One of Washington's great growth industries -- the trade association -- is suffering a small but noticeable erosion to the suburbs, caused by a growing advantage in rental costsoutside the city limits, officials of those groups say.

The shift isn't new, notes Frank Martineau of Association Trends magazine, but it has taken place quietly because the city and surrounding counties haven't aggressively competed with each other. "We try to think of economic development on a regional basis," said a Montgomery County official.

The Fairfax County Economic Development Authority, however, is tooting its horn to trade associations about the appeal of Reston and the Tysons Corner area.

The number of trade and professional associations moving into Fairfax County grew 11 percent in 1980, to a total of 176 -- compared with about 850 in the District -- and that trend continued in 1981, the county's development authority reported.

Some of that growth is coming at the District's expense, as new arrivals pass up the city to settle in the county and as old associations leave the city to take advantage of lower rental costs in the county, the authority said, although it does not yet have specific numbers on the shift.

"Office rents for new construction in downtown Washington are expected to reach $30 per square foot by mid-1982," the authority said, citing statistics gathered by the Coldwell Banker real estate firm. "Prices for equivalent top-quality office space in Fairfax County will range from $14 to $18," the report added.

"Generally associations moving to the metropolitan area initially locate in downtown Washington. Then, during their third or fifth year in the area when old leases expire they often relocate again, this time to the suburbs," the Fairfax authority said.

The Reston Land Corp. recently counted 54 national trade associations based in Reston. Ten years ago, there were none. The Future Homemakers of America and the Academy of Model Aeronautics are two that moved from Washington to the Fairfax County new town because of rent, Reston officials said.

Falls Church has two new arrivals from the District, the American Production and Inventory Control Society, and the National Association of Plumbing, Heating and Cooling Contractors, whose building is under construction across from the historic church that gave the city its name.

The National Association of Rehabilitation Facilities moved from Chevy Chase, just on the District line, to Tysons Corner last fall. "Our lease was up and we were looking at a very large rental increase," said association executive Rich Edelman. One option was a relocation to Georgetown, but the rental charges there exceeded $30-per-square-foot -- too much, it turned out. The Tysons location puts the association close to Dulles Airport and reduces the travel time to National Airport, as well, he said.

"It's been going on for a long time," said Martineau. There are human as well as economic factors, he added. "A lot of good people available in the outlying areas just don't want to fight the traffic into the city," he said. Martineau says the city's political and bureaucratic climate can work against the District's competitive position. The American Trucking Associations Inc. gave up a frustrating, three-year struggle to expand its 16th and P streets property in the face of "incredible bureaucratic delays" on the city's part, said association official Donald Tender. Instead, it willbuild an 11-story building in Alexandria.

Cost is most important, though. "Commercial development in surrounding jurisdictions is taking place on cheaper land which results in cheaper rental costs," said Lawrence P. Schumake, executive director of the District's Office of Business and Economic Development. Cheaper rent is an attraction, particularly to smaller associations that don't have to be close to the federal agencies and Congress, he said.

"We recognize that. We're very sensitive to that." The city's answer, he said, is to encourage the redevelopment of less expensive land along the New York Avenue corridor, the Union Station area and the development of outlying Metrorail stations as magnets for new association growth.

"We're going to have to break loose areas that can accommodate business and industry at lower rental prices," said Schumake.

The District also suffers from a shortage of office condominium space that permits associations to own rather than rent space, Fairfax development officials said. In 1980, there were 40 office condominiums available in Northern Virginia, compared to 10 in suburban Maryland and none listed in the District, in a study cited by the Fairfax authority.

Washington's position isn't seriously threatened, however, says Richard H. Mansfield III, an attorney representing the American Society of Association Executives -- the "association of associations" located in downtown Washington.

"There is a stampede by trade associations to Washington,"said MAnsfield,who recently conducted a study on this question.