Washington Revised Code Title 83. Estate Taxation § 83.100.095. Examination by department of returns, other information--Assessment of additional tax, interest

(1) If upon examination of any returns or from other information obtained by the department it appears that a tax or penalty has been paid less than that properly due, the department shall assess against the taxpayer an additional amount found to be due and shall add interest as provided in RCW 83.100.070 on the tax only. The department shall notify the taxpayer by mail of the additional amount, and the additional amount shall become due and shall be paid within thirty days from the date of the notice, or within such further time as the department may provide.

(2) Interest shall be computed from the original due date of the Washington return until the due date of the notice. If payment in full is not made by the due date of the notice, additional interest shall be computed until the date of payment.

(3) No assessment or correction of an assessment for additional taxes, penalties, or interest due may be made by the department more than four years after the close of the calendar year in which a Washington return is due under this chapter, including any extension of time for filing, except upon a showing of fraud or of misrepresentation of a material fact by the taxpayer or as provided under subsection (4) or (5) of this section or as otherwise provided in this chapter.

(4) For persons liable for tax under RCW 83.100.120, the period for assessment or correction of an assessment shall extend an additional three years beyond the period described in subsection (3) of this section.

(5) A taxpayer may extend the periods of limitation under subsection (3) or (4) of this section by executing a written waiver. The execution of the waiver shall also extend the period for making a refund as provided in RCW 83.100.130.