Recent increases in mortgage interest rates, which have resulted in a wider gap between fixed interest rates and adjustable rates, have once again made adjustable-rate loans more popular, according to a new survey by the National Association of Realtors.

Last spring, when the gap between the two types of loans was closer, borrowers chose an adjustable-rate mortgage only 8 percent of the time. But during June and July, 26 percent of borrowers picked an ARM. In the Washington area, 30-year fixed-rate mortgages are hovering around 10 1/2 percent, with starting adjustable rates about 3 percentage points lower.

The Realtors' trade group said its study of 750 house sales also showed that buyers financing with 30-year, fixed-rate mortgages bought houses costing an average of $138,800 and made down payments averaging $39,500. Those using Federal Housing Administration-insured loans typically bought $70,600 homes and paid $5,300 down, while those obtaining Veterans Administration-insured mortgages paid an average of $83,300 for a house and made $6,800 down payments.

The average annual income of those with 30-year, fixed-rate loans was $53,700, while FHA borrowers had $40,500 and VA borrowers $44,600.

A total of 85 percent of the buyers bought detached houses and 89 percent of the houses that were purchased during June and July had three or more bedrooms.

IN THE BUSINESS ... Linda G. Hollemon, vice president of Smithy Braedon's market analysis and communication division, has been elected president of Commercial Real Estate Women-National, a newly created group formed from chapters of the organization in the Washington area, Baltimore, Philadelphia, Northern Virginia and Detroit... . The Loudoun County Planning Commission has rejected, on a 4-to-2 vote, a developer's plan for a 550-home subdivision on the 2,100-acre Beacon Hill tract northeast of Leesburg. The development group, called Beacon Hill Farms Associates II Limited Partnership, includes former Secretary of State Alexander Haig. The group said it will submit plans for development of the land under existing conventional residential zoning rather than the clustered plan it sought ... Mulligan/Griffin and Associates Inc., a Rockville-based developer, has completed construction of its 78,000-square-foot, three-story Decoverly office building at 15200 Omega Dr. The building is the third of seven the developer plans at the 42-acre Shady Grove area complex ... The Northern Virginia Board of Realtors is sponsoring its third annual "Smart Moves for Seniors," a free lunch and seminar on real estate financing and investment for the elderly. Reservations are required. For more information call Bobbie Haydon at 560-7350 ... Andrea Grenadier has been named executive director of Clarendon Alliance, a consortium of developers, property owners, local businesses and residents in Arlington County that was incorporated last year to promote the development and redevelopment of Clarendon ... The Consumer Housing Library, which provides housing and real estate information for consumers, has opened an office in Silver Spring at 12501 Prosperity Ave. The group also operates a Fairfax office ... The local boards of Realtors in Prince George's and Caroline counties and Ocean City have won the Maryland Association of Realtors' top honors for community service projects this year. The Prince George's group won among large boards for the $26,000 it raised and spent on 10 projects, including the purchase of $13,000 worth of smoke detectors for poor families ... A total of 3.3 million square feet of office space was leased in the District in the first six months of the year, which is comparable to the 1986 figures, according to Julien J. Studley Inc., a commercial leasing firm. A total of 3.6 million square was leased during the same period in Montgomery and Prince George's counties (two-thirds of it in Montgomery) and 3.8 million square feet in Northern Virginia (two-thirds in Fairfax County) ... The National Housing Conference, which lobbies for affordable housing, is giving its Carl A.S. Coan Sr. Award this year to Gerald R. McMurray, staff director for the House Subcommittee on Housing and Community Development. The conference is commemorating the 50th anniversary of the passage of the U.S. Housing Act of 1937 with a Sept. 15 luncheon at the Capital Hilton Hotel ... Neighborhood Housing Services Inc. of Washington, which has helped finance the rehabilitation of houses for owner occupants since 1972, has started its first new construction project, building three town houses on a vacant lot in the Uniontown section of Anacostia. The group ultimately plans to build 10 town houses on scattered sites in Uniontown and sell them at below-market interest rates ... TC Communities, a Northern Virginia residential builder, has started work on the 52-unit Ashburton Oaks single-family house development on Ashburton Avenue in Reston and a six-unit project called Kelley's Green in Vienna.

PERSONNEL FILE ... Thomas J. D'Alesandro IV has been promoted to vice president for commercial marketing and development for Reston Land Corp., Reston's developer. He replaces J. Hunter Richardson Jr., who on Tuesday becomes development manager for Himmel/MKDG, Reston Land's joint venture partner in construction of the Town Center project in Reston ... James Myles has been appointed vice president in charge of production for TC Communities