Cal. Welf. & Inst. Code § 355
(b) A social study prepared by the petitioning agency, and hearsay evidence contained in it, is admissible and constitutes competent evidence upon which a finding of jurisdiction pursuant to Section 300 may be based, to the extent allowed by subdivisions (c) and (d).
(c)
(1) If a party to the jurisdictional hearing raises a timely objection to the admission of specific hearsay evidence contained in a social study, the specific hearsay evidence shall not be sufficient by itself to support a jurisdictional finding or any ultimate fact upon which a jurisdictional finding is based, unless the petitioner establishes one or more of the following exceptions: