As part of an effort to improve bus maintenance in the Washington area, Metro officials began construction yesterday of a new bus garage in Prince George's County.

The $12 million garage, scheduled to open in mid-1985, will be the second bus maintenance facility built by Metro since the transit authority took over the region's privately owned bus companies 10 years ago. Last September, Metro opened a $10 million bus garage in Montgomery County.

The garage will be on a 10-acre site in the Ardwick Industrial Park in Landover. It will accommodate 200 buses traveling chiefly on Prince George's routes and will replace a 41-year-old garage on Southern Avenue in the Coral Hills area.

The existing Prince George's Division garage, formerly owned by the old WMA Transit Co., was described yesterday by Metro officials as outmoded, cramped and poorly equipped.

Metro officials are also searching for a site in Southeast or Southwest Washington to build a new bus garage and are studying possible sites for garages in Northern Virginia. The transit authority, which owns nine bus garages, also plans to renovate several maintenance facilities in the District.

The start of construction at the Landover site was marked by Metro and Prince George's County officials at ground-breaking ceremonies yesterday afternoon.

Because the site is in a commercial area surrounded by warehouses, the proposed garage drew no significant opposition from county residents at public hearings, officials said. "It will enhance the transportation system in that area and, as a result, I think everybody's supportive of it," said Floyd E. Wilson Jr., vice chairman of the Prince George's County Council, who represents a district that includes Landover.

Metro officials said the new garage will be streamlined to allow faster bus repairs. "It's going to be more cost effective," said Shirley DeLibero, Metro's bus operations chief. Officials also said it is designed to be safer and is likely to help boost morale among bus mechanics.

When the new garage opens, a Metro spokesman said, it will take 5 1/2 to 7 minutes to refuel, clean and service each bus--about half the time required in older facilities.

The new garage, financed mainly by federal funds, is also expected to help cut costs, officials said, by reducing the amount of time needed to drive buses to the starting points of their routes, because it is closer to the beginning of some Prince George's routes than existing facilities.