The State Department's great barn of a ballroom is to be remodeled at an estimated cost of $1 million, Clement E. Conger, curator of State's Diplomatic Reception Rooms and of the White House, announced last night.

Conger said the Benjamin Franklin State Dining Room would be redesigned "as a wonderful Palladian room paralleling the Thomas Jefferson State Reception Room. For this we will have to raise at least $1 million." He spoke, appropriately, at a party for donors of $500 or more to the State Department Fine Arts Committee Americana Fund.

Conger said private donors would raise the money to transform the 98-by-45-foot room, now walled with St. Genevieve gold vein marble, separated at intervals by green marble pilasters. Since the 1961 opening of the diplomatic reception rooms on the eighth floor of the State Department building, Conger has raised $1.9 million in private donations for architectural embellishments to 11 smaller rooms. The entertaining floor originally was in a late '50s institutional style, as is the State Department building in Foggy Bottom.

The American antique furnishings and accessories collected for the reception rooms are now valued at $26 million.

"At this point," Conger said, "we are apparently more popular than I thought for a recession year . . . 1982 was our second most successful year in the history of our fund-raising," with $1,108,943 donated and another $1,181,795 in objects given or lent. However, Conger admitted "the bad news is we spent $1,237,367.20, of which $639,895 was for acquisitions and $384,032.62 was for architectural improvements--$33,424.06 more than we took in."

Secretary of State George Shultz, speaking to the group, pointed through the windows. "Beyond the monuments and across the Potomac River," he said solemnly, "we can see the Pentagon, which, together with this Department of State, symbolizes our national strength and the resolve of this administration to keep the peace.

"Some people will wonder whether military power and diplomatic practice must not clash. Well, Cap Weinberger has the power and I still need a lot of practice. But our guests tonight, as connoisseurs of fine arts, know that integration and balance of ostensibly divergent elements often are the essential ingredients of beauty. Balance also is pretty important in politics."

Kay Folger was the year's biggest contributor with $135,000 for the renovation of the James Monroe Reception Room. Former ambassador to Great Britain Walter H. Annenberg and his wife, former chief of protocol Leonore Annenberg, gave $50,000 for the Thomas Jefferson Reception Room improvements. Other major donors were: Lois and Henry McNeil of Philadelphia, $47,000 (unspecified); Joseph M. and Kathleen Bryan of Greensboro, N.C., $38,000 for a chandelier in the Henry Clay room; and Janet Annenberg Hooker, Walter Annenberg's sister, $30,000.

Almost 500 guests dined on a feast of American and international food--blintzes, crab, oysters, crepes--on tables dispersed around the ballroom. The estimated $28,000 cost of the party came out of donations to the project. Last year, after some criticism of the use of State Department funds for the annual party, the cost of the party was paid out of the Americana fund of the Fine Arts Committee, the umbrella organization that accepts donations. A Conger aide this year said, "The fund will pay for the party and then the receipt will be submitted to the secretary. We don't know if he'll pay any of it."