The new year promises a spurt of development in Washington's dingy downtown such as has not been seen since construction of the Federal Triangle was begun half a century ago.

Developers from all parts of the country are talking about several hundred million dollars worth of new hotels, apartments, offices and stores in key spots between the White House and Capitol. The site for one such complex - at No. 1 Thomas Circle - is already being cleared.

The Metro subway functions and may soon be considered old enough to stay up a while after supper. Work on Pennsylvania Avenue will finally get under way. So will the remodeling of the old Post Office and the renaissance of the venerable Willard Hotel. The East Wing of the National Gallery is scheduled to open next summer.

The wide open question is the quality of the Beaux Arts classicism on the Mall, turned out to be petrified bureaucracy. Its Grand Plaza was never anything but a parking lot.Its columns and pediments form a massive wall that keeps 25 million tourists a year confined to the Mall and stops them from spending money downtown.

The architecture that followed the Beaux Arts temples after World War II did nothing to penetrate the wall. The government continued to build monumental monsters but scraped off the col- umns and pediments to make them "Modern." Free enterprise office boxes progressed from desultory Modern to Modernistic kitsch.

West-of-Connecticut K. Street, in partcular, is a parade of jerry-built office boxes in architectural giftwrapping as tasteful as candy-flavored potato chips.

Except for the superb design of the Metro subway stations, architecture, by and large, has let us down these past 50 years. It may be just as well that relatively little was built in the centrol city.

But something else has happened that promises to start turning Washington into a capital city , a cosmopolitan metropolis representing not only the nation's politicians, but also its culture, its esprit.

Washington has become a leading cultural center. This New Year's Eve we can toast not only the growing number our museums and galleries and their staggering attendance. We can take great pride in the creativity and vitality of their displays and special offerings.

Washington is also becoming an important international center. There are more embassies and larger diplomatic staffs and special missions.

Some day, I am sure, our downtown merchants wil discover that Garfinckel's need not be the only downtown store catering to a cosmopolitan clientele. And all those fancy new New York and Dallas emporia out in the sticks will discover that a journey to White Flint is as inconvenient, if not frustrating, for people who work downtown, as a journey downtown is for a suburban homemaker.

In other words, the burgeoning cultural and international life has touched the physical appearance and, to use that vague but cozy word, the "ambience of Washington hardly at all. There are the new shopping centers in the suburbs - paces that serve up the merchandise and decor some New York or Dallas impresario imagines we would like.

There are a still increasing number of sidewalk cafes and a growing list of good restaurants. There is a new touch of elegance on Connecticut Avenue, although the display windows there would hardly win a design award in Peoria.

But downtown, the area planners call "the central business district," has only seen the demise of a few department stores, a giant blight known as the FBI Building and a vast proliferation of porno shops. It has also seen a minor disaster called "Streets for People."

"Streets for People" are two pedestrian mall (one in front of the Martin Luther King Library, the other in front of the National Portrait Gallery) with a third one under construction between 8th and 9th Streets, which makes me wonder if the proposed new development will give us more charm or more K Street . They are not malls so much as granite quarries - arty compositions of waterless fountains, conversationless conversation pits, uncomfortable benches and puddled pavements - all of relentless, hard, hostile granite.

If we get more gimmicky and misanthropic urban design like that, the new downtown development will be of no help at all.

Luckily, however, there are a number of factors that give grounds for optimism:

Foremost among them is a new mood in the land. People have had it with bare and square granite benches and glass boxes. We have re-discovered the past and the charms of Victorian architecture, of variety, whimsy, ornament and cityscape that appeals to the heart. This new mood turned the Pennsylvania Avenue plan around from a pompous abstraction to a simple sprucing up - the renovation of nice old buildings, a few more trees, a pavement that is pleasant to walk on and look at, and a few new buildings that promise more people and more life on the avenue rather than linear mau soleum.

An important part, furthermore, of the downtown development which appears ready to get going in 1978 is the lively new use for cherished old buildings: The Old Post Office will become an bustling tourist cetner as well as an office for the National Endowment for the Arts. The Willard Hotel will be restored as a hotel worthy of its name, I hope.

The Pension Building, surely one of the most dramatic works of Victorian architecture in the city, may be turned into a museum of building and planning - a place to stimulate and inform the new interest in man-made America.

You can't go wrong with old buildings like that.

As to the new ones, the proposed National Press Club-John Portman complex between 13th and 14th Streets and fronting on Pennsylvanian Avenue is undoubtedly the largest. With John Portman as architect, the city should be safe. At his numerous Hyatt-Regency Hotels with their dramatic atrium courts and shopping arcades (the Washington one is a bad copy) Portman has proven that he can give even large buildings a human and endearing quality. He builds stage sets for the human drama.

Except for the proposed Thomas Circle complex there are no architectural designs as yet for the other downtown promises. (The Thomas Circle job looks pleasantly innocuous.) We can only hope that their architects find ways to combine our discovery that "small is beautiful" with the economic fact that "large is inevitable," as Wilfred Owings of Brookings put it.

This hope is reinforced by the fact that Washington's new Municipal Planning Office is caught up in the new mood. It is prepared to prevent more K Street monoliths and drafted new "goals and policies" by which the city will trade zoning favors for set-backs, trees, inner courts, mid-block passages, art embellishments and other elements of architectural design that make a city interesting and livable.

I'll drink an extra glass of champagne for their success tonight.