Pub. Util. Code, § 1418

After a hearing, the commission shall make and file its finding fixing, as of the date on which the finding is made and filed, the extent to which the just compensation theretofore fixed should be increased or decreased by reason of the matters alleged in the petition or petitions. If the claim is made that the just compensation theretofore fixed by the commission should be increased by reason of expenditures made by the owner subsequent to the date of the filing of the original petition with the commission for the purpose of preserving or improving the lands, property, and rights the commission may increase the just compensation only to the extent to which the commission determines that such expenditures were beneficial to the lands, property, and rights, and reasonably and prudently made. The finding of the commission fixing the extent to which the just compensation theretofore fixed should be thus increased or decreased shall be final and shall not be subject to modification, alteration, reversal, or review by any court of this State.