A trailer being used by Fairfax County as a temporary tax assessment office on a residentially zoned piece of land has created a hornet's nest in McLean.

The trailer is set on what appears to be a permanent foundation on a vacant site at the intersection of Lewinsville Road and Balls Hill Road, across from the Dranesville District government center and McLean area police headquarters. The trailer will house the local tax assessment office while a new wing is added to the nearby government center.

Innocent as it all sounds, local residents are worried.

This week, several residents said "the county is violating its own zoning laws" by allowing the space to be used as an office.

The site is vacant, except for the trailer and paved parking which the county has provided. It abuts additional residential land owned by local developer Kenneth Thompson who also owns the site where the trailer is located.

Several residents said they feared the precedent that could be set for future commercial rezoning of the site because the county is now using it as an office. The site is across Lewinsville Road from the Farm Credit Bureau and Planning Research Corp. (PRC) facilities which were developed by Hazel/Peterson Cos.

During a meeting of the McLean Citizens Association's planning and zoning committee, residents said they were concerned and worried that the action could be the first step toward bringing commercial development to the north side of Lewinsville Road. Lewinsville has generally acted as a dividing line between residential and commercial use.

"They the county did not have to even get a permit," said one committee member.

Linda O'Brien, a resident of the area, said, "Our neighborhood is concerned about the trailer."

Several residents of McLean Knolls, a nearby subdivision of $250,000 homes, who were not at the meeting also questioned the trailer this week. Stephen Hubbard, chairman of the committee, said that Fairfax has an 18-month lease on the site and tried to assure worried residents that "just because the county is leasing the site from Thompson, it does not make the county beholden to him in future zoning cases."

Other disagreed. Several recalled an old proposal by Thompson to build a total life-care senior citizens residence on the site. "There has been some thought given to rerouting Balls Hill Road through that site. Then they the county would have to give him something in return," said one committee member.

Joan DuBois, an aide to Dranesville Supervisor Nancy Falck, said Wednesday that the site was leased as of Aug. 1 through Jan. 31, 1986, for a total cost of $12,000. After that time, the contract can be extended by mutual agreement.

"It is a simple lease arrangement. He pays taxes on the property. We pay the utilities," DuBois said.

"It does not set a precedent for any rezoning," she said.

The site was picked after what she called "an extensive search" and was selected because of its proxmity to the exsiting government operations including the police department.

"A contract for construction of a new wing on the government building has been let," she said. That wing is scheduled to be headquarters for Fairfax police operations in the McLean area. The interior of the present center will be renovated to house the assessment office, a community meeting room and the office of the Dranesville supervisor.