A large new office building will be started this fall at 1717 De Sales St. NW to house the Washington news operations of the American Broadcasting Co.

The building being planned by builder-developer Melvin Lenkin for the site owned by the Taishoff (Broadcasting magazine) family will be tailored to meet the specifications of what ABC wants for its expanding news operations here.

A long-term lease has been negotiated for approximately 140,000 square feet of space in the new structure, in contrast to the 40,000 now used by ABC on Connecticut Avenue NW., opposite the Mayflower. Kohn, Pederson & Fox of New York City is working on the building design for ABC, a spokesman said here.

Reports are circulating that Columbia Broadcasting System is negotiating for new quarters downtown.

Another major lease agreement has been made recently by the American Bankers Association, which plans to occupy space to be built by 1980 in the vicinity of 18th and L. streets NW. ABA, which now has a staff of 350 persons in the Bender building, 1120 Connecticut Ave. NW, plans to occupy five floors in the new structure on a long-term lease, according to Willis Alexander, executive vice president ABA moved its headquarters here from New York in 1971.

Another new office building is planned for the northwest corner of Connecticut Avenue and Hillyer Place NW. The American Psychiatric Association plans a spring start on a six-story structure being designed by Hartman & Cox.

A spokesman for APA confirmed that the professional association has purchased the 9,600-square-foot site that now has two small, older buildings and a parking lot. APA bought the site for $870,000 from Martha Dezendorf.

APA, which has a staff of 130 in two brick buildings at 1700 18th St. NW and at two other leased locations, is a 24,000-member organization that has been based here since 1954.

Another downtown move, scheduled Saturday, will put the small Mathematical Association of America into 75-year-old townhouse-offices at 1527-29 18th St. NW. The MAA bought the properties from the Association of the United States Army for $735,000. Barnes, Morris & Pardoe represented the seller and Boss & Phelphs, the purchasers.

The army association has relocated to a building in Arlington.