed wouldn’t require expert testimony, or that you hadn’t anticipated. If you didn’t disclose experts on the subject, you can serve a Supplemental Expert Disclosure within 20 days of the exchange of expert witness disclosure.

Code of Civil Procedure section 2034.280 titled Submission of supplemental expert witness list; When permitted; Expert witness declaration; Availability of experts for deposition reads as follows: 

(a) Within 20 days after the exchange described in Section 2034.260, any party who engaged in the exchange may submit a supplemental expert witness list containing the name and address of any experts who will express an opinion on a subject to be covered by an expert designated by an adverse party to the exchange, if the party supplementing an expert witness list has not previously retained an expert to testify on that subject.

(b) This supplemental list shall be accompanied by an expert witness declaration under subdivision (c) of Section 2034.260 concerning those additional experts, and by all discoverable reports and writings, if any, made by those additional experts.

(c) The party shall also make those experts available immediately for a deposition under Article 3 (commencing with Section 2034.410), which deposition may be taken even though the time limit for discovery under Chapter 8 (commencing with Section 2024.010) has expired.

Beware, you cannot use the supplemental expert disclosure procedure if you did not participate in the original exchange procedure.  Fairfax v. Lords (2006) 138 CA4th 1019. Courts have also held that an attorney’s decision to ignore statutory requirements does not constitute mistake, inadvertence, surprise, or excusable neglect. (See Bettencourt v. Los Rios Community College Dist. (1986) 42 Cal.3d 270, 276; Garcia v. Hejmadi (1997) 58 Cal.App.4th 674, 682;  Luri v. Greenwald (2003) 107 Cal.App.4th 1119, 1129).

You also may not use the supplemental disclosure procedure set forth in CCP 2034.280 in the following instances:

In each of these cases, the parties should bring a motion under C.C.P. §2034.610 et seq. to augment or amend the expert witness list and declaration.

For a further discussion on improper supplemental disclosures see the blog  When Opposing Counsel Tries to use Improper Supplemental Disclosures to its Benefit, Here’s How to Fight Them.