Cal. Sts. & Hy. Code § 720
If any encroachment exists in, under or over any State highway, the department may require the removal of such encroachment in the manner provided in this article.
Except as otherwise provided in sections 680 and 721, notice shall be given to the owner, occupant or person in possession of the encroachment, or to any other person causing or suffering the encroachment to exist, by serving upon any such person a notice containing a demand for the immediate removal of such encroachment from within such highway. Any such notice shall describe the encroachment complained of with reasonable certainty as to its character and location. In lieu of service upon such person, service of such notice may also be made by registered mail and by posting, for a period of five days, a copy of the notice on the encroachment described in the notice. In the case of an owner, occupant or person in possession, who is not present in the county, the notice may be given to his agent in lieu of service by mailing and posting.
Ca. Sts. and High. Code § 720