Cal. Rev. & Tax. Code § 253

If, because of active military service of the United States in time of war, sickness, or other cause found to be unavoidable in the judgment of the assessor, an applicant for the veterans’ exemption is unable to attend in person before the assessor, and a deputy is not available to go to the place where the applicant is located, then the applicant may make and subscribe the affidavit before any person authorized to administer an oath. If, during time of war, the applicant is in active military service of the United States or of any nation with which the United States is allied, or is outside of the continental limits of the United States, or if the person entitled to the exemption has a mental health disorder or lacks mental capacity, a member of the applicant’s immediate family or the applicant’s guardian or legal representative, having personal knowledge of the facts required to be set forth, may appear before the assessor and may make and subscribe the affidavit on the applicant’s behalf.