The chief Fairfax County jailer pleaded guilty yesterday to using prisoners to build a shed at the home of Sheriff James D. Swinson and was fined $25 by a county General District Court judge.

Substitute Judge Robert C. Watson said after the court hearing that he fined John O. Feehan $25 because "I thought that's what he deserved." Feehan could have received a maximum penalty of $1,000 and a year in jail.

Feehan, who testified earlier this month that he had not used prisoners at the sheriff's home, had been scheduled to appear in court tomorrow, but Fairfax Commonwealth's Attorney Robert F. Horan Jr. said Feehan told him he wanted to plead guilty earlier than scheduled Feehan could not be reached for comment.

Swinson said he suspected Feehan for one week as a result of the incident. Feehan will lose more than $300 in pay, Swinson said. Feehan has retained his position as chief correctional officer at the jail, Swinson added.

"I think it got blown out of proportion." Swinson said of the incident. "It never should have happened. It was a case of bad judgment," by Feehan.

The incident began with a Dec. 21 hearing at which Ronald Lawhorne asked to have his eight-year man-slaughter sentence reduced. Two deputy sheriffs gave him strong recommendations and Circuit Court Judge Richard J. Jamborsky reduced his sentence by 2 1/2 years.

Earlier this month Jamborsky requested a further hearing into Lawhorne's case because a deputy sheriff had told another judge that the deputies' recommended the sentence reduction for reasons other than his good work as an inmate.

Jamborsky also called for the Jan. 10 hearing to determine why Lawhorne was still in the county jail rather than in the state prison system.

Feehan testified at that hearing that he had not used Lawhorne or another inmate jailed for robbery to work at Swinson's house. Swinson said at that time he hired Feehan and Feehan's uncle to build a 12-foot-by-16-foot shed at his home on their own time and that he did not know whether any inmates had been used at his home.

Lawhorne testified that he had poured concrete for the shed for about two hours.

At that hearing, Feehan testified that Lawhorne asked him several times to let him do the work at Swinson's house so he decided to let him do it. Lawhorne once was a foreman at an Alexandria construction company.

Horan said the county police are investigating furthur allegiations of illegal of prisoners outside the count y jail.