Tenants will begin moving this month into 1850 K St. NW. the first phase of the largest private building complex here since L'Enfant Plaza opened in 1968.

Called International Square and projected to cost some $50 million, the office and retail center will cover virtually all of a city block when the planned second and third office towers are opened in 1979 and the early 1980s.

An underground shopping mall will have a direct connection to the subway system's Farragut Square station, and the center's developer, Oliver T. Carr Co. of Washington is planning a 10,000-square-foot, covered atrium in the center of the buildings.

Oliver T. Carr Jr., president of the Carr company, said he believes the "scale" of the International Square project is an important statement about the future of the District, whose governmental budget has been strapped for money because of a generally weak private tax base.

The decision to build International Square was made late in 1974, during what Carr said in an interview was "a low point in the D.C. outlook." Equitable Life Assurance Society of the U.S. agreed with Carr that a huge new office complex would be successful and became a partner in the development at a time when recession was causing most investors to shy away from any real estate projects.

International Square will occupy the lock bounded by 18th, 19th, K and I streets NW; the only existing building that will remain in the International Club. By almost any measure, the new complex represents a major new element in the District economy:

The three buildings will include 1.2 million square feet of office and retail space. On two ground levels, a unique downtown shopping arcade (for Washington) will include 50 retail stores, restaurants, banks and other services; exterior glass elevators eventually will run from the top of the interior atrium to the two-level arcade.

The first building, on which interior work is being completed, contains about 350,000 square feet of office space and 43,000 square feet of retail and commercial space. More than 65 per cent of the office space, 85 per cent of the ground-floor commercial space and 30 per cent of the mall space have been leased.

Between 6,000 and 7,000 persons are expected to work in the offices or stores at the International Square when all three buildings are completed.

Tenants who will occupy the first building starting in the next few weeks include the World Bank (two and one-half floors), the American Council of Life Insurance (two and one-half floors), the first District office for the investment firm of Drexel Burnham & Co., Union Camp Corp., British Airways, Equitable Life, and the new main office for United National Bank, one of the city's minority-owned financial institutions.

According to Carr Co. leasing representative John J. Donovan Jr., one of the most unusual elements in the International Square buildings has been attention to energy-saving construction.

All windows, for example, are double-paned with glare-reducing solar bronze for the outer glass. The exterior walls are double the thickness of normal building in recent years, to provide extra insulation.