Plans for an additional 3,500 seats at RFK Stadium, including 1,300 in private $20,000-a-year luxury boxes above the mezzanine, were announced yesterday by RFK Stadium management.

The expansion and related improvements, which would cost about $4 million, only will be done with private capital, Armory Board General Manager Robert H. Sigholtz told a news conference. He said he hoped to have plans ready by Dec. 1 for potential investors, who should be able to recover their investments plus interest from the fees and income generated by the new facilities and seats.

"The exercise is to modernize our facility to provide these services without any cost to the District of Columbia Armory Board," Sigholtz said.

As envisioned by stadium management, with the approval of the Armory Board, the improvement and expansion plan includes:

Expansion of the bleacher section at the east end zone of the stadium to include an additional 1,008 seats. Access cuts would be made to connect the bleachers with the second level of the stadium, permitting holders of bleacher tickets to get to their seats without going on the field or entering through the truck entrance.

Construction of a cantilevered deck for 1,230 seats projecting from the third level over the bleachers.

Establishment of a private, members-only club with two luxury dining rooms each capable of seating 250 in the area now occupied by the Starplex Club above the west end zone of the stadium. This would also include a 500-seat, theatre type viewing area from which to watch the games at the mezzanine level. tFor Redskin contests, it would displace about 500 ticket holders in mezzanine sections through 28. Sigholtz said these ticket holders would be offered club memberships, at an approximately $500 a year or, if they do not want that option, seats in the cantilevered deck at the other end of the field.

"They are comparable seats and even closer to the field," Sigholtz said.

Construction of the 52 private boxes, each 15 feet wide by 30 feet long, along the stadium's third level in areas currently occupied by concession and restroom facilities. Those facilities would be relocated to abut the exterior of the stadium walls.

Sale of the tickets for 1,700 of the new seats would be left to the Washington Redskins and the Diplomats. The Redskins currently have a waiting list of 10,000 for season tickets.

However, the 1,800 tickets that would go with the private boxes and club membership would involve negotiations that have not been completed yet.