In a year when office construction nationally has reached a record--with 519 major buildings under way--less than a year's supply of unrented space is available in downtown Washington, reports the Office Network Inc., a Texas-based research organization.

Nearly 10 million square feet of office space is currently being built in this area and the vacancy rate in the central business district is under one percent, compared with an average of 3.48 percent nationally, the organization said.

Office construction is up an average of 66 percent from a year ago in 17 major cities in the survey. For the first nine months of this year, nonresidential construction--which soared while home construction dwindled--was up by 81 percent here, the McGraw-Hill Information Systems Co. reported.

The Society of Real Estate Appraisers noted in its recent newsletter that the record construction has failed to alleviate the shortage of downtown office space in other cities as well as Washington. Nearly 4 million square feet of space is being built downtown here, while nearly 2.4 million square feet were leased city-wide through August, according to Julian J. Studley Inc.

The shortage of space in central cities is being felt by large companies needing 100,000 square feet of contiguous space for 500 employes or more, the Office Network said.

Office rents in Washington are averaging $24 a square foot downtown and $13.80 outside the central business district, the newletter reported. In buildings under construction the averages are $25.90 and $16.36, respectively.

Recent realty transactions here:

* A parking lot at 1023 15th St. NW was sold by Atlantic Garage to Savage Fogarty Companies, representing a Dutch company, for construction of a 12-story office building. The $600-a-square foot price was one of the highest paid for land here.

* Meisel and Cohen Properties has bought 28 acres next to the NASA Goddard Space Flight Center in Prince George's County and plans to build a 1-million-square-foot office and "high technology" park. The firm says it intends to include a 300-room hotel in the complex.

* Construction began recently on a 13-story, trapezoidal office building at 1655 N. Fort Myer Drive in Rosslyn for a partnership headed by J. W. Kaempfer Jr. of the Kaempfer Co.

* B-K Dynamics Inc., a research and consulting firm, has leased 48,000 square feet at 3204 Monroe St., Rockville.

* The law firm of Levitan, Erzin, West, Weinstein and Kerxton has moved to the Air Rights North Building in Bethesda, where it is leasing 40,000 square feet.

* Westinghouse's Defense and Electronics Systems has moved a laboratory and computer development operation into a 45,580-square-foot building in Columbia's Guilford Industrial Park owned by McCormick Properties.

* The Arthur D. Little corporation has signed a lease for 18,470 square feet in the new Capital Gallery at 600 Maryland Ave. SW, where Dun & Bradstreet Inc. has also leased space.

* First American Bank of Maryland has moved its operations center to a 35,900-square-foot building owned by the Manekin Corp. in the Oakland Ridge Industrial Park in Columbia.

* Deak-Perera, dealer in foreign exchange in precious metals, has leased 16,000 square feet at 1120 Vermont Ave. NW and will relocate its headquarters.

* The three-story Grace-Graham Building in Falls Church was sold recently for $1.55 million by Dr. Ray A. Soltany to J.R. Associates.

* The former Gilpin Co. building on Southern Avenue was sold for $1.6 million to Jack R. Trible & Associates, an appliance and electrical components distribution firm.