Fairfax County board chairman John F. Herrity said yesterday he will ask county attorneys to seek an injunction preventing the District from expanding Lorton Reformatory and forcing it to dismantle any buildings put up or reconstructed in violation of Virginia and Fairfax land-use laws.

In a statement to be read to county supervisors this morning, Herrity says he wants the county attorneys to determine what control the supervisors can legally exert over the 3,000-acre barbed-wire enclave in southern Fairfax.

Herrity, whose statement followed a surprise visit to the complex Saturday, said the county may, for the first time, be able to force the District to seek building permits, planning permission, and health, fire and safety approval from the county. Such approval, Herrity noted, may be "difficult or impossible" for the District to obtain from county officials, "and it could force them into housing their own prisoners."

Herrity's land-use questions come at the end of a week-long assault on Lorton by Northern Virginia politicians determined to see the prison and its 2,800 inmates moved across the Potomac.

Last Tuesday, Rep. Stan Parris (R--Va) asked the General Accounting Office to inspect prison expenditures to make sure they were not being used for expansion. On Wednesday, Herrity sent a letter to D.C. Mayor Marion Barry protesting expansion at Lorton and asking for an "immediate" meeting to discuss the problem. And on Friday, Fairfax Supervisor Sandra L. Duckworth, who represents the Mount Vernon District in which Lorton is located, inspected the prison with Barry.

Herrity arrived unannounced at Lorton's gates Saturday, accompanied by a television news crew. The on-site inspection confirmed suspicions that the District is expanding the prison complex and not just beefing up security, Herrity said.

District corrections officials have said that any expansion plans are "tentative," but Herrity said he found ample evidence that expansion is already in progress. Officials there, he said, have already partially converted an alcohol-treatment center into cell space and he has learned of other expansion plans from Lorton guards.