California Code, Revenue and Taxation Code - RTC § 10753.6

(b)(1) The department, pursuant to the request of a qualified disabled owner of a vehicle which was registered prior to the effective date of this section, shall exclude from the market value of such vehicle the cost of any modification or alteration required to adapt such vehicle to such disabled person's needs as either a driver or passenger, if such cost was previously included in the determination of the market value of such vehicle.

(2) There shall be no reduction in the amount of vehicle license fees or market value determination pursuant to this section with regard to fees imposed prior to the effective date of this section.

(3) For purposes of paragraph (1), a “qualified disabled owner” means a disabled person who qualifies for a distinguishing license plate or placard under Section 22511.5 of the Vehicle Code whose vehicle was registered prior to the effective date of this section.