The Federal Home Loan Mortgage Corp. is moving its national headquarters staff of 600 employes from the District, as well as 65 workers from its Northeast regional office in Crystal City, to new facilities in Reston.

This decision delights Reston officials who see it as another sign that the town -- once viewed as a seeding ground in the outer suburbs for new businesses in the high-technology field -- finally can compete with such commercial centers as Tysons Corner for prominent national companies.

"Reston now is an integral part of the Washington market," said Hunter Richardson, vice president of Reston Land Corp., which oversees development of the 20-year-old town in western Fairfax County.

But Reston's victory is a disappointment for D.C. officials, who in recent years have aggressively sought to attract new businesses to the city.

"Quite honestly, I believe they have not taken the time to look at the advantages of relocating in the District," said Fred L. Greene, the District's planning director.

"We challenge any firm that wants to compare land development costs in the District with the suburbs. We can meet or equal those prices," he said.

Mark Goldharber, a vice president for Freddie Mac, as the firm is widely called, disputed Greene's claim, saying the corporation spent a "long and exhausting time" assessing the cost advantages of the District versus the suburbs, and opted to consolidate its operations in Reston.

Although Freddie Mac could provide no precise figures on how much money the move supposedly would save, spokeswoman Helen Dalton said the corporation found office rents to be 107 percent higher in the District than in the suburbs, land costs 400 percent greater and building costs 33 percent more.

Now that Freddie Mac has shed its quasi-federal status and must compete in the market as a private corporation, these cost differences become a "tremendous factor," Dalton said.

Freddie Mac, congressionally chartered in 1970 to create a large secondary market for mortgages issued by savings and loan institutions, became subject to taxation in the Deficit Reduction Act of 1984.

In addition, Freddie Mac became a private corporation last December, which meant it had to start worrying for the first time about shareholders' profits as well as taxes.

These changes, Dalton said, meant that buying office space made more sense than continuing to rent space in a building at 1776 G St. NW, Freddie Mac's current location.

Reston looked attractive because the company, lured in 1983 by the large labor pool of computer technicians in Northern Virginia, already had moved its sophisticated computer operations there, Goldharber said. The accounts section followed suit this year, putting 200 Freddie Mac employes in Reston in more than 100,000 square feet of office space at the Lake Fairfax Business Center. Freddie Mac plans to move its 65 Crystal City employes to the same building, Dalton said.

For its national headquarters, the company is negotiating a contract to buy 250,000 square feet in the Campus Commons office park less than a mile away from the Lake Fairfax center, according to a source familiar with the negotiations. The building on Wiehle Avenue would offer Freddie Mac excellent visibility from the Washington Dulles International Airport access road.

Representatives from Freddie Mac; Samis Construction Co., the building's developer; Coldwell Banker, the real estate broker handling the property, and Reston Land Corp. declined to comment on the negotiations.

Although District officials expressed regret at losing a nationally known company, Curtis R. McClinton Jr., the District's deputy mayor for economic development, said Freddie Mac's departure does not signal a trend.

D.C. office space has increased from 13.9 million square feet in 1980 to 22 million square feet today, according to Madelyn Andrews, a McClinton aide. Even with an average vacancy rate for commercially leased properties of 19.06 percent in the District's major business center, the city still enjoys a net growth in occupied office space.

The federal government also has slightly increased its presence in the District over the past decade. The General Services Administration, the government's landlord, said office space it controls in the District has risen 2.8 percent from 35.2 million square feet in June 1975 to 36.2 million square feet today. Northern Virginia, meanwhile, enjoyed a larger increase in federal offices over the same period. In June 1975, GSA counted 16.1 million square feet in Northern Virginia. Today that figure is 8 percent higher.

Freddie Mac, which bought and sold $25 billion worth of mortgages and mortgage-related securities in the last year, is the second major financial company this fall to choose Reston as home. Allstate Insurance Co. announced last week it will locate its regional headquarters in Reston.

Richardson credited the opening last October of the Dulles toll road, which shortened the driving time to downtown Washington to 25 minutes, for the increased interest in Reston.

"Before the Dulles toll road, we had a hard time persuading people to come here," Richardson said.

In the past decade, Reston had leased an average of 500,000 square feet of office space per year, but the figure has doubled in the last year, Richardson said. To date, Reston has a total of 6.5 million square feet of office space, about 1,000 companies and about 22,000 jobs.