The General Services Administration is undertaking a study of whether to build a massive new complex of Defense Department office buildings near the Pentagon, a project that would be almost equal in size to that building.

Arlington officials said yesterday that they are wary of the proposal because of the danger that it would overload the region's already crowded roads and bridges. GSA officials disputed those concerns.

"There's no reason to say: 'This plan won't work, this plan won't work,' " said Jack Finberg, GSA's regional deputy assistant administrator for real estate, yesterday. But Finberg conceded that "there'll be a lot of fear" about traffic problems created by the new office buildings.

"What we have to do is make a case that what we propose is feasible," he said.

Finberg's comments followed disclosure that the agency began advertising this month for a consultant to undertake a year-long study of about 700 acres of federal property surrounding the Pentagon, and bounded by Shirley Highway (Rte. 395), Washington Boulevard, Boundary Channel Drive and Army-Navy Drive.

The GSA has tried for years to move government agencies from leased space to government-owned facilities in the Washington Navy Yard and elsewhere in the region to save money.

In particular, the Defense Department has been interested in moving thousands of its employes, housed in 5.5 million square feet of office space, to new offices near the Pentagon.

Until now, however, the government has not conducted a thorough study of any such plan, officials said.

A preliminary GSA study in 1979 said that the 700 acres around the Pentagon could accommodate up to 3 million square feet of new Defense Department office space.

In 1981, the Defense Department estimated that it would need 2.1 million square feet of new office space in the Pentagon area. GSA officials said that because of growth in the Defense budget, the agency is asking for considerably more than 2.1 million square feet.

Finberg said GSA believes 3 million square feet is the maximum that can be built on the 700 acres, which includes, in addition to the Pentagon, parking lots, the Navy Annex, and other buildings.

The Pentagon, the largest office building in the region, contains3.7 million square feet of office space and houses 22,000 employes.

GSA officials said that, besides the Pentagon, they do not know which other facilities would remain on the 700 acres after new construction, or the height or density of any future office development.

Finberg said he believes that the best plan would be for low-rise offices of about two or three stories.

The GSA has not decided that it wants to build new offices at the site, but only that it likes the idea in the abstract, Finberg said.

"The study will be an analysis of what the Pentagon site is capable of holding . . . what's developable there," Finberg said.

GSA officials said that the study will take into account the need for new roads to accompany any development in the Pentagon area.

Joseph Durkee, a traffic engineer with Arlington County's Public Works department, said that traffic on such roads as Shirley Highway and Jefferson Davis Highway (Rte. 1) already is "more or less at capacity . . . . But that hasn't stopped development in the area so far."

Arlington County Board Chairman John Milliken said that because of concerns about traffic, Arlington officials will "stay in very close touch" with GSA on the proposal.