Developers have announced that construction on the World Bank "J" Building, a new, 11-story, 530,000-square-foot office building at the southeast corner of Pennsylvania Avenue and 18th Street NW, should be completed by the fall of 1986.

The "J" Building project includes three underground parking levels and a tunnel under 18th Street that will link the new building to the existing World Bank complex on H street. The George Hyman Co. began the project for the International Bank for Reconstruction and Development in mid-Janurary. Cushman and Wakefield Inc. is the project consultant, and the firm of Hellmuth, Obata and Kassabaum is the architect.

Other recent developments in commercial realty in the Washington area:

* Omni Construction Inc. has begun work on a 10-story office building at 20th and L streets NW., with completion scheduled for next spring. The building will have 137,000 square feet of office space and 16,500 square feet of retail space, including a three-level retail arcade facing 20th Street.

The building, at 2001 L St. NW, is a project of the Markborough-Viking Partnership, a joint venture of Markborough Columbia Inc. and Viking Capital Inc., both of Washington. Skidmore, Owings and Merrill is the architect.

* The American Enterprise Institute for Public Policy Research has signed a 40-year lease for 86,956 square feet at 1275 Pennsylvania Ave. NW., in a transaction handled by Smithy Braedon.