Washington's historic preservation agent yesterday cleared a major hurdle in the path of the planned downtown convention center by approving the demolition of the 73-year-old Elks Lodge building at 9th and H Streets NW.

Carol B. Thompson of the D.C. Department of Housing and Community Development, ruled that the controversial $99 million convention center project is one of "special merit" that warrants the destruction of the lodge, an officially designated historic landmark located on the edge of the convention center site.

The city's preservation law is designed to protect buildings such as the Elks Lodge, which was built in the electric Beaux-Arts style which dominated Washington at the turn of thecentury. But the law also provides for landmarks to be altered or demolished if the mayor's preservation agent decides the change is necessary to make way for a project of special merit, or if special circumstances exist.

Thompson said in a statement that thenew convention center "will be the single most important development project in the history of Washington, which will have a profound impact on the entire economy of the city."

The center will provide social and "other high priority benefits," she said. "This is the first time the District has ever built a convention center, and it is the only such facilityit will build," Thompson predicted.

Thompson's ruling came less than two weeks before scheduled groundbreaking ceremonies for the convention center, which are to be held April 23, according to a city planner.

But representatives of citizens' and preservation groups that have opposed construction of the center yesterday said they plan to appeal Thompson's decision and continue their fight to preserve old structures in the courts, in Congress and before the D.C. City Council

Anne Sellin, a spokesman for the Citizen's Planning Coalition, called the ruling "really appalling."

By sitting n judgement of a case essentially involving its own proposal for a civic center, the city engaged in a conflict of interest that "not only makes a mockery of fairness, but destroys the integrity of the historic preservation act," Sellin said.

Donald Creamer, secretary of the Washington Lodge No. 15 Benevolent Protective Order of Elks in Washington, said his organization has been renting temporary office space downtown while searching for a new permanent home in upper Northwest Washington since the end of last year.