Washington Homes Inc. of Waldorf, confronted with an $84.6 million buyout offer from a small, private home-building firm in Clinton, announced yesterday that it had hired Thomson McKinnon Securities Inc. of New York as its investment banker and adviser.

William J. Harnett, chairman of Washington Homes, also said he had asked Sonny DeCesaris & Sons Development Group to clarify its $18-a-share offer for all of Washington Homes' 4.7 million shares. DeCesaris offered a combination of cash and securities but did not specify how the $18-a-share package would be arranged.

Geaton A. DeCesaris Jr., who is heading up his family's efforts to acquire Washington Homes, did not return phone calls seeking comment. Harnett also did not respond to requests for comment.

Thomson McKinnon has acted as an underwriter of stocks and bonds for Washington Homes on several previous occasions. A Thomson McKinnon official said his firm could not comment on its role in the Washington Homes discussions.

Washington Homes is a residential builder in the Washington area and owns 89 percent of the Washington Savings Bank in Waldorf, which has $113 million in assets.

The company earned $8.8 million on sales of $89.5 million for the year ending July 31.

The surprise offer by the DeCesaris group two weeks ago came while the two top officials of Washington Homes were selling a large chunk of the company to a British industrial firm, the Scottish Heritable Trust, based in York, England.

The trust recently bought an option to acquire 900,000 shares of Washington Homes stock owned by Harnett and President Lawrence M. Breneman.

The trust now controls 25.6 percent of the company through the shares they hold and options to buy additional stock.

The trust offered $17 a share, or a total of $15.3 million. The option is good for 90 days and expires in November.

The DeCesaris plan was disclosed when the Clinton group told the Securities and Exchange Commission that it had bought 376,700 shares, or 8 per cent of Washington Homes' stock. Investors are required to notify the SEC when they acquire more than 5 percent of a company's stock.

DeCesaris said it invested $4.1 million in the stock, with about half coming from company and personal funds and half bought on margin. An investor who buys on margin puts up 50 percent of the cost of the stock in cash and borrows the rest.

Sonny DeCesaris & Sons was formed in 1972 by Sonny DeCesaris, now 56, and his four sons, according to a family member.

The firm began by building custom homes and later acquired tracts of land for larger groups of homes, including town-house developments, according to those familiar with suburban Maryland homebuilding.