Fairfax Commonwealth's Attorney Robert F. Horan Jr., calling the county's public disclosure law "a defense attorney's delight," said yesterday that he would not use it in his investigation into whether County Board Chairman John F. Herrity illegally participated in a recent zoning case.

Instead, Horan said his investigation would rest solely upon whether Herrity violated the Virginia state conflict-of-interest law. Unlike the county ordinance, the state code requires a prosecutor to prove intent -- that the official "knowingly and willfully" violated the law.

"If I had to prosecute under that statute, I'd quit," Horan said of the county code, which was enacted in the early 1970s to guard against abuses in Fairfax planning and zoning decisions. "It's just a badly written ordinance. It doesn't do what they thought it would do."

In a letter he wrote to Horan last week, Herrity said he was not aware of a potential conflict when he participated in the board's review of the zoning application, filed by a developer with whom Herrity was a joint partner in a real estate venture. He has hired lawyer Richard E. Dixon to represent him in the case.

In an interview yesterday, Dixon confirmed that he and Horan were partners in a Fairfax City law firm -- Dixon & Horan -- from 1965 until 1967, when Horan left to take the commonwealth's attorney's post. During the previous two years, Dixon and Horan worked together as assistant prosecutors under former commonwealth's attorney Ralph G. Louk.

Dixon said this case was the first time he had been involved in "direct negotiations" with Horan on behalf of a client and he did not believe Horan's involvement in the case represented a conflict of interest. "Our relationship would not deter him from doing what he is supposed to do," Dixon said.

Horan could not be reached yesterday afternoon for comment on his past relationship with Dixon.

In an earlier interview, Horan said the question of Herrity's intent would be a major factor in determining whether the Fairfax board chairman violated the law. Horan said he also was trying to determine the nature of Herrity's relationship with the developer, Hersand Builders Inc., and whether their business association is covered by the state statute.

Horan, who initially said he would reach a decision on the case by today, said yesterday that he might not complete his investigation until next week.

Horan is investigating Herrity's recent participation in a rezoning application by Hersand to build 137 town houses on a 41-acre tract in the Springfield district. Herrity voted against the application, which the board approved, 6 to 2, but then supported a motion granting Hersand several waivers to county zoning standards.

Herrity issued a $10,157 check to Hersand for his one-quarter interest in an office-condominium on Dec. 18, one month after the board first considered the zoning application. The president of Hersand, Herbert L. Aman III, and his wife, Sandra, the company's sole stockholder, own a 50 percent interest in the condominium unit.

Fairfax board members, before acting on a land use issue, are required under the county code to disclose whether they, either directly or through a partnership, have had any business or financial relationship with an applicant during the preceding five years.

The state law includes similar requirements, but differs from the Fairfax ordinance in that it covers business relationships that go back one year from the time an application is filed and states that an official must have intentionally failed to disclose to be found in violation.

Herrity, in his letter to Horan requesting a legal opinion on his involvement in the Hersand case, said an affidavit filed by Hersand in conjunction with its application did not mention that the company had a business or financial relationship with any board members.

Fairfax board Vice Chairman Martha V. Pennino (D-Centreville) said yesterday that the supervisors passed the ordinance because they believed the state law did not go far enough in addressing conflict-of-interest problems.

She said the county regulations are necessary because of the "rampant development" Fairfax officials must oversee and the potential that exists for planning and zoning abuses. She said that although the Fairfax board "is very honest," county officials must do "whatever has to be done" to ensure that the ordinance is enforceable.