Army officials unveiled a $2 billion development plan for Fort Belvoir yesterday calling for construction of offices, stores and houses that would bring an additional 35,000 people to live or work at the southeast Fairfax County site in the next 15 years.

The plan, some of it subject to congressional financing, calls for more than 10 million square feet of commercial office space and hotels and more than 7,000 housing units.

A large development at Fort Belvoir has been expected because the Army's engineering school was moved to Missouri, and the land used for it is one of the last federally owned parcels in the region with enough space for a major building program. Some officials have called Belvoir "Crystal City South" because it could rival that development in size and building intensity.

Local government officials who attended an Army briefing on the plan yesterday expressed concern about its potential impact on the region's traffic congestion, economy, education facilities and environment.

Fairfax County Supervisor Gerald Hyland (D-Mount Vernon), whose district includes most of Fort Belvoir, cited a 1986 report by an Army consultant claiming that transportation improvements necessitated by new base development could cost $350 million. He said the federal government would have to pay for those and other public facility improvements.

"We're going to have to be incredibly creative and innovative in devising a transportation network to handle the intensity of that development, and it's going to cost big dollars," he said.

Alexandria Mayor James P. Moran Jr. said he was particularly concerned about the impact on his city. Army officials said that one of the driving forces behind the plan is to save $43 million in annual lease costs by moving personnel out of private buildings in the area and into Army-owned ones.

Given the economic slump and office glut, Moran said, "The last thing we need is for the Army to be relocating out of privately leased space. We will lose millions of dollars in property taxes alone."

Col. Robert R. Hardiman, head of military base development in the Washington area, promised that the Army would pay for any off-site transportation and other improvements required by the base's growth. "The Army will stand four-square for that," he said. "The Army is putting its reputation and credibility on the line."

Pressed on how much money would be provided for transportation improvements, he said the amount was "subject to negotiation based on the impact" of additional traffic caused by the base expansion.

Hardiman said the Army will conduct an economic impact assessment of the development, adding that retail stores will be kept to a minimum so local shops would not have new competition. The Army will keep at least 216 acres of environmentally sensitive land free of development, he said, and is exploring construction of a new $12 million, 1,000-student school.

"We are going to put a solution together that will solve the problems," he vowed.

The base expansion will occur in two major areas. One, the mostly vacant, 820-acre Engineer Proving Ground between Rolling Road and Interstate 95 near Springfield Mall, would become a $1.5 billion office park. The Army wants to develop it as a "public-private partnership" under which a builder would construct 3.1 million feet of office space for Army use in exchange for permission to develop the rest of the site with at least 4.4 million square feet of private offices, 1.5 million square feet of hotel space, and 5,000 housing units. The development would accommodate about 10,000 federal employees, 10,000 civilian workers and 10,000 residents. The Army hopes ground can be broken by August 1991.

The second part of the expansion would be at the main base, on 8,656 acres straddling Route 1. A variety of housing units, recreation facilites, offices and stores would be built, pushing Belvoir's population from roughly 13,000 today to more than 18,000 by the year 2004.