What's a shopping center developer to do, when his marketplace is saturated with boutiques, discounters and department stores?

Convinced that there's not much to be gained for now from developing new regional malls in the congested retail mecca called metropolitan Washington, Theodore N. Lerner is switching his talents to office parks.

The Lerner Companies, developers of such diverse complexes as Wheaton Plaza, Tysons Corner Center and White Flint Mall, is planning to break ground Oct. 1 for an entirely different kind of real estate venture, a 40-acre office campus at the intersection of Route 123 and Jermantown Road near Interstate 66 between Vienna and Oakton in Fairfax County.

Lerner said last week that the new center, to be called Flint Hill Office Park, is the first in a series of possible office centers, with other land parcels under study. Flint Hill is being developed as a joint venture with Mar Kap Enterprises of Bethesda.

It should be emphasized that Lerner is not retiring entirely from his retail specialty. Within a few weeks, he is expected to announce major tenants and detailed plans for one project already on the architects' drawing boards, a long-awaited retail and office center at Connecticut Avenue and L Street NW, site of the old LaSalle Building and on top of a subway entrance.

Although Lerner has declined to discuss this D.C. venture, retail insiders insisted last week that Saks Fifth Avenue will open its second area store at the major intersection, adding to the stylish retail competition already visible in downtown Washington and helping to relieve growing pressure on the crowded but very profitable Saks store in Chevy Chase.

One retail project abandoned by Lerner and partners was a projected second mall at Tysons Corner on 117 acres. A huge office complex how is projected for the site, but Lerner and the other owners are at odds on selling the land to a Boston developer, with Lerner interested in developing the project himself (he owns 25 percent).

While that situation remains in dispute, Lerner is moving ahead on the Flint Hill office park. Initial tenants will be able to move in a year from now, with 142,000 square feet of space available on six floors. A second building, to open early in 1982, already has been leased fully to a California-based international consulting firm, Engineering-Science Inc.

Lerner's son, Mark D. Lerner, has been focusing his attention on Flint Hill for his father's company. Mark visited a similar center in Atlanta for ideas about landscaping and design, and both men claimed last week that what they are bringing to Washington is a "perfect work environment."

All together, the office complex will have only six office buildings and one hotel when completed. The maximum office space for all of the structures, under current zoning, is 800,000 square feet. "It's a perfect spot for a corporation headquarters," said Lerner, who is well-versed in recent history of the immediate area, which has included construction by American Telephone & Telegraph of a regional headquarters in Oakton and by Mobil Corp. of its domestic headquarters close to the Capital Beltway.

The center's first building, designed by the Weihe, Black, Jeffries, Strassman & Dove architecture firm, is a blend of glass and brick, with a two-story lobby of bronze columns and travertine floors. Buildings will have 24-hour security and parking will be concealed from roadways by extensive landscaping, wooded grounds and a lake.

Lerner said a major attraction of the center should be Interstate 66, providing non-stop highway access to downtown Washington shortly after the first building is opened at Flint Hill. Tiber Construction Co. of Alexandria has contracted to build the first offices, and Shannon & Luchs will handle leasing.