California Code, Revenue and Taxation Code - RTC § 24651

(a) Income shall be computed under the method of accounting on the basis of which the taxpayer regularly computes its income in keeping its books.

(b) If no method of accounting has been regularly used by the taxpayer, or if the method used does not clearly reflect income, the computation of income shall be made under such method as, in the opinion of the Franchise Tax Board, does clearly reflect income.

(c) Subject to subdivisions (a) and (b) and Section 24654, a taxpayer may compute income under any of the following methods of accounting--

(d) A taxpayer engaged in more than one trade or business may in computing income, use a different method of accounting for each trade or business.

(e) Except as otherwise expressly provided in this part, a taxpayer who changes the method of accounting on the basis of which it regularly computes its income in keeping its books shall, before computing its income under the new method, secure the consent of the Franchise Tax Board.

(f) If the taxpayer does not file with the Franchise Tax Board a request to change the method of accounting, the absence of the consent of the Franchise Tax Board to a change in the method of accounting shall not be taken into account for either of the following: