The old-boys' club of architecture has a new member.

It conferred chic on James Stirling, 55, a Scottish-born architect who lives and works in London. Philip Johnson pronounced him "the Wunderkind of modern architecture."

Stirling stirred things up in Europe ever since 1959, when, with his then-partner James Gowan, he designed a 1920s-ish Russian Constructivist petrochemical plant for the Leicester University engineering building.

Twenty-one years later he was awarded the Gold Medal of the Royal Institute of British Architects. He also got several commissions in Germany.

But that was in Europe.

In America, hardly anyone heard of him. He was talked about only by the kids in architecture school and their more literate teachers, particularly at Yale where he served as a visiting critic.

Now he suddenly receives the 1981 Pritzker Architecture Prize, which comes with a $100,000 tax-free check and a Henry Moore sculpture.

He was also recently commissioned to design the new school of architecture at Rice University at Houston, the chemistry building for Columbia University in New York City and an addition to Harvard's Fogg Art Museum at Cambridge, Mass.

The easiest thing to say about Stirling is that he is a "third-generation modernist," i.e., a stylistic grandchild of Le Corbusier, Vladimir Tatlin, the Constuctivist, and Piet Mondrian, the de Stijl painter. God knows who his stylistic parents are. For a while he seemed remotely related to the comic-strip school of British architecture which called itself "archigram" and produced the Georges Pompidou arts center in Paris.

Stirling, at any rate, was never an architectural pioneer. He practices modern architecture much like a bright heir would conduct an inherited family business.

When he started with the Leiscester plant (which was soon followed by another ruthless steel-and-glass factory-like building for the Cambridge University history faculty building) English critics called him a "new brutalist." That rang true to the British public, but it was meant as a term of endearment.

More recently, Stirling announced that "the shapes of a building should indicate -- perhaps display -- the usage and way of life of its occupants, and it is therefore likely to be rich and varied in appearance, and its expression is unlikely to be simple . . . the total building should be thought of as an assemblage of everyday elements [staircases, windows, corridors, rooms, entrances, etc.] recognizable to a normal man and not only an architect."

His most famous building, perhaps, of his "normal recognition" period, is the Olivetti Training School in Haslemere, England. It is made of rounded, tan-and-yellow-colored, molded plastic panels and looks very plastic indeed.

Most recently, like most architects, Stirling has discovered history and the need to preserve urban continuity. He now aims, he says, to place new buildings in the context of their environment.

For the as-yet-unbuilt Wallgraf-Richartz Museum in Cologne, Germany, for instance, he would create new space by connecting the old Cathedral, the railroad station and the Hohenzollern Brucke. At St. Andrews University in Scotland, he framed an 18-th century house with a bland and curved-glass structure to create more space for an arts center.

The addition to the Fogg Museum at Harvard to judge from first sketches, presents a five-story-high, fairly innocuous brick facade that turns the corner of Quincy and Cambridge streets -- innocuous, that is, if the polychrome brick stripes that are to reflect the polychromed brick of the nearby Memorial Hall don't get too garishly poly.

The square shape and the simple fenestration provide a transition between the present, neo-neo-Georgian Fogg and late-late-Modern Gund Hall. The exterior appears as contextually accommodating as it is skillful.

In the interior, however, hellish post-modernism breaks loose. There is, to be sure, a marvelous staircase. But the sketches, currently on display at the Fogg, also show a not-so-marvelous Egyptian temple entrance, columns that don't support anything, Louis Kahn-Baroque skylights and Venturi-esque (that is to say, needlessly contrived) complexity.

The Fogg addition, which will replace the obsolete Allston Burr lecture hall and a vacant frame house, will house the museum's notable collection of Oriental, ancient and Islamic art as well as space for special exhibitions, classrooms, a library, research and offices. It will be the largest Harvard building open to the public.

Stirling, who is currently also at work on an extension of the Staatsgalerie at Stuttgart, a Berlin Science Center and an addition to the famous Tate Gallery in London, won the Fogg commission in competition with 70 other architects.

The Pritzker Architecture Prize was established in 1979 by the Pritzker family of Chicago, which owns the Hyatt Hotel chain. The idea, according to Jay A. Pritzker, is to make up for Alfred Nobel's omission of architecture (as well as the performing and visual arts, for that matter) when he launched the Nobel Prize.

The Pritzker is "to reward outstanding creative endeavors [by a] living architect or architectural group whose work demonstrates a combination of talent, vision and commitment that has produced a consistent and significant contribution to humanity and the environment."

The 1979 winner was Philip Johnson, who led the way of modern American architecture from his own glass house in New Canaan, Conn., to AT&T's Chippendale-topped skyscraper in New York City.

The 1980 winner was Luis Barragan of Mexico, the exquisitely esoteric designer of exquisitely esoteric private gardens, which the Pritzker jury called "architecture as a sublime act of poetic imagination."

The jury consists of Sir Kenneth Clark, the British art historian; Carter Brown, the director of the National Gallery of Art; Arata Isozaki, a Japanese architect who will build the new Museum of Contemporary Art in Los Angeles; Irwin Miller, the industrialist who turned his home town, Columbus, Ind., into a veritable museum of contemporary architecture; Cesar Pelli, architect and dean of the Yale school or architecture; and Philip Johnson.

Arthur Drexler, director of architecture and design for the New York Museum of Modern Art and the architectural old-boys' club manager, also manages the Pritzker jury's affairs.

By selecting Philip Johnson, this impressive jury honored effervescent excellence. By selecting Baragan, it honored pristine elitism. This year, by selecting James Stirling, it put the spotlight on a brilliant mirror of today's architectural confusion.

The formal presentation to Stirling will be made May 19 at Washington's old Pension Building, which Congress has recently designated as the future site of a National Building Museum.