George Washington University broke ground yesterday at a 50-acre site in Loudoun County for the university's first classroom building in Northern Virginia.

The new building, scheduled to be completed in time for fall 1991 classes, is part of a long-term, 576-acre project called University Center that would include twice as much office space as the Pentagon and more than 800 apartment units when it is completed in about 20 years.

The four-story GWU building will house classrooms, faculty offices, computer teaching facilities, a student lounge, three levels of laboratories, a teleconferencing room and a large gallery for catered dining. Initially, the Loudoun campus will provide space for the school of engineering and for scientific research.

It is the university's first foray into the lucrative education market in Northern Virginia, although President Stephen J. Trachtenberg said that is not GWU's sole purpose for building a campus near Leesburg.

"In the long run, it will be a lucrative market," Trachtenberg said. "That part of Virginia is a projected growth area. But it will take five to 10 years of patience."

The move south of the Potomac River also provides one solution to a space problem at the Foggy Bottom campus, which exists on 45 acres. "Over time, as technology continues to grow," he said, "there will be no reason for accounts payable to be in Foggy Bottom. At some points, other offices like the registrar also could move."

The estimated cost to build and furnish the building is $15 million to $20 million, Trachtenberg said. The first 50 acres of land was donated to GWU by the project developers.

The project has had a controversial history in Loudoun. Some county officials initially balked at the joint venture between GWU and Charles E. Smith Cos., fearing that the massive development would worsen traffic congestion in the area.

Developers lowered the amount of office space from 14.7 million square feet to 8.3 million square feet and almost doubled the number of housing units from 433 apartments to 826.

The development site is bounded by Routes 7 and 606, the Potomac River and Broad Run near Route 28.

It is close to several large residential, office, commercial and retail projects, including Xerox's 2,267-acre Lansdowne and the Lerner Cos.' Dulles Town Center, which will feature a 1.2 million-square-foot shopping mall.

When completed, the development is expected to generate 25,000 jobs.