Marriott Corp. is planning to build a corporate headquarters complex containing three million square feet of office space -- nearly the size of the Pentagon -- on 210 acres it purchased last year in Germantown, sources said yesterday. The complex, which the company plans to finish by the mid-1990s, could mean an additional 9,000 jobs for Montgomery County, county officials said.

The building site is in the area of the county known as Germantown East, along I-270 near the Rte. 118 exit, a spot Montgomery County officials have projected as the next area to experience a development boom.

Real estate development officials characterized the plan -- whose price tag has not been revealed -- as a huge undertaking that is perhaps unprecedented in size in the Washington area for one corporation. The space would be spread over several buildings, and plans for the complex also include a hotel, according to county officials.

Although the Tysons II development in Fairfax County now being built by Homart Development Co. will be about the same size, it has as tenants a number of companies and retail operations and is not planned for completion until after 2000. Crystal City in Arlington County contains about 10 million square feet of office space, while the Pentagon occupies 3.7 million square feet of space.

"That's a very large development," said Robert S. McGarry, director of the Montgomery Country Transportation Department. "We're talking about a very top-quality office complex that most any government would kill to get -- a corporate headquarters that they'd be proud of and we'd be proud of."

Marriott -- one of the nation's largest employers, with 220,000 workers in its hotel and food-service empire -- now has about 2,500 employees at its Bethesda headquarters off Democracy Boulevard. The new complex would house more employees because of corporate growth and the consolidation of operations now located elsewhere.

The company, whose more than $5 billion in annual revenue makes it the largest corporation in the Washington area, is in the process of adding 300,000 square feet of space to the half-million-square-foot building it now occupies. That addition will accommodate another 500 to 1,000 employees and should be ready by spring.

Marriott officials confirmed the plans for the Germantown site and that they have been involved in conversations with state and county transportation officials, but they declined to elaborate further on their plans. Company spokesmen previously said the site would be used for a corporate learning center and for future expansion.

Because a three-million-square-foot complex could house as many as 12,000 workers, state and county officials have been involved in discussions with Marriott officials about new roads and schools and whether county services planned for the area would meet their needs.

"The question of how the Marriott proposal can or cannot be accommodated is being discussed," said Richard Tustian, director of the Montgomery County Planning Department. He said the planning department staff has not yet made a recommendation on the plans submitted by Marriott.

McGarry was optimistic that the roads already included in state and county transportation department plans would be sufficient. "The good news is the roads can handle it," said McGarry, who added that a major road improvement already is scheduled in the master plan for that area because of anticipated growth that includes retail and office complexes.

Timing of the development will be important, said McGarry. "Certainly it's not going to be built over a weekend," he said. Although the road capacity for a complex that size will not be available when the site plan is approved, it would be available by the time the development is finished, he said.

Montgomery County Executive Sidney Kramer last week outlined an annual growth policy for the county that would allow for more jobs and housing in several areas where limits on development have been in effect. The proposal is now waiting for approval of the county council. Kramer could not be reached for comment late yesterday.

The council's approval of the policy could be important for Marriott and others who want to build in the Germantown area. However, McGarry said that if the council does not approve the plan, it would mean only that Marriott would have to take longer to complete the project.

Marriott sources said yesterday that it was possible that if the total three-million-square-foot project is not approved, the company may change its plans to relocate there.

Marriott, best known for its hotels and Roy Rogers restaurants, has undergone phenomenal growth in the past 10 years, more than tripling the number of workers it employs.