Washington Bancorporation, parent of the National Bank of Washington, yesterday said it plans a $15 million public offering, the holding company's first sale of stock to the public since its incorporation in 1983.

Officials of the Washington bank holding company said proceeds from the sale of approximately 800,000 shares of common stock will be used for general corporate purposes, which may include additions to its subsidiary banks' capital or investments in other financial institutions. Until that decision is made, proceeds from the sale will be placed temporarily in short-term investments or used to reduce short-term debt, the company added.

Although officials declined to be specific about investments that may be made with proceeds from the stock sale, Washington Bancorporation may apply a portion to its planned acquisition of Center Savings and Loan, a state-chartered S&L in Baltimore.

Washington Bancorporation signed an agreement in principle to acquire the S&L, but the transaction is subject to adoption of a definitive agreement and enactment of enabling legislation by the Maryland General Assembly.

Washington Bancorporation announced a special $12 million addition to its allowance for loan losses in June, to reflect loans to Latin American countries. As a result, the company reported a net loss of $9.2 million for the six months ending June 30. There is no indication, however, that the proposed stock sale has any connection with the increase in loan-loss reserves.

In addition to the National Bank of Washington, Washington Bancorporation, which has assets of $1.9 billion, owns The Washington Bank (WBV), a Northern Virginia bank that it acquired in 1986.

Although stock in the holding company is publicly traded, few shares are actually in public hands. Colson Inc., a New York investment group, acquired controlling interest in 1985, then merged with the bank holding company in 1986.