Responding to criticism from the International Trade Commission that its headquarters building is dilapidated and should be repaired, General Services Administration officials said yesterday that they plan to relocate the agency a block eastward and sell its 148-year-old headquarters to the Smithsonian Institution.

James Whitlock, GSA's assistant regional commissioner of public buildings, said the Smithsonian has a decade-old interest in renovating the so-called Old Tarriff Commission Building at 701 E St. NW for use as a museum, but ITC officials have refused to agree to a relocation site during negotiations that have stretched over the past two years.

Now, some 78,000 square feet of space in the Bicentennial Building at 600 E St. NW will "probably be agreeable to the ITC," Whitlock says. The ITC has a small block of computer-room space there now. The space is currently occupied by the Department of Housing and Urban Development, which plans to vacate by Sept. 30.

ITC officials were not available for comment late yesterday.

Previously, GSA had offered the ITC space in the Century Building in Germantown, and space in the Presidental Building, adjacent to Prince George's Plaza, in Hyattsville. ITC officials insisted the agency remain in downtown Washington.

GSA officials have drafted a letter to Sen. John C. Danforth (R-Mo.) explaining that, contrary to the views expressed by ITC Chairman Alfred E. Eckes Jr. before the Senate subcommittee on international trade, GSA "does deeply care about the ITC building and keeping it in good repair," Whitlock said.

After taking Danforth on a tour of the building recently, Eckes complained to the Senate subcommittee at a hearing that problems range from falling plaster to leaking roofs and from dead rats bloated by rat poison to inadequate plumbing.

Through a dozen years of discussions between GSA and the Smithsonian, GSA maintained that space in Washington was at a premium. In Oct. 1979, then-GSA administrator Roland G. Freeman III told S. Dillon Ripley, secretary of the Smithsonian Institution, that "due to serious shortage of rental space in the Washington metropolitan area, I do not believe that this transfer will be possible in the near future." GSA officials said space now was avaliable because of the current office glut in the Washington area.