Nineteen eighty-four was "a banner year" for commercial construction in the Washington area -- and especially in Northern Virginia -- as 510 major projects valued at $2.1 billion got under way, according to a report released yesterday by the Metropolitan Washington Council of Governments.

The number of projects was up sharply from 1983's total of 314, and the total cost was double that of the previous year, the report said. Northern Virginia accounted for 279 of the 510 projects, and 54 percent of all new office space across the region.

Fairfax County alone registered 154 projects valued at $597 million and totaling 12.3 million square feet.

"I am here today to tell you that 1984 was in fact a banner year for commercial construction, a key measure of growth in the region's economy," said R. Scott Fosler of the Montgomery County Council, who presented the report to the COG board.

But amid the good news, Fosler noted, there are other "important development issues to be addressed in the future." He pointed especially to the questions of whether "the suburban road systems can sustain the traffic this development will bring," whether growth around Metro stops will generate more transit use, and what, if anything, the 1984 total of 34 million square feet of commercial construction means "to the unskilled, unemployed man or woman not sharing in the region's prosperity?

"We must keep these concerns in mind as we look toward the completion of these projects," he said.

Otherwise, the report was largely a compendium of upbeat data.

Although Fairfax County's growth was the most striking, almost every jurisdiction showed some improvement. Only Arlington and Alexandria recorded fewer starts in 1984 than in 1983. And Fosler noted that "Arlington was down . . . because of so much development already under way."

The largest percentage increase was in Prince William, where square footage begun climbed 243 percent, to 2.1 million.

The figures showed almost half of the space started in the quarter was near Metro stations. For the year, Metro stops got nearly a third of the development -- 91 projects totaling 10 million square feet.

Leasing activity was also brisk, the report found, with 14 million square feet taken during the year, areawide. Again, Fairfax County led, with 5.2 million feet, followed by the District with 3 million. The report did not, however, examine vacancy rates, which, though improving, have remained high by historical standards.

While offices accounted for the largest share of the new space, mixed-use projects -- combinations of office, hotel, retail or residential uses -- were the second-largest category. The report showed 24 of these valued at $253 million, excluding land costs.

Montgomery and Prince George's showed notable retail growth in the fourth quarter, recording between them $25 million worth. For the year, however, Virginia led with $75 million, compared with the two Maryland counties' $60 million and the District's $5 million.

The report was based primarily on data from the F. W. Dodge Construction Division of McGraw-Hill, which tracks construction contracts in major markets around the nation. Under its definitions, commercial construction includes offices, retail space, hotels, medical buildings and other nonresidential buildings. A project is any such construction work valued at more than $75,000.