Roger W. Snyder, Prince William County's director of planning for eight years, announced yesterday that he is leaving to become the head of Northern Virginia's leading development lobby.

Snyder, who earned wide respect as Prince William's chief planner during some of the county's most rapid growth, said he will take over Dec. 7 as chief executive officer of the politically potent Northern Virginia Building Industry Association.

He will succeed Michael Forbes, who left after a short tenure as head of the association amid reports of administrative problems.

With about 1,500 members, including large and small players from all quarters of the thriving development industry in Alexandria and Arlington, Fairfax, Loudoun and Prince William counties, the association is one of the region's most powerful interest groups.

As its head, Snyder will be its leading spokesman, a role that may take on added importance with the elevation next year of Audrey Moore to chairman of the Fairfax County Board of Supervisors and the election Tuesday of other candidates in Fairfax that seem to herald a slower-growth era in Northern Virginia's most populous locality.

Snyder struck a conciliatory tone yesterday. "There are many goals of the building industry and local government that are truly one and the same," he said.

A 44-year-old Dartmouth graduate who lives near Manassas, Snyder was paid about $63,000 as Prince William's planning chief. He declined to reveal his salary at the association.

Snyder said he is concerned that his departure will be perceived incorrectly as a response to Tuesday's election results. In fact, he said, he had been planning for some time to enter the private sector.

Prince William County Executive Robert S. Noe Jr. said yesterday he will appoint an interim planning director soon. Like many development-related government agencies across Northern Virginia, the Prince William planning office has had a chronic problem of losing employes to the private sector.

Although Snyder had sometimes clashed with Prince William supervisors, those contacted yesterday said they had benefited from his expertise. Supervisor Edwin C. King (D-Dumfries) said Snyder is an "excellent, hard-working, tremendously qualified employe who will be sorely missed."