The valuation for the purpose of public acquisition of property used and useful, or in the discretion of the city or city and county prospectively useful, and owned by the grantee at the time application is made for a resettlement franchise, permit, or privilege shall be fixed by the Public Utilities Commission. The valuation of such property as fixed by the commission may be set forth in the resettlement franchise, permit, or privilege, in which case a readjustment from time to time of this valuation by the addition of the cost of extensions and betterments and by the deduction of the value of property sold or abandoned, and of the amount of depreciation sustained by the
property used or useful or prospectively useful, shall be made in such manner as may in the resettlement franchise be provided.