Civ. Code, § 4751

(a) Any covenant, restriction, or condition contained in any deed, contract, security instrument, or other instrument affecting the transfer or sale of any interest in a planned development, and any provision of a governing document, that either effectively prohibits or unreasonably restricts the construction or use of an accessory dwelling unit or junior accessory dwelling unit on a lot zoned for single-family residential use that meets the requirements of Article 2 (commencing with Section 66314) or Article 3 (commencing with Section 66333) of Chapter 13 of Division 1 of Title 7 of the Government Code, is void and unenforceable.

(b) This section does not apply to provisions that impose reasonable restrictions on accessory dwelling units or junior accessory dwelling units. For purposes of this subdivision, “reasonable restrictions” means restrictions that do not unreasonably increase the cost to construct, effectively prohibit the construction of, or extinguish the ability to otherwise construct, an accessory dwelling unit or junior accessory dwelling unit consistent with the provisions of Article 2 (commencing with Section 66314) or Article 3 (commencing with Section 66333) of Chapter 13 of Division 1 of Title 7 of the Government Code.