Although Northern Virginia commercial developers and real estate brokers still are riding the crest of 1985 -- the best year in their history -- industry leaders were warned this week that "some judicious belt-tightening is clearly the order of the day for 1986" as office vacancies increase and leasing rates decline.

Lewis Bolan, managing director of Colliers Leggat McCall Consultants Inc., a consulting firm based in the District, predicted vacancy rates for commercial office and research and development space in Northern Virginia will jump from the 11 percent registered in 1985 to between 16 and 20 percent by the beginning of 1987.

Speaking to 500 brokers, developers, lenders and market analysts, Bolan said Northern Virginia "is well on the way to some widespread overbuilding."

According to James V. Whipp, senior vice president of PaineWebber Mortgage Finance Inc., "The No. 1 concern expressed by lenders is overbuilding."

Bolan, Whipp and other commercial real estate executives made their assessments at the Northern Virginia The opening this year of one million square feet of new office space near the Ballston Metro subway station in Arlington "will pull users from the District," giving Arlington a big share of those moving into the suburban office market. -- John P. McEvilly, Coldwell Banker chapter of the National Association of Industrial and Office Parks' first annual market forecast conference.

According to data released by Colliers Leggat McCall, vacancy rates in suburban Maryland will hit 17 to 21 percent by 1987, up slightly from last year's 15 percent rate. In the District, projected 1987 vacancies are between 10 and 13 percent compared with 8 percent in 1985.

In spite of the projected vacancy increases, Bolan said the regional market in Washington is in good shape compared with such cities as Houston, Dallas and Denver, all way overbuilt with offices.

"We leased 6 million square feet for an all-time record" in Northern Virginia in 1985, Bolan said. His company reported that 3 million square feet were leased in the District and another 2.6 million in suburban Maryland. Last year's regional vacancy rate was 11 percent.

The Rosslyn and Crystal City areas have the lowest vacancy rate in Northern Virginia, a trend Bolan said will continue. But he said Springfield, with a 17 percent vacancy rate, and Reston-Herndon, at 30 percent, might be overbuilt.

But Centennial Development Corp. President Pete Scamardo, the largest builder-developer in Reston, disputed the Reston vacancy figure. He said it is closer to 10.7 percent, a figure that almost matches estimates by the Fairfax County Economic Development Authority.

Reston Land Corp. official Hunter Richardson said Reston's "worst case" vacancy rate today is 8.7 percent, a figure many attending the briefing agreed was more accurate than the 30 percent figure, even if Reston and Herndon vacancies were lumped together.

John P. McEvilly, senior sales consultant for Coldwell Banker Commercial Real Estate Services, predicted that the opening this year of one million square feet of office space near the Ballston Metro subway station in Arlington "will pull users from the District," giving Arlington a big share of those moving into the suburban office market.

Scamardo, Bolan and others on the panel agreed that the regional construction boom is "money driven," accelerated by the availability of financing at attractive interest rates rather than sparked by a demand for additional space.

Bolan predicted developers would keep using "cheap" money to build in spite of increasing vacancy rates and the "near absence of restraints in financing."

He said the industry has faced "a complete revolution in financing" with "the distinction between lenders and owner-developers blurry because lenders have taken an equity position" in many of the projects they now finance.

"Lenders used to cut off the spigot" when the market was glutted, he said. "Nobody wants to turn off the spigot."

However, Whipp said getting money to build a project is likely to become more difficult as lenders take a closer look at proposals. He predicted lenders will begin looking at potential tenants, absorbency rates in the area in which a building is to be situated, potential cash flow and equity invested by a developer before making a decision to get involved in a particular project.

Once a project is under way, McEvilly told developers and brokers to "remember the user is keen. Go after every deal." His remarks reflected the market's concern that the vacancy rate will intensify the already competitive broker market.

The growing trend by building owners to give free rent and tenant benefit packages is increasingly a worry to lenders and market analysts, panelists agreed.

Scamardo, however, said his company always has given three to 12 months free rent as part of leasing packages. He said such policies "go with the territory" in the development business.