A report on office space vacancy rates in downtown areas of major U.S. cities showed that the Washington area continued to have the lowest rates in the nation -- only one-tenth of one percent.

That ultra-low figure has been stable since last September, according to the quarterly suyrvey made by Coldwell Banker, a major realty firm with offices here.

Nationally, the vacancy factor increased to 3.5 percent in June, from a low 3.4 percent in March. Two years ago the national vacancy rate was 6.9 percent -- but only 0.7 percent here. Only Los Angeles and San Francisco, with vacancy rates of 0.2 percent, approach the low Washington mark. Atlanta has a 12 percent vacancy rate and Chicago, 4.1 percent.

Meanwhile, a report from the Washington Board of Realtors' commercial leasing committee shows 1.2 million square feet of new space available, with most of it in three buildings being completed at 1110 and 1120 Vermont Ave. NW at 500 C St. SW. A total of 1.3 million square feet is listed as available for 1981 delivery.

Julian J. Studley Inc. reported that all of the 90,000 square feet of office space is leased in a building to be completed next month at 2501 M St. NW. The nine-story structure, known as the Chemical Manufacturers Association building, also includes three top floors of condominium apartments -- making it the first such mixed-use building in downtown.

Arthur Rubloff & Co. of Washington announced that 16,000 square feet has been leased to National Con-Serv Inc. in the new Exchange Place building at 451 Hungerford Dr., Rockville. The tenant handles federal contract work for the Federal Crime Insurance Program of the Federal Insurance Administration.

Weaver Bros. Inc. has been named exclusive leasing agent for the Park Center Office Building one just off Shirley Highway in Arlington. Scheduled for occupancy in January, the 218,000-square-foot structure is the first of five buildings planned for the 24-acre site being developed by Stone Tract Associates. Erkiletian Construction Co. is the general contractor.

Bendix Corp. will occupy the top three floors of a 30-story office building being built at 1000 Wilson Blvd. in Rosslyn. Environetic D.C., Inc. has been selected to plan and design the 65,000 square feet of space to be occupied by Bendix next summer.

The Howard Research and Development Corp. announced that construction will begin this fall on the Lake Elkhorn Office Park, an $11 million complex of 28 small one-story office buildings that Birtcher-Butcher Partnership is developing. Individual buildings will be sold to occupants through Coldwell Banker at prices ranging from $310,000 to $510,000.

Long & Foster Real Estate recently handled the leasing of 4,400 square feet of space in a new building at 144 Church St., Vienna, to the eastern regional headquarters of Dynalectric Co., a subsidiary of the McLean-based Dynalectron Corp., which handles electric contracts for large projects.