A group of 26 Loudoun County residents, charging that the county illegally is permitting a developer to build an additional 112 houses in a large subdivision near Dulles International Airport, has filed suit against the County Board of Supervisors in an effort to block construction of the houses.

The suit, filed in Loudoun County Circuit Court Friday by the CountrySide Citizens Committee, asks the court to overrule a Nov. 5 decision by the supervisors. That 5-to-3 vote by the board granted a developer permission to build the extra homes in return for giving the county conservation easements on 350 acres of farmland in western Loudoun.

"We want it overturned," said Benjamin Hicks, a member of the CountrySide group. The group is also asking $500,000 in damages, alleging that the added homes will devalue their property, part of a planned subdivision of more than 2,000 homes.

Mary Gayle Ashley, an attorney for the group, said her clients believe their rights were violated by the board and that their only recourse is to sue. In addition to the board, the group named as defendants the five supervisors who voted to allow the additional construction.

County Board Chairman James Brownell was quoted by United Press International yesterday as saying he stands by the board's action. He said the suit is an attempt to discredit the county land management plan.

Under the plan, adopted to preserve Loudoun's rural land, a landowner may place conservation restrictions on property that could be otherwise developed. In return, the county board can transfer the old building rights from the undevelopable land to other property, in this case, CountrySide.

The board's November vote allowed 437 Land Co. Inc. of Fairfax City to build the additional single family town houses in the CountrySide development.

In exchange for the extra houses, the developers agreed to pay nearly $600,000 to buy conservation easements on three tracts of open land, said regional manager James DeFrancia. Under the conservation agreement, he said, that land will remain practically undevelopable, or zoned only for one housing unit per 50 acres.

Hicks said residents are concerned that the additional units at CountrySide will change the "profile" of the subdivision, which is currently zoned for about four housing units per acre.

Edward J. Finnegan, attorney for Loudoun County, as of yesterday had not seen the lawsuit, but said he expects to prepare a report on the suit for the supervisors' next meeting, Jan. 7.