RCW 82A.04.530: Method of accounting.

(1) A taxpayer's method of accounting for purposes of the tax imposed under this chapter is the same as the taxpayer's method of accounting for federal income tax purposes. If no method of accounting has been regularly used by a taxpayer for federal income tax purposes or if the method used does not clearly reflect income, tax due under this chapter is computed by the cash method of accounting.

(2) If a person's method of accounting is changed for federal income tax purposes, it must be similarly changed for purposes of this chapter.