Fairfax Commonwealth's Attorney Robert F. Horan Jr. said yesterday that he will expand his investigation of County Board Chairman John F. Herrity to determine whether Herrity should be prosecuted for failing to disclose campaign contributions he received from individuals involved in three development projects that he voted to approve.

"We're certainly going to look at this," said Horan, referring to a report in yesterday's Washington Post. The article cited four instances in the past 13 months in which Herrity voted to approve projects without disclosing that the applicants helped finance his 1983 reelection campaign.

Horan, who is investigating Herrity on an unrelated conflict-of-interest allegation, said he will examine county records relating to the votes and to the chairman's 1983 political contributions.

The Fairfax prosecutor, who earlier this week said parts of the county's disclosure ordinance were too vague to enforce, said the section covering the disclosure of political donations and other gifts "doesn't appear to have the same flaws" and seems to be enforceable.

Herrity has declined to be interviewed on the issue. Richard E. Dixon, the attorney hired by Herrity to represent him during the investigation, could not be reached for comment yesterday.

Some members of the county board yesterday criticized Herrity for his apparent failure to comply with the county law.

"The law says you have to disclose," said Supervisor James M. Scott (D-Providence). "The law is strict, but it has been on the books for 10 or 11 years now and it should be complied with."

The law requires that before participating on a land use issue or a request for tax-exempt government bonds, members of the board must disclose gifts or donations they received in excess of $50 during the preceding five years from anyone with a financial interest in the application. Board members may vote on the application after disclosing the donation.

Since May 20, 1985, Herrity has reviewed and voted in favor of three land use applications without disclosing that the developers or representatives of the developers contributed a total of $450 to his 1983 campaign, according to county records. Those records show that Herrity also supported a developer's request for $8 million in industrial revenue bonds without revealing the campaign donation he received from the developer.

Horan said his investigation would not include an apparent disclosure violation by Herrity in a vote last May 20 because the one-year statute of limitations on prosecution has expired. He said he will review the other three votes.

The latest questions surrounding Herrity's disclosure practices drew mixed reactions yesterday from fellow board members.

Supervisor T. Farrell Egge (R-Mt. Vernon) said Herrity should have disclosed the contributions "if he had known" about them.

"It's difficult keeping track," said Egge, who said he finds it difficult to fault a supervisor for occasionally overlooking a donation in light of the large numbers of campaign contributors and land use applicants.

Supervisor Audrey Moore (D-Annandale) said board members have an obligation to keep track of their campaign contributions and to inform the board before reviewing the application of a donor.

"We should all be able to do it," Moore said. "If you take contributions, you ought to report them."

Board Vice Chairman Martha V. Pennino (D-Centreville) attributed the apparent violations to possible "sloppy staff work" by Herrity's aides.

Horan said his investigation will include Herrity's vote in January authorizing the payment of $8 million in industrial revenue bonds to finance a computer facility in the Springfield district. Herrity reviewed the bond request by Artillery Hill Limited Partnership, then voted for it without disclosing that he had received a $200 campaign donation in 1983 from Herbert N. Morgan, a general partner in the project, according to county records.

Herrity again failed to disclose the donation from Morgan on March 10, when he supported Dulles Corner Associates' request for an exemption to county height limits for its office park and hotel project near Dulles International Airport. Morgan is a partner with the Webb Sequoia Co., an owner of the development site.

Because of the expiration of the one-year time limit on prosecution, Horan said he will not examine Herrity's support on May 20, 1985 for Webb Sequoia's request for another exception to the county's height limitations for the Dulles-area office park project.

On Oct. 21 Herrity voted in favor of an application from Hallmark Bank & Trust Co. of Virginia for a zoning waiver to permit construction of a drive-in bank in southern Fairfax. Herrity did not disclose a $250 political donation he received in 1983 from John P. DiGiulian, an officer of Hallmark, county records show.

Horan said he is continuing his investigation into whether Herrity violated state or county public disclosure laws when he failed to reveal his joint ownership of an office condominium unit with the principals of Hersand Builders Inc. Herrity participated in the board's consideration earlier this year of the rezoning of property in the Springfield district to allow the Fairfax development firm to build 137 town houses.