The director of the Prince William County building inspections department resigned yesterday, saying allegations of payoffs to his inspectors and ineffective prosecution of builders accused of shoddy construction made his job difficult.

Charles J. Vincent, the inspections chief, said his decision was influenced by the failure of county prosecutors to obtain convictions of builders charged with violating the Virginia building code.

"I went to court seven times [as a prosecution witness] on 12 different charges involving two builders, said Vincent, who is taking a $5,200 pay cut to become public works director in a suburb of Miami.

Only one of the cases resulted in a conviction. That was overturned on appeal.

"I haven't been able to accomplish what I set out to," said Vincent, a county employe for five years wanted builders guilty of violations to get held responsible."

County Manager Robert S. Noe Jr. yesterday praised Vincent's work and said he accepted his resignation from the $28,700-a-year post, with regret. Noe said Vincent did not mention the alleged payoffs or his criticism of county prosecutors when he submitted his resignation, which takes effect March 21.

Fast-growing Prince William County, on the southern edge of metropolitan Washington, has been beset by homebuyers' complaints of shoddy construction work and a state building code that critics say is riddled with loopholes.

A year-long investigation of the county's building industry also has resulted in bribery charges against five builders and one county inspector. A second inspector resigned after being implicated in the investigation and reportedly is cooperating with county and state investigators.

Vincent said yesterday he is not implicated in the probe, a statement confirmed by county prosecutors.

Vincent said he had sent out warning notices to several contractors accused of building code violations, but stopped seeking more criminal charges after he was unsuccessful in court.

Prosecution of the bribery cases so far has been equally unsuccessful. The first case to come to court was thrown out during a preliminary hearing Feb. 5.

General District Court Judge William R. Murphy dismissed the charge against Manassas plumber Luther Anderson after the key presecution witness, inspector Eugene Raymond Bradley, failed to identify Anderson as the person who allegedly offered him a payoff.

Chief county prosecutor Paul B. Ebert, defending his assistants' prosecution of the builders, said yesterday. "It's hard to obtain convictions because the building code is an administrative code that is not designed to be enforced criminally."

Ebert said he feels confident convictions will be obtained in the remaining bribery cases, but cautioned that "a lot of the testimony is one man's word against another."

One former Prince William builder, Stephen J. Siegel, whose controversial development of homes in the Monclair Country Club Lake subdivision triggered the payoff probe, surrendered to authorities last week following his indictment on two bribery and three false pretense charges.

Siegel, who moved to Florida after his company, Tray-Marc Construction Co. Dumfries, lost its state contractor's license, was released on bond. His trial is scheduled for May 19.