California Code, Code of Civil Procedure - CCP § 2033.260

(a) The party requesting admissions and the responding party may agree to extend the time for service of a response to a set of admission requests, or to particular requests in a set, to a date beyond that provided in Section 2033.250.

(b) This agreement may be informal, but it shall be confirmed in a writing that specifies the extended date for service of a response.

(c) Unless this agreement expressly states otherwise, it is effective to preserve to the responding party the right to respond to any request for admission to which the agreement applies in any manner specified in Sections 2033.210, 2033.220, and 2033.230.

(d) Notice of this agreement shall be given by the responding party to all other parties who were served with a copy of the request.