FIVE AND ONE-HALF years after opening to a razzle-dazzle of world premieres and glittering first-night audiences, the Kennedy Center expects to begin work this summer on a small theater-library complex that will fill the last of its empty spaces and expand its activities as a performing arts center.

The money ran out before a 500-seat theater in the original plans could be built on the roof-terrace level. A roughed-in theater shell was left along with unfinished plans for experimental drama, chamber music, opera and dance.

Now, with a $3-million Bicentennial gifts form Japan, the Studio Theater, redesigned from a revolving three-quarters circular stage to proscenium, will be finished off. Nearby will be the Performing Arts Library, linked by computer terminal to the rich collections of the Library of Congress.

The Kennedy Center opened on Sept. 8, 1971, but patrons still are getting acquainted with the internal geography of the great marble box on the Potomac that encloses 1,488,590 square feet of floor space.

On curtain-time deadline, there is the question of whether the Opera House box office with the waiting tickets is off the Hall of Nations or the Hall of States. And on which side are the elevators that take you to the roof-terrace restaurants? The accompanying schematic floor plans may be a help to those still disoriented amid the ins and outs of the Center.

So far, during the first five years of its operation, the Center has made use of its vast scale with matching grand style on special occasions. There was the midnight party for hundreds on the river overlook on the opening night of La Scala's historic first visit to America last September.

On Jan. 20, 1973, for a Nixon second inaugural party, more than 10,000 people fitted quite nicely, if closely, into the 630-foot-long Grand Foyer, with half again that number spilling into the Hall of Nations and Hall of States.

THe vastness of the Kennedy Center also can be measured by such statistics as these:

Behind the public facade of three major stage halls lies a labyrinth of more than 400 rooms - offices, dressing rooms, rehearsal halls, paint shops, wardrobe storage storage areas, reception lounges, mechanical rooms, press facilities.

On nights when the three halls are sold out, which is not so rare, 6,100 people come to the Kennedy Center for theator, opera, dance and concerts. Its three restaurants can handle upward to 550 patrons at a time.

In the cavernous backstage areas, entire sets are laid out on air-glide platforms ready for instant scene shifts.

And the Center's three stage doors have been discovered by autograph-seekers and stage-door johnnies. There is one regular, a middle-aged bespectacled man, who stations himself outside one or the other of the stages' doors each opening night, carrying a brief case with books, records and programs to be signed by arriving star personalities.

Another regular is President Carter, who is dropping in so foten that the Kennedy Center no longer is arranging a special greeting party of its White House guest.

Perhaps no one - except Mosby, the gray ghost of a cat - knows his way around the vastness of the Kennedy Center better than Edward Schessler, the building manager, who has lived with plans, construction and operation of the Center for 11 years.

And, Schessler vouchsafes, Mosby is no phantom of the opera.

"He's gray - pearl gray. I've helped feed Mosby for years. It's a high-profeed Mosby for years. It's high protein diet supplied by Mrs. Stevens (wife of Roger Stevens, the Center's director). I remember when Henry Fonda was here a couple of years ago and Mosby wandered on the stage. Fonda patted his head and Mosby meowed like a ham."

Each morning at 5:30 Schessler arrives in darkness at the Kennedy Center to have the building ready for an influx of thousands of tourists and theater-goers.

What they do not see is the warren of back-stage support areas behind the 2,750-seat Concert Hall, the 2,200-seat Opera House and the 1,50-seat Eisenhower Theater. It is a Grand Hotel house of culture with a contained existence of its own.

This is where it is all put together before the actor, dancer, singer or musician steps on the stage. In contrast to the opulence of the foyer and stage halls, there are spartan rehearsal rooms bare except for a piano and few straight chairs, rows of green lockers for musicians and chorus members, paint and scenery shops, stuffed wardrobe rooms, a stagehand lounge with a reminder. "Stagehands Do It on Cue," a canteen where stars and "supers" for crowd scenes snack side by side.

A conductor star or soloist rates a separate dressing room. But members of the corps of ballet or chorus line make up at a long line of lighted mirrors. And everywhere along the corridors there are mirrors for the last-minute check before stepping on the stage. The sound of a tuba may be heard from a dressing room as a musician warms up.

The theater, concert hall and opera house are designed as self-contained units, each with its own house manager, stage-door, dressing rooms and service areas, with a pre-curtain green room for the Eisenhower Threater and Opera House.

For the Eisenhower Theater, there is one large rehearsal hall. The Opera House has seven rehearsal rooms, the largest equipped with wall-to-wall mirrors and bars for dancers to practice. In the Concert Hall, the stage itself is the rehearsal hall for orchestras, although there are small coaching rooms for the conductor to go over the score with a soloist or ensemble.

In addition to its own administrative offices - the original design made no provisions for offices and these have been carved out of corridors and open spaces - the Kennedy Center leases space to the American Film Institute, with its own small movie house, the National Symphony, The Washington Opera and adjunct arts organizations like the Opera Institute.

Other office space is set aside for the National Park Service, which administers to the Kennedy Center as a memorial to the late President John F. Kennedy.

As a gift to the presidential memorial, nations have decorated three of the reception lounges. Twenty-four African nations have contributed $83,000 to furnish the African Room with its handsome carved teak doors and changing exhibits as a showcase of native crafts. There are also the Israel and Chinese Rooms. For the 5 million tourists who come to the presidential memorial and culture center each year, these rooms offer a glimpse at other national cultures and arts.

The Chautauqua Tent, left over from a Bicentennial exhibit, will come down to make way for the new Studio Theater complex at one end of the North Gallery Corridor on the roof-terrace level.

Work is expected to begin about the end of July on the redesigned proscenium theater, with haigh-rake seats and design to handle tricky acoustical problems for a multi-purpose stage for drama, chamber music, opera and dance. It also will be used for new Kennedy Center projects like the Children's Theater and national festivals.

Nearby is the Musical Theater Lab, with its bleacher seats that can be shifted for proscenium of theater-in-the-round performances. Here, before small audiences of 100, composers, choreographers and performers can try out experimental work in musical theater that will serve as a "feeder" to the Studio Theater.