Cal. Sts. & Hy. Code § 10100.1
If the written consent of the owner of the property is first obtained, work may be done on private property to eliminate any disparity in level or size between the improvement and private property, provided that the legislative body determines in the resolution of intention to order the improvement that it is in the public interest and more economical to do such work on private property than to adjust the work on public property to eliminate such disparity. The actual cost of such work may be added to the assessment of the lot on which the work is done. Nothing in this section limits or restricts the authority of the legislative body to make agreements authorized by Section 1263.610 of the Code of Civil Procedure.
Ca. Sts. and High. Code § 10100.1