Mobil Land Corp. has been waiting more than a year to begin construction on its massive Colonial Village office project near Arlington's Courthouse Metro stop.

But this week's word that the Bell System had leased a 360,000-square foot office building right down the street led Colonial Village officials to predict that construction would begin within the next year.

What that means, say Arlington officials, is that the county's Metro corridor office market is about to take off.

"All it takes is one company saying, 'I want to be in the courthouse area,' and the others will look at it too," said Thomas C. Parker, director of Arlington's Economic Development Division. "The psychology of the Bell decision is what's important."

Bell's announcement that it will bring some 1,200 employes to Arlington by the end of the year, Arlington officials say, will enhance demand for office space along the Rosslyn-Ballston corridor and spur developers to pursue some of the plans they have on the shelf.

Other commercial real estate experts, however, mindful of high vacancy rates and stalled construction plans in the District and the Maryland suburbs, are more cautious about the impact of Bell's planned move to Arlington.

"It's a nice plum in Arlington County's hat, but it wouldn't change my plans and I doubt that it would change anybody else's," said Charles E. Smith Co. vice president Stephen B. Altman, adding that his company has no intention of speeding up its plans to build a courthouse office building.

"I'm seeing the demand, but it isn't great enough to make me jump into the courthouse market," he said.

According to county figures, the leasing of the 10-story Courthouse Plaza building this week to Bell's Middle Atlantic operating company will reduce Arlington's office vacancy rate from 8 percent to 5 percent. About 750,000 square feet of office space is currently under construction in the county, and county officials said they hope another million square feet will be under construction by the end of the year.

By contrast, a recent survey by Coldwell Banker, a large leasing firm, found that about 3 million square feet of office space in the District, about 8 percent, is standing vacant. Another 5 million square feet is due to be added to the market in the next year.

"The problem in D.C. is not a demand problem, it's a supply problem," said Ken McVearry, vice president of Coldwell Banker's suburban leasing office. "We've got an overbuild situation."

The same survey found a 10 percent office vacancy rate in Montgomery County, and 7 percent in Prince George's. Fairfax County officials estimate their county vacancy rate at 4 percent to 5 percent.

Parker predicted that the county's relatively low vacancy rate, coupled with new interest in office space along Metro's Orange Line, is likely to advance construction schedules on some major county projects by up to a year. Seventeen major office projects, comprising more than 8 million square feet, are currently approved but unbuilt in the county. All but one are near subway stops, and most are along the Orange Line.

Several developers said yesterday, however, that signed preleasing agreements--not public perceptions of the Arlington office market--will determine their construction schedules. And while they hoped the news of the Bell move would improve the market, none had seen an increasing interest from tenants.

"We're very pleased that the leasing took place, but we don't have any plans to speed up," said Neel Teague, whose District-based Quadrangle Development Corp. is trying to get advance leases for two planned 12-story office buildings in the Ballston area.

A corporate leasing executive saw the situation differently. "Arlington is a lot more competitive today than it was yesterday," said Edward I. Geisinger, vice president of Julien J. Studley. "And to the extent that their rents are a little cheaper there than in the District , they have an advantage." $130; Map, no caption, By Dave Cook--The Washington Post