This spring, at long last, the much talked-about "revitalization" of Pennsylvania Avenue will get under way with the ground-breaking for a privately financed office building next to the National Theater. The National League of Cities will be the major tenant.

Before the year is out, we should also know who, of several interested developers, is to restore the Willard Hotel to its former glory. The empty hotel has just been bought by the Pennsylvania Avenue Development Corporation.

The Corporation has furthermore promised to start construction, work on a better traffic pattern and two new plazas along the Avenue between 13th and 15th Streets. General Persuing will get his monument at last and a sculpture by avantgarde artist. Richard Serra has been commissioned to give tourists a suitable foreground for their snapshots of the "grand avenue" vista and far distant Capitol dome.

Meanwhile the Municipal Planning Office, together with the Corporation, is seeking zoning revisions to encourage residential development and better architecture to liven things up along the Avenue.

It has taken 16 years and a complete reversal in architectural aspirations to get this far. Planning to turn the Avenue into a "grand axis" between the Capitol and White House began on June 1, 1962, when President Kennedy appointed a "President's Council on Pennsylvania Avenue," headed by Nathaniel A. Owings, founding member of the large architectural firm Skidmore, Owings and Merrill.

Daniel Patrick Moynihan, a member of the original council and now senator from New York, warned against "a solid phalanx of office buildings" and excessive monumentality. But that, plus the purely symbolic vacuity of a "National Square" is precisely what Owings offered.

The first version of that square was to be bounded by a White House Gate and the Treasury Building on 15th Street and extend as far as F Street to the north and 13 1/2 Street to the east, saving the National Press Club, the National Theater, the Washington and Williard Hotels and many other useful buildings.

Officials were stunned and critics criticized, so Owings retreated to 14th Street. But there, on the ruins of the cherished, historic Willard, he made his stand, and that is where I suspect that, deep in his heart, Owings still stands.

Successive Presidents and their officials supported the grand plan, not because they necessarily shared Owings' dreams of architectural glory, but because they rightly felt that opment. It had Owings on it. And its this capital disgrace.

But the people of this city and the people of this country, as representer on Capitol Hill, balked.

In the end, the grand plan yielded nothing but the demise of the Willard, the FBI Building and other grief.

In 1974, the Pennsylvania Avenue Development Corporation was established to come to the rescue and the whole concept was turned around from cataclysmic urban renewal to careful urban conservation. Renewal, here as elsewhere, meant totally rebuilding large parts of the city - a totalitarian approach. Conservation, here as elsewhere, means preserving and remodeling old buildings, improving and enhancing what we have in close concert with the residents of the city - a democratic approach.

The turn-around was hardly the achievement of the new Corporation, however. It is headed by Gen. E. R. Quesada, who was in charge of the L'Enfant Plaza urban renewal develplanning was directed by John Woodbridge, an amiable San Francisco architect, who directed all previous planning for the Avenue.

No. What turned the Owings plan around were the people of this city: Downtown merchants who did not want their businesses bulldozed; "Don't Tear It Down," a group of mostly young people whose concern for the environment includes the urban environment; the Washington Housing and Planning Association, long a guardian of livability for all; citizen associations; architecture and planning students, who made a film to show that Pennsylvania Avenue should be lively like the Champs Elysees, not sterile like Albert Speer's Siegesallee. And public-spirited architects, such as John Wiebenson and Arthur Cotton Moore, who offered positive suggestions.

Just then, the city's Municipal Planning Office, a result of home rule, came into its own. In this instance, it became the spokesman of citizens, their protests and their wishes. The Municipal Planning Office's "counterplan," written by planner John Fondersmith, largely became the Corporation's plan.

The turning point came about two years ago. Yielding to public clamor, the General Services Administration stopped talking about rotting pilings and started to figure out how to save the marvelous Old Postoffice. The Pennsylvania Avenue Corporation stopped paying lip service to the resurrection of the Willard Hotel and started in earnest to make the resurrection part of its plan.

With the new direction, the Corporation eventually got a new director, William A. Barnes, who had helped developer James Rouse build Columbia, the new town in Maryland.

The new spirit is evident in architect Frank Schlesinger's design of the new office building at the corner of 13th Street. The site is now occupied by a parking lot and a movie house.

To judge from his drawings, Schlesinger and his client, Quadrangle developers, are giving us not just another humdrum speculative office building, but a special place, a place designed to help make Pennsylvania Avenue what it should be.

There is a generous arcade on the Avenue side as well as a sizable restaurant (a return of the old Occidental?). Inside there is a spacious concourse lined with two stories of shops and leading to the 13th Street entrance. It should make a pleasant shortcut for visitors and shoppers.

The Avenue facade has an open loggia on each floor from which to enjoy the view and watch the inaugural parades. On the 13th Street side a terrace emphasizes "carved out" recesses in the building that add daylight to the interior and interest to the exterior.

The office floors have two-story high elevator lobbies to provide a breather, so to speak.

All of this promises to add up to an inviting, dignified, attractive and self-assured building although much will depend on how well this competent design is built and how it will fit in with the $100-million office, hotel and shopping complex the flamboyant architect John Portman will design and build on the same block.

The good news about the two proposed plazas between 13th and 15th Street is the people chosen to design them: One was given to Robert Venturi, the other to Paul Friedberg.

Venturi's architectural credo is just about the exact opposite of what Owings stands for. Venturi started the rebellion against modern formalism and monumentality. He expounds the esthetic validity of neon-lit strip developments. He has, to my knowledge, never designed a plaza. It will be interesting to see what he comes up with. It will hardly be conventional.

Friedberg, a landscape architect, has designed some of the most imaginative playgrounds in America. He has designed small urban parks and intimate places for people to sit and relax, places that are at once innovative and familiar. Again, it is hard to imagine him designing a formal setting in Washington's classic tradition.

The zoning revisions the planners are seeking would permit developers along Pennsylvania Avenue to build up to the full 160-foot height limitation imposed by Congress in the Height of Buildings Act of 1910. Present zoning allows only 130 feet. The planners also want to remove certain zoning obstacles to apartment-house development along the Avenue.

In addition to the 30-feet bonus, developers are to be encouraged to introduce innovative amenities in their buildings, to use rooftops for tennis courts and swimming pools, cafes and restaurants, and to build underground concourses to Metro stations.

In short, Pennsylvania Avenue is finally beginning to advance from abstract architectural concepts to the practical things that attract builders and their customers.