(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 such additional amount found to be due and shall add thereto interest on the tax only. The department shall notify the taxpayer by mail, or electronically as provided in RCW 82.32.135, 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.

(a) For tax liabilities arising before January 1, 1992, interest shall be computed at the rate of nine percent per annum from the last day of the year in which the deficiency is incurred until the earlier of December 31, 1998, or the date of payment. After December 31, 1998, the rate of interest shall be variable and computed as provided in subsection (2) of this section. The rate so computed shall be adjusted on the first day of January of each year for use in computing interest for that calendar year.

(b) For tax liabilities arising after December 31, 1991, the rate of interest shall be variable and computed as provided in subsection (2) of this section from the last day of the year in which the deficiency is incurred until the date of payment. The rate so computed shall be adjusted on the first day of January of each year for use in computing interest for that calendar year.

(c)(i) Except as otherwise provided in this subsection (1)(c), interest imposed after December 31, 1998, shall be computed from the last day of the month following each calendar year included in a notice, and the last day of the month following the final month included in a notice if not the end of a calendar year, until the due date of the notice.

(ii) For interest associated with annual tax reporting periods having a due date as prescribed in RCW 82.32.045(3), interest must be computed from the last day of April immediately following each such annual reporting period included in the notice, until the due date of the notice.

(iii) For purposes of computing interest under (c)(i) and (ii) of this subsection (1):

(A) The same computation of interest applies regardless of whether the department grants additional time for filing any return under RCW 82.32.080(4)(a)(i).

(B) If the department extends a due date under subsection (3) of this section or RCW 82.32.080(4)(b), and payment is not made in full by the extended due date, interest is computed from the last day of the month in which the extended due date occurs until the date of payment.

(iv) If payment in full is not made by the due date of the notice, additional interest shall be computed under this subsection (1)(c) until the date of payment. The rate of interest shall be variable and computed as provided in subsection (2) of this section. The rate so computed shall be adjusted on the first day of January of each year for use in computing interest for that calendar year.

(2) For the purposes of this section, the rate of interest to be charged to the taxpayer shall be an average of the federal short-term rate as defined in 26 U.S.C. Sec. 1274(d) plus two percentage points. The rate set for each new year shall be computed by taking an arithmetical average to the nearest percentage point of the federal short-term rate, compounded annually. That average shall be calculated using the rates from four months: January, April, and July of the calendar year immediately preceding the new year, and October of the previous preceding year.

(3) During a state of emergency declared under RCW 43.06.010(12), the department, on its own motion or at the request of any taxpayer affected by the emergency, may extend the due date of any assessment or correction of an assessment for additional taxes, penalties, or interest as the department deems proper.

(4) 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 tax year, except (a) against a taxpayer who has not registered as required by this chapter, (b) upon a showing of fraud or of misrepresentation of a material fact by the taxpayer, or (c) where a taxpayer has executed a written waiver of such limitation. The execution of a written waiver shall also extend the period for making a refund or credit as provided in RCW 82.32.060(2).

(5) For the purposes of this section, the following definitions apply:

(a) "Due date of the notice" means the date indicated in the notice by which the amount due in the notice must be paid, or such later date as provided by RCW 1.12.070(3).

(b) "Return" means any document a person is required by the state of Washington to file to satisfy or establish a tax or fee obligation that is administered or collected by the department and that has a statutorily defined due date. "Return" also means an application for refund under RCW 82.08.0206.

Application—Effective date—2022 c 282: See notes following RCW 82.32.057.

Retroactive application—2020 c 139 ss 60 and 61: "Sections 60 and 61 of this act apply both prospectively and retroactively to January 1, 2020." [ 2020 c 139 s 62.]

Effective date—2020 c 139 ss 60-62: "Sections 60 through 62 of this act are necessary for the immediate preservation of the public peace, health, or safety, or support of the state government and its existing public institutions, and take effect immediately [March 25, 2020]." [ 2020 c 139 s 63.]

Part headings not law—2008 c 181: See note following RCW 43.06.220.

Part headings not law—2007 c 111: See note following RCW 82.16.120.

Findings—Intent—1996 c 149: "The legislature finds that a consistent application of interest and penalties is in the best interest of the residents of the state of Washington. The legislature also finds that the goal of the department of revenue's interest and penalty system should be to encourage taxpayers to voluntarily comply with Washington's tax code in a timely manner. The administration of tax programs requires that there be consequences for those taxpayers who do not timely satisfy their reporting and tax obligations, but these consequences should not be so severe as to discourage taxpayers from voluntarily satisfying their tax obligations.

It is the intent of the legislature that, to the extent possible, a single interest and penalty system apply to all tax programs administered by the department of revenue." [ 1996 c 149 s 1.]

Effective date—1996 c 149: "This act shall take effect January 1, 1997." [ 1996 c 149 s 20.]

Effective date—Applicability—1992 c 169: "(1) This act shall take effect July 1, 1992.

(2) This act is effective for all written waivers that remain enforceable as of July 1, 1992." [ 1992 c 169 s 4.]

Effective date—1991 c 142 ss 9-11: "Sections 9 through 11 of this act shall take effect January 1, 1992." [ 1991 c 142 s 13.]

Effective dates—Severability—1971 ex.s. c 299: See notes following RCW 82.04.050.