Cowlitz County appeals a summary judgment requiring it to refund $84,456.45 in taxes plus interest to Longview Fibre Company. This case presents the following issue: When the taxpayer, who pays its property taxes in semiannual installments, deems its taxes, or any part thereof, excessive, must each installment be paid under protest to preserve the right to a refund of the excessive tax? We conclude that each installment must be paid under protest to preserve the right to a complete refund of an overpayment. Therefore, we reverse the Superior Court.
This case arose from a valuation dispute between Cowlitz County and Longview Fibre over the County Assessor's 1980 appraisal of Longview Fibre's paper mill for property taxes payable in 1981. Longview Fibre appealed the property tax assessment to the Cowlitz County Board of Equalization, which upheld the assessment. It then commenced a refund action in superior court. Longview Fibre repeated this process each year from 1980 to 1983. The parties resolved the dispute in 1985 and agreed that Longview Fibre's property had been overvalued by $17 million during each of the years in question. The overvaluation resulted in overpayment of taxes, which in 1981 amounted to $168,912.90.
Longview Fibre paid its taxes in two installments each year, as permitted by RCW 84.56.020.1 In 1981, Longview *311Fibre's property taxes were $2,544,968.66, of which it paid half on April 30, 1981, and half on October 31, 1981. Pursuant to the protest statute, RCW 84.68.020, Longview Fibre paid the second installment under protest.2
When the parties were finalizing the settlement, the County refused to refund more than half of the 1981 overpayment, because of Longview Fibre's failure to protest the first installment of its 1981 taxes. The amount of the refund for taxes paid in 1981 was the only remaining issue. The parties agreed that there was no question of fact and on cross motions for summary judgment the court entered judgment for Longview Fibre in the amount of $84,456.45, half of the 1981 overpayment, plus interest.
Statutes that are clear and unambiguous are not subject to judicial construction. PUD 1 v. Public Empl. Relations Comm'n, 110 Wn.2d 114, 118, 750 P.2d 1240 (1988) (quoting Roza Irrig. Dist. v. State, 80 Wn.2d 633, 635, 497 P.2d 166 (1972)). We find neither RCW 84.56.020, which permits taxes to be paid in installments, nor RCW 84.68.020, which provides the means to protest excessive tax, to be ambiguous.
Longview Fibre contends that the protest statute explicitly permits a taxpayer to pay "any part" of its taxes, such as one installment, under protest and thereby become entitled to a refund of any overpayment, up to the amount *312protested. We disagree. The plain language of the statute provides that a taxpayer may pay the tax under protest, meaning the entire tax, or any part thereof that it deems unlawful, and then seek to recover the tax so paid under protest.
The approximate 8 percent reduction in valuation resulted in an across-the-board reduction in the 1981 taxes. Viewing the entire 1981 property tax as a unit, as we must, pursuant to Morf v. Johnston, 173 Wash. 215, 216, 22 P.2d 663 (1933), we hold that the reduction applies pro rata to each installment. Having paid only half of its 1981 taxes under written protest, Longview Fibre is entitled to only half of its 1981 overpayment. This result is dictated by both the protest statute and the long-standing rule that void taxes voluntarily paid cannot be recovered. Pacific Fin. Corp. v. Spokane Cy., 170 Wash. 101, 102, 15 P.2d 652 (1932).
We find National Bank of Detroit v. Detroit, 272 Mich. 610, 262 N.W. 422 (1935) to be persuasive authority. There, the bank paid without protest the first half of its taxes, which were based on an incorrect valuation by assessors. The bank paid the second half of the taxes under protest. Although the bank prevailed on the valuation question, the court held that it was entitled to recover only that portion of overpayment paid under protest. The court relied on the general common law rule, identical to that of Washington's, that " [a] voluntary payment of tax, even though it be void, is a bar to a subsequent recovery." National Bank of Detroit, 614-15.
The decision of the Superior Court is reversed.
The former RCW 84.56.020, in effect when the facts of this case arose, provided in part: "That when the total amount of tax on any lot, block or tract of real property payable by one person is ten dollars or more, and if one-half of such tax be paid on or before the said thirtieth day of April, then the time for payment of the remainder thereof shall be extended and said remainder shall be due and *311payable on or before the thirty-first day of October, following after which date such remaining one-half shall become delinquent, and interest at the rate of eight percent per annum shall be charged upon said remainder from the date of delinquency until paid ..."
RCW 84.68.020 provides in part as follows:
"In all cases of the levy of taxes for public revenue which are deemed unlawful or excessive by the person, firm or corporation whose property is taxed . . . such person, firm or corporation may pay such tax or any part thereof deemed unlawful, under written protest setting forth all of the grounds upon which such tax is claimed to be unlawful or excessive; and thereupon the person, firm or corporation so paying, or his or its legal representatives or assigns, may bring an action in the superior court. . . against the state, county or municipality by whose officers the same was collected, to recover such tax, or any portion thereof, so paid under protest..."