Heavy demand for space by high-technology firms moving to Fairfax County has resulted in planned and approved industrial construction of more than 1.26 million square feet for commercial leasing. Another 317,154 square feet of usable space in eight buildings, with 53,570 square feet already leased, is under construction, according to a Fairfax County Economic Development Authority (FCEDA) report just released. All eight buildings are scheduled for completion by March 1982.

Calling the industrial space market in Fairfax "speculative," the report goes on to say that the current vacancy rate for available space hovers around 2 percent. New leases in Fairfax total between 700,000 to 800,000 square feet annually, and the county has about 13 million square feet of commercial space on inventory. In the 10 months since the last FCEDA survey, 587,045 square feet of planned space has been added.

In addition, another 166,233 square feet of space previously occupied in existing buildings, and 372,693 square feet in recently completed projects, became available in Fairfax this year, bringing the county's total (existing, under construction, and planned and approved) in 1981 to 2.06 million square feet.

Although the FCEDA report says warehousing and distribution firms--the "traditional tenants of industrial space"--are still predominant, "a growing segment of high-technology firms is seeking a more flexible mix of office and industrial space."

To meet that demand, the report says, "industrial developers are constructing buildings with assembly/operations space, moderately high ceilings, tile floors and air conditioning." Lease rates for this "high-tech" space generally fall between $6 and $8 a square foot, with some leases as low as $3.75 and others as high as $10.50 a square foot, depending on location. Developers are also planning upgraded industrial parks with integrated building design and landscaping "to create a business park environment attractive to industrial and office users."

* Development officials in Prince George's County have issued a study that predicts the addition of nearly 2,000 new hotel and motel rooms by 1990, virtually double the number of existing accommodations. The study cites the county's "transformation from a Washington, D.C., suburb into a booming office and light industrial park center." Almost half of the projected hotel/motel rooms are already in the planning stage.

* Arthur Rubloff & Co. has been appointed exclusive agent for the subleasing of 125,000 square feet of available space on the top five floors of 400 Army Navy Drive. The 10-story structure is the closest private building to the Pentagon. It was built for highest security, and is approved for top-secret presentations.

* The Jacyn Development Group has opened a new 35,000-square-foot office complex, Three Lakefront North, in downtown Columbia. More than 70 percent of the building has been leased by a diverse collection of businesses, including Management Science Associates, Manufacturers Financial Services and BDM International Ltd.

* The Grace-Graham Building, a three-story, 28,500-square-foot office complex in Falls Church, has been sold for $1.55 million to J. R. Associates, a Washington-based general partnership. Pauline Thompson of Tysons Realty represented both the buyer and the seller, Dr. Ray A. Soltany, in the transaction.

Westinghouse's Defense and Electronics Systems has located a major laboratory and computer development facility in the Owen Brown Business Center Building in Columbia's Guilford Industrial Park. By leasing the entire 45,580 square foot building, Westinghouse brings its total office and industrial space in Columbia to more than 200,000 square feet. The property is owned by McCormick Properties Inc. Coldwell Banker was the broker.

1000 Thomas Jefferson Street NW, a new 138,000 square foot, six-story Georgetown structure designed by Vlastimil Koubek, is scheduled for completion in April 1982. The first four floors have been leased to Syncom. The remaining two floors, with 50,000 square feet, are still available. The Fred Ezra Co. is the exclusive leasing agent.