The Kennedy Center, unquestionably the biggest marble box in town, will get even bigger if architect George Hartman's "preconceptual presentation" at yesterday's meeting of the Commission of Fine Arts becomes a reality.

Hartman, partner in the Hartman/Cox firm here, stressed that his "feasibility study" was not a final design. But he argued that the best way to meet the Kennedy Center's needs for additional parking and its desire to create a national conservatory for music and the theater arts would be to add onto Edward Durell Stone's huge structure -- 60 feet at either end, so that the 630-foot-long building would run 750 feet from north to south.

The study was made at the behest of Kennedy Center chairman Roger Stevens, who explained that the shortage of parking spaces in the underground garages has been especially onerous to patrons of the National Symphony Orchestra, whose performances customarily begin later than those at the Eisenhower Theater or the Opera House.

"At the same time," Stevens said, "the maestro NSO music director Mstislav Rostropovich has been after me to do something about the conservatory." The conservatory was originally proposed as part of a cultural center package by a presidential advisory commission that Stevens also headed nearly two decades ago. It was eliminated because of costs when the cultural center became the John F. Kennedy Center for the Performing Arts.

"A lot of us in Washington feel that the National Symphony should be the greatest in the country," Stevens told the commission. Despite improvements in the symphony, he said, "it needs the infusion of new talent" that a conservatory would bring. As currently conceived, the conservatory would function as a study center for advanced students in the creative aspects of music and theater -- "a bridge between the academic and the professional worlds," in the description of Thomas Kendrick, director of operations for the Kennedy Center.

Hartman's proposal envisions three separate building projects: the extension of the Kennedy Center at each end to accommodate the conservatory and additional underground parking; a new parking garage buried beneath a triangular plot of land in front of the east facade of the building; and a relatively small structure for student housing at the western corner of this plot.

Functionally, the conservatory would be split into two parts -- music at the southern end, adjacent to the Concert Hall, and theater at the northern end, adjacent to the Eisenhower Theater. These ends would be linked by a string of classrooms, practice rooms and service spaces stretched across the building's western facade (facing the Potomac River) in an area currently occupied by the uppermost of the Kennedy Center's three parking garages (level A).

In terms of parking, Hartman said the new scheme would provide a "cleaned-up operation of three beautifully accessible parking garages" -- levels C and B under the existing structure and the new garage in the front providing a net gain of about 500 spaces.

In terms of design, the chief problem will be how to expand the Kennecy Center without doing damage to Stone's scheme. Hartman's drawing suggest a simple extension using the same marble facing and adding two columns to each end. Yet he has also considered other materials -- opaque glass panels, for instance -- as a way of framing the existing structure with the new additions.

Whether it gets built or not depends in large measure on Stevens' skill in raising money from the private sector. Stevens seemed mildly optimistic yesterday. "We've made a fairly good start," he said, "certainly much better than the last time I appeared before this commission, about 18 years ago. Then, I had nothing."