California Code, Government Code - GOV § 57329

(a) If unincorporated territory was, or is hereafter, annexed to a city, all roads and highways or portions of a road or highway in the territory which had been accepted into the county road system pursuant to Section 941 of the Streets and Highways Code are, or shall become, as the case may be, city streets on the effective date of the annexation.

(b) Subdivision (a) does not apply to a road or highway which had been accepted into the county road system pursuant to Section 941 of the Streets and Highways Code after the date of the first signature on a petition for annexation or incorporation, the adoption of a resolution of application by an affected local agency, or a date mutually agreed upon by the city and the county.

(c) Nothing in subdivision (a) requires a city to improve the affected road or highway to city standards.