California Code, Code of Civil Procedure - CCP § 1250.340

(a) Subject to subdivisions (b) and (c), the court may allow upon such terms and conditions as may be just an amendment or supplement to any pleading. In the case of an amendment or supplement to the complaint, such terms and conditions may include a change in the applicable date of valuation for the proceeding and an award of costs and litigation expenses which would not have been incurred had the proceeding as originally commenced been the same as the proceeding following such amendment or supplement.

(b) A public entity may add to the property sought to be taken only if it has adopted a resolution of necessity that satisfies the requirements of Article 2 (commencing with Section 1245.210) of Chapter 4 for the property to be added.

(c) Property previously sought to be taken may be deleted from the complaint only if the plaintiff has followed the procedure for partial abandonment of the proceeding as to that property.