The Fairfax County Planning Commission last night unanimously recommended approval of a developer's $400 million plan to build a new downtown for Centreville, now a mostly rural town in one of the last largely undeveloped areas in the county.

If the County Board of Supervisors approves it after a July 20 public hearing, the project's primary developer, Cadillac-Fairview Co., will build a 125-acre complex called Trinity Centre.

It would include nine office buildings ranging from five to 10 stories, a 300-room hotel, 764 garden apartments, a movie theater and a three-acre lake.

The county planning staff initially recommended rejection of the project, but reversed that stand after the developer made some concessions.

Among its major concessions, Cadillac-Fairview now offers to provide for the design of the interchange at Rtes. 28 and 29 and promises to phase in the construction of no more than half of the project over the next six years.

Planners had been concerned that the project did not offer enough residential units compared with commercial space, but county officials said last night that they would not press the point since the traffic problems had been resolved.

The final product that came before the commission last night was the result of more than a year and a half of meetings among the developer, citizen groups that are seeking to control the area's growth, and the county.

During a two-hour hearing, the project was supported by most county officials and resident speakers.

Richard Korink, chairman of a task force that drafted a comprehensive plan for the Centreville area, praised the project and said he was impressed at the millions of dollars in road improvements planned by the developer. Korink, one of eight speakers, called the project "outstanding."

In an unusual move, Board of Supervisors Chairman John F. Herrity appeared at the hearing and told the commissioners: "I'm suggesting we move the show one way or the other." Herrity, who is running for reelection, emphasized that he has formed no judgment on the merits of the plan.

The plan has been scaled back considerably from the time citizens learned of it more than a year and a half ago and mounted a battle against it.

The proposed project has been reduced by about 500,000 square feet of office space, and the maximum height of the office buildings has been reduced from 16 stories to 10. Community activists said the development now is in line with the county's comprehensive plan for the area. That plan acknowledges that major development in Centreville is inevitable, but urges that it be controlled and reflect the area's predominantly rural character.

The Cadillac-Fairview project is one of several developments planned for the Centreville area that could generate more than 6,000 residential units and several million square feet of office, retail and hotel space over the next 20 years, planners said.

Ten percent of the 764 apartments in Trinity Centre would be reserved for low- and moderate-income families.

The commission's vote for the project was followed by applause from some of the more than 100 citizens who attended the meeting.