Fair Oaks Mall, benefitting from the boom in retailing in Northern Virginia, plans to add between 25 to 50 new stores to the already massive shopping center that has 125 retailers, including six major department stores.

"We indeed plan to expand at Fair Oaks," said Christopher J. Tennyson, vice president of corporate affairs for the Detroit-based Taubman Co., which owns and operates the mall.

Confirming reports circulating in the retailing community, Tennyson said Fair Oaks was planning to add more small-scale specialty stores. Construction could start as early as spring, he said. "Many retailers wanted into Fair Oaks, and we had no space," he said. "

For now, he added, plans call for an additional 25 to 50 stores. "This does not eliminate the opportunity to build another major store," which Fair Oaks had been anticipating ever since it opened on I-66 and Route 50, 20 miles from the White House, five years ago. "But there is no impending announcement" about a new department store.

Fair Oaks already has one of the most concentrated number of large stores competing in one mall.

They include: Woodward & Lothrop Inc., The Hecht Co., Sears Roebuck & Co., J. C. Penney Co., Lord & Taylor and Garfinckel's.

The expansion of Fair Oaks will come at a time when retailing is going through a major expansion in Northern Virginia.

The long-awaited Tysons II center is now under construction, due to open in the fall of 1987. It will include Macy's Bambergers and Saks Fifth Avenue as two of the mall's three main anchors for 130 stores.

Meanwhile, at the 20-year-old Tyson's Corner shopping mall, the new ownership is planning to add more retail space, including more major stores, by converting the sprawling parking lot into double-decker parking lots.

Another new mall, near Leesburg, is also being planned.