Cal. Rev. & Tax. Code § 74.3

(a) For purposes of subdivision (a) of Section 2 of Article XIII A of the California Constitution, “newly constructed” does not include the construction, installation, or modification of any portion or structural component of an existing single- or multiple-family dwelling that is eligible for the homeowner’s exemption as described in Section 218, if the construction, installation, or modification is for the purpose of making the dwelling more accessible to a severely and permanently disabled person who is a permanent resident of the dwelling.
(b) For purposes of this section, “a severely and permanently disabled person” is any person who has a physical disability or impairment, whether from birth or by reason of accident or disease, that results in a functional limitation as to employment or substantially limits one or more major life activities of that person, and that has been diagnosed as permanently affecting the person’s ability to function, including, but not limited to, any disability or impairment that affects sight, speech, hearing, or the use of any limbs.
(c) For purposes of this section, “accessible” means that combination of elements with regard to any dwelling that provides for access to, circulation throughout, and the full use of, the dwelling and any fixture, facility, or item therein. The construction of an entirely new addition, such as a bedroom or bath, that duplicates existing facilities in the dwelling that are not otherwise available to the disabled resident solely because of his or her disability, shall be deemed to make the dwelling more accessible within the meaning and for the purposes of this section.
(d) The exclusion provided by this section shall apply only to those improvements or features that specially adapt a dwelling accessibility by a severely and permanently disabled person. The value of any improvement, addition, or modification excluded pursuant to this section shall not include any other functional improvement, addition, or modification to the property unless it is merely incidental to the qualified improvements or features.
(e) The exclusion provided by this section shall not apply to the construction of an entirely new dwelling.
(f) The construction, installation, or modification, with regard to an existing building, for purposes of making the structure more accessible to a disabled person, shall be eligible for exclusion pursuant to this section only if the disabled person, or his or her spouse or legal guardian, submits to the assessor both of the following:
1. (1) A statement signed by a licensed physician or surgeon, of appropriate specialty which certifies that the person is severely and permanently disabled as defined in subdivision (b), and identifies specific disability-related requirements necessitating accessibility improvements or features.
2. (2) A statement that identifies the construction, installation, or modification that was in fact necessary to make the structure more accessible to the disabled person.
(g) The assessor may charge a fee to the disabled person or his or her spouse or legal guardian sufficient to reimburse the assessor for the costs of processing and administering the statement required by subdivision (f).
(h) This section shall apply to construction, installations, or modifications completed on or after June 6, 1990.