This capital's rapidly changing skyline has a new addition that raises the architectural level at least a thousand degrees. The new East Building of the National Gallery of Art, designed by I.M. Pei, is proof that architecture, even monumental architecture, can have a soaring beauty as well as the ingenious use of interior space for a variety of purposes.

It happens that the new building is across Pennsylvania Avenue from that hideosity the J. Edgar Hoover FBI building. You couldn't ask for a more striking contrast between bureaueratic government architecture with all its sterility and architecture privatley commissioned for public use.

The new gallery and art study center cost $94.4 million. The money came entirely from three foundations: th Paul Mellon Foundation, the Ailsa Mellon Bruce Foundation and the Andrew W. Mellon Foundation. The orginal gallery and the nucleus of the collection came from funds provided by Andrew Mellon.

The provision in the tax law allowing deductions for charitable and educational gifts is one of the chief reasons for the richness of American museums that in a relatively short time have come to equal some of Europe's greatest treasure troves. That contrasts with Britain, where repeatedly valuable art works have been sold out of the country and only frantic last-minute fund raising efforts have occasionally saved a particularly cherished work with long associations.

By a curious irony, Paul Mellon helped to save two pictures for Britain threatened with export. They were "The Haymakers" and "The Reapers" by George Stubbs, one of the stars of British 18th-century painting. Mellon gave four paintings from his private collection to be auctioned off, for an estimated $125,000, to keep the Stubbs works.

Mellon has acquired several Stubbses for the Yale Center for British Art, which he endowed. Altogether, the collection is said to be worth close to $100 million. Stubbs was noted above all for his paintings of horses.

The new building is to open on June 1 with a great fanfare and seven separate exhibits. The centerpiece is the Splendor of Dresden, a loan of 700 objects from the fabulous state museums of that East German city. In his youth, J. Carter Brown, director of the National Gallery, studied in Dresden. He is said to have resolved at the time that if he were ever able he would do all he could to bring collection to the United States.

Except for occasional pieces it has never been out of Dresden. The initiative for the loan came from East German Ambassador Rolf Sieber. In a rare example of cooperation with one of the Eastern European communist states.

In 16th century Dresden the Elector of Saxony, August I, became one of the earliest and greatest collectors of precious objects. Paintings, bronze statuettes, carved ivories, shells, precious stones and an unrivaled jewel collecton filled his private Kunstkammer, a room in the electoral palace. One of the most curious objects is a carved black figure emblazoned with cut emeralds and holding a tray of uncut emeralds that came from the New World across the sea.

Fortunately the collection was hidden in salt mines a hundred miles from Dresden and thereby saved from a final Allied bombing that leveled a large part of the city. Insured for $82 million, it will go from Washington to the Metropolitan Museum in New York and then to the Palace of the Legion of Honor in San Francisco.

But the East Building is itself almost as much of a rarity, showing what steel, glass and marble can do to create light and grace. Ingenious triangular skylights illuminate curved stairways from one floor to another. The main hall is a stunning expanse, with contemporary art objects used sparingly to accentuate space and the warmth of the Tennessee marble walls.

Along with that are small galleries that give a sense of quiet and seclusion. They are to hold the six exhibits other than the Splendor of Dresden, which, with its armor and robes and weaponry embossed with gold, takes a larger area.

While the National Gallery collection is almost entirely classic art, the new building will contain contemporary American work. One of the exhibits is American Art at Mid-Century with paintings by DeKooning, Rothko, Pollock and Motherwell. One gallery is devoted to Picasso and Cubism, with a remarkable collection of Picasso's early work.

Pei, who was born in Canton in 1917, has been the architect of many notable buildings both in this country and overseas. They range from the John F. Kennedy Library in Boston to the National Center for Atmosphere Research in Boulder, Col., to the Industrial Credit Bank in Tehran and the Indiana University Art Museum in Bloomington. But the East Building, with its austere masses and soaring planes, breaks new ground.