California has no law directly addressing DPC. We are not aware of any planned legislation, although the state has attempted to pass DPC legislation in the past as described in this California Healthline article by Mari Edlin from September 6, 2012. There are multiple DPC practices operating in the state.
When designing your contract be sure to review the California Insurance Code (some of the more important provisions are included below) and the laws & regulations of the Department of Managed Healthcare (so that you can demonstrate you are not an HMO). The Knox-Keene Act presents a potential hurdle to any prepaid practice models in California. Please make sure you use a California attorney familiar with these issues - here is a quick overview from the California Society for Healthcare Attorneys. The California HealthCare Foundation has compiled a helpful overview of the Act. Here is some of the problematic language:
(f) "Health care service plan" or "specialized health care service plan" means either of the following:
(1) Any person who undertakes to arrange for the provision of health care services to subscribers or enrollees, or to pay for or to reimburse any part of the cost for those services, in return for a prepaid or periodic charge paid by or on behalf of the subscribers or enrollees.
(2) Any person, whether located within or outside of this state, who solicits or contracts with a subscriber or enrollee in this state to pay for or reimburse any part of the cost of, or who undertakes to arrange or arranges for, the provision of health care services that are to be provided wholly or in part in a foreign country in return for a prepaid or periodic charge paid by or on behalf of the subscriber or enrollee.
INSURANCE CODE - DIVISION 1. GENERAL RULES GOVERNING INSURANCE - PART 1. THE CONTRACT
CHAPTER 1. Classes of Insurance 
(a) Disability insurance includes insurance appertaining to injury, disablement or death resulting to the insured from accidents, and appertaining to disablements resulting to the insured from sickness.
(b) In statutes that become effective on or after January 1, 2002, the term “health insurance” for purposes of this code shall mean an individual or group disability insurance policy that provides coverage for hospital, medical, or surgical benefits. The term “health insurance” shall not include any of the following kinds of insurance:
(1) Accidental death and accidental death and dismemberment.
(2) Disability insurance, including hospital indemnity, accident only, and specified disease insurance that pays benefits on a fixed benefit, cash payment only basis.
(3) Credit disability, as defined in subdivision (2) of Section 779.2.
(4) Coverage issued as a supplement to liability insurance.
(5) Disability income, as defined in subdivision (i) of Section 799.01.
(6) Insurance under which benefits are payable with or without regard to fault and that is statutorily required to be contained in any liability insurance policy or equivalent self-insurance.
(7) Insurance arising out of a workers’ compensation or similar law.
(8) Long-term care.
(c) In a statute that becomes effective on or after January 1, 2008, the term “specialized health insurance policy” as used in this code shall mean a policy of health insurance for covered benefits in a single specialized area of health care, including dental-only, vision-only, and behavioral health-only policies.
(d) “Insurance” means primary insurance, excess insurance, reinsurance, surplus lines insurance, and any other arrangement for shifting and distributing risk that is determined to be insurance under the laws of this state.