Those giant construction cranes swinging back and forth across downtown Washington's skyline are clearly visible signs of an office-building boom that began toward the end of the previous decade.

But they may be as deceiving as they are symbolic.

Time will tell--about two years, in fact--whether the boom can be sustained through the 1980s.

Most commercial real estate developers and leasing specialists estimate 11 million to 14 million square feet of office space will be added to the market by the end of 1984.

But nobody is willing to make any projections beyond then, indicating the boom will give way to a slowdown that could prove troublesome for some developers.

In fact, there is growing concern over three factors that shape the market--financing, space absorption and rental rates.

Financing for most major projects being built now was obtained at least three years ago when interest rates were considerably lower. But higher interest rates point to less activity three years from now.

For the time being, at least, the key to the future of the market lies in the state of the national economy and its effect on lenders.

"Lenders, not tenants, control space," observed Thomas G. Owens, vice president and director of commerical leasing at Shannon & Luchs Co.

And lenders, because of high interest rates, are out of the market, a factor which is likely to postpone some developers' plans for some time.

Generally, most projects under construction today will be delivered for occupancy in 1984, but beyond that, cautioned Owens, "Lenders are taking a look at the economy."

"It's very difficult to obtain financing," said James L. Eichberg, president of Smithy Braedon, a major District-based leasing and property management company. "Insurance companies are out of the market. They're really in the bond market."

"I think interest rates, where they are today, are going to have a tremendous impact" on developers of office buildings, said Ken McVearry, vice president and resident manager for Coldwell-Banker in Washington.

Meanwhile, other major factors may affect the local market long before the office space currently under construction becomes available.

Eichberg, for example, predicts only half of the projected 14 million square feet will be added by the end of 1984.

In some cases, projects have been put on hold even though land has been assembled, Eichberg explained. In others, deals that appeared certain at one stage were never consummated. "It's not a matter of people being afraid that no more tenants will be coming to Washington or expanding," Eichberg said. He added that there is a "little downturn," which he doesn't find "unhealthy" for the local economy.

That might be true. Nonetheless, "some people may have a problem," within the next year, suggested Philip R. Carr, vice president/operations for the Oliver T. Carr Co., one of the area's biggest developers.

Carr says the problem is that the space scheduled to come on the market by the end of 1983 may be too much to absorb under current conditions.

For example, 2 million of the 4.6 million square feet of space being constructed for delivery this year are yet to be leased. And only 1.3 million square feet of the 4.3 million being built for delivery in 1983 have been leased.

The absorption rate over the past two or three years has been roughly 2 million square feet annually. However, with Washington riding the crest of a construction boom, the number of buildings and square feet have exceeded the absorption rate, Carr explained.

Moreover, he said, the president's program for dealing with the economic downturn has had a "dribbling-over effect" on the office-user market here.

Indeed, the normal absorption rate could slip to one million square feet, said Carr, whose company has four projects worth $180 million under construction in the District.

The vacancy rate, which was 2 percent two months ago, is approaching 4 percent, but Owens and Eichberg, among others, refuse to call it a glut. "What we consider a glut other cities would like to have," Owens quipped.

Still, the national average vacancy rate is 5 percent.

Despite the decline in the absorption rate and the building increase, rents for prime downtown property increased substantially over the past year. In fact, Owens says, "They escalated so fast in '80 and '81, they overescalated."

As a result, some projects, which were leased for as much as $30 a square foot, are being leased at $27, and prospective users are waiting for them to drop further, Owens observed.

"It's now a user's market," McVearry said. "The user is king.