California Code, Government Code - GOV § 65853

A zoning ordinance or an amendment to a zoning ordinance, which amendment changes any property from one zone to another or imposes any regulation listed in Section 65850 not theretofore imposed or removes or modifies any such regulation theretofore imposed shall be adopted in the manner set forth in Sections 65854 to 65857, inclusive. Any other amendment to a zoning ordinance may be adopted as other ordinances are adopted.

When the legislative body has requested the planning commission to study and report upon a zoning ordinance or amendment which is within the scope of this section and the planning commission fails to act upon such request within a reasonable time, the legislative body may, by written notice, require the planning commission to render its report within 40 days. Upon receipt of the written notice the planning commission, if it has not done so, shall conduct the public hearing as required by Section 65854. Failure to so report to the legislative body within the above time period shall be deemed to be approval of the proposed zoning ordinance or amendment to a zoning ordinance.