The newly approved Tysons II development in Fairfax County is expected to spark a transformation in the county's burgeoning commercial core, contributing to an economic explosion that some county officials say will give the suburban county a distinctly urban veneer.

Developers of the $550 million project, one of the largest ever built in the Washington metropolitan area, say the commercial center will bring big city department stores -- Saks Fifth Avenue and Macy's -- and national corporate headquarters to the Northern Virginia county.

At the same time, critics argue that the development just off the Capital Beltway near the existing Tysons Corner shopping mall also will bring big city problems, including traffic jams that will strangle the county's already-congested highways.

The Tysons II development is the second massive commercial project approved by the county in less than six months. The first was the $460 million Fair Lakes development in western Fairfax County. The approval of the two developments represent a major political shift in a county that hasvacillated in its relationships with developers. That switch leads the county away from residential development and into commercial projects, a move officials say will pump millions of dollars into the county rather than drain millions from its budget.

"The county is going to fill up sooner or later," said Republican Supervisor Thomas M. Davis, who represents the Mason District. "I'd rather it be with commercial development."

Developers estimate that the project will contribute $696,000 in taxes to the county in its first year of operation in 1987. By the year 2000, developers say, their contributions will reach more than $17 million. The county's costs of providing transportation, sanitation and other services to the project are expected to be half that amount.

The forest of 11 new office towers, two hotels and a "galleria" style shopping mall, modeled after the Galeria Mall in Houston, will be the last and largest remaining unfinished piece of the commercial center in what has become known as Fairfax County's downtown.

The board's action this week ends a 12-year debate over how the valuable property would be developed. Some of its original owners opposed building a shopping center on the property that might compete with the existing Tysons Corner Center.

Some county officials say the two first-class hotels with their 1,200 rooms and the chic three-tiered mall will bring more urban amenities to the county.

"Can you imagine me going home and explaining to my wife that we turned down a chance to get Neiman-Marcus?" quipped Davis after the board approved the controversial project late Monday night.

But some county officials warn that all those benefits will be outweighed by the monumental traffic problems that will be created by adding as many as 7,550 automobiles to area highways during peak hours.

"We will have a traffic disaster," said Annandale Supervisor Audrey Moore, a Democrat who frequently duels with developers. "You can't snap your fingers and produce roads."

The developers went before the Board of Supervisors this week with one of the county's most prominent land lawyers, John T. (Til) Hazel Jr., who said the project's owners would pour $14 million into road improvements in the area.

The developers of the project are Theodore N. Lerner, the shopping mall magnate who brought the original Tysons Corner, White Flint and Landover malls to the area, and Homart Development Co., a subsidiary of Sears, Roebuck and Co.

The developers already have paid the state $70,000 to provide ramps off the Dulles Access Road to their project, said E. Wayne Angle, project director. They plan to provide an additional $130,000 to erect toll booths near the exits, he said.

"We've got to get out there and lay some asphalt to correct these problems," said Angle. "They were generated by existing development, but we have to clean it up for the community as well as our own marketing effort."

Developers say they have not yet begun recruiting tenants for the commercial center, but say they will be attempting to lure high-technology corporations from throughout the nation, as well as large legal and accounting firms from the District.

Attorney and developer Hazel also is beginning work on his own Fair Lakes Development project in western Fairfax, a project that some say could siphon away potential tenants from the Tysons Corner area.

But Hazel said the two centers will be offering different commercial environments. Hazel's 5.1-million-square-foot Fair Lakes project will include an office park setting on 620 acres. The Tysons II project will include 13 high-rise buildings, ranging from 10 to 25 stories in height, on about 107 acres.

Hazel has said he will contribute about $22 million in road improvements surrounding the Fair Lakes project. Tysons II developers say the $14 million in road improvements for their project will be completed by the end of 1985 and that the shopping mall is scheduled to open in the spring of 1987.