Wash. Admin. Code § 458-20-203
Each separately organized corporation is a "person" within the meaning of the law, notwithstanding its affiliation with or relation to any other corporation through stock ownership by a parent corporation by the same group of individuals.
Each corporation shall file a separate return and include therein the tax liability accruing to such corporation. This applies to each corporation in an affiliated group, as the law makes no provision for filing of consolidated returns by affiliated corporations or for the elimination of intercompany transactions from the measure of tax.
Each unincorporated association organized under the Massachusetts Trust Act of 1959 (chapter 23.90 RCW) is likewise taxable in the same way as are separate corporations.
Revised June 20, 1959.
Wash. Admin. Code § 458-20-203
Order ET 70-3, § 458-20-203 (Rule 203), filed 5/29/70, effective 7/1/70.