Something extraordinary happened in Congress just before it recessed. ''The Federal Triangle Development Act'' passed both houses by unanimous consent. It authorizes a $350 million public building on the vast asphalt steppe that surrounds the District Building. One-third of a billion dollars in a time of anxiety about budgetary deficits -- and not a word of dissent.

The reason is not that the watchdogs were asleep but that the project is good for Washington, economical for the federal government and potentially useful for citizens throughout America.

The building will provide a place for 8,000 to 10,000 federal workers currently scattered in leased office space and for an international cultural and trade center for the commercial offices of foreign governments and American traders.

This latter function is what first attracted the interest of the Federal City Council, whose task force chairman, Don Brown, argued that Washington's unique world role and diverse population required some entity within the District to facilitate international transactions, cultural as well as commercial. It would be a place in which a businessman could get his passport renewed, obtain visas and seek information and appointments abroad, and where visitors could enjoy the art, inventions, show and food of other countries.

Terence Golden, administrator of the General Services Administration, saw the center's potential to respond to several different needs. Golden has a background in development, and he pays a colossal bill to lease 30 million square feet of office space for federal workers in the Washington area. State is in 16 locations around Washington, Treasury in 38, Justice in 26.

Golden and Joe Wright, deputy director of the Office of Management and Budget, thought that reducing the number of these outposts would bring gains in both efficiency and rent costs. And the building into which the federal diaspora would be moved could fill out the last piece of the Federal Triangle. So much the better for downtown Washington and particularly for Pennsylvania Avenue. If a bureaucratic work place could be combined with an international center for the promotion of trade and international understanding, everyone would benefit: the government building would have the color of life, and the center, paired with such an irresistible money saver, would have magnetism.

What was needed was a member of Congress who knew the history of downtown Washington and cared about it, who could see the political value of joining several worthwhile purposes in one project, and who was in a position, owing to seniority, to move it forward. Fortune suddenly smiled. Sen. Daniel P. Moynihan chaired the right committee and, working with House Public Works Committee Chairman Jim Howard, secured bipartisan support for the legislation and saw the legislation through.

The bill requires that the building "shall be designed in harmony with historical and government buildings in the vicinity, shall reflect the symbolic importance and historic character of Pennsylvania Avenue and the Nation's Capital, and shall represent the dignity and stability of the Federal Government." Well and good; one can hope that it will also have flair and grace, and that it will contain, on the public floors, colorful and inviting exhibition areas that millions of tourists will be glad to invade.

Besides its beauty and utility, the building will set a new course for the acquisition of office space for government workers. Since 1960 the proportion of leased to owned space has increased from roughly 5 to 50 percent. The rush to find cheaper space has produced development pressures in the wrong places, made a mess of regional planning, dislocated and inconvenienced the federal work force and set off a senseless competition among the economic development units of the local government.

The bill authorizes the GSA to enter into a 30-year lease with a private development team that will design, construct and finance the center. Once the building is in place, the government's basic rental charge will remain constant throughout the lease term. After 30 years, the government will own the building free and clear -- having saved, during the period, over $270 million in rent. Land costs typically account for 20 to 30 percent of lease charges on downtown properties. In this case the government already owns the land, so lease costs will be low.

From a purely local point of view, the center will offer job opportunities -- perhaps 2,000 to 3,000 of them -- in its exhibition and retail areas. It will thrust a major visitor attraction north from the Mall, toward the revived downtown business area. All good for Washington and every American who enjoys it.

Now for the last, tough part, to be managed by the Pennsylvania Avenue Development Corp.: overseeing the design of the building and the shaping of the cultural and trade center. If the good luck of the past few months can hold for a while, we should all have a winner. The writer is president of the Federal City Counci