ANNAPOLIS, DEC. 11 -- Paying heed to the ever-expanding Washington suburbs, Maryland's Mass Transit Administration plans to begin its first commuter bus service to the District, with routes from Annapolis, Crofton, Columbia, Frederick and Charles County starting in about six months.

Until now, the MTA has focused its attention on Baltimore commuters, leaving residents of fast-developing Howard, Anne Arundel and Frederick County, as well as southern Maryland, to use private bus companies or automobiles. Areas such as Crofton in western Anne Arundel have found themselves in a land of no public transportation, outside both the MTA and Washington Metro.

"We've been stuck in the mode where MTA was part of Baltimore and had nothing to do with D.C.," administration manger Ronald Hartman told a group of Crofton residents last night. "We've finally concluded that the area is more connected to Washington than to Baltimore."

Officials in Washington's outlying suburbs praised MTA's decision today. "It's terrific," said Crofton Civic Association President David Lombardo. "You really need a car to live in Crofton. This really is the ultimate suburbanization of West County -- by finally recognizing we need public transportation."

Frederick County's transportation services coordinator, Nita Heidler, said: "It's a godsend. We're becoming quite a bedroom community of Washington, D.C., and about one-third of the work force goes in to Montgomery County or Washington."

MTA officials said they were still planning specific routes, but expect buses from Annapolis to stop at Crofton and at the New Carrollton Metro station. Buses from Columbia would end at the Silver Spring Metro, while those from Frederick would take passengers to the Rockville station. The Southern Maryland buses would probably take passengers into downtown Washington.

Hartman said he expects there to be several buses on each route for morning and evening trips, and possibly a mid-day trip on some routes.

Fares would be lower than those offered by private bus companies that now run along some of those routes, Hartman said, because the MTA is legally allowed to subsidize operating costs by up to 50 percent. Hartman could not provide an estimate of fares.

Hartman and other officials noted that the private companies carrying commuters to the District have often faced serious financial problems. Fares have risen in recent years, while the number of buses have been trimmed.

Currently, the Eyre Bus Co. runs 11 weekday commuter buses from Columbia to the District at $12.20 for a round trip. Two small bus companies run a service from northern Frederick County to Montgomery County at prices ranging from $65 to $125 a month, while the Dillon Bus Service runs five buses from Annapolis to the District and charges $30 for 10 one-way tickets. The MTA subsidized the Dillon service for four months last year, after a large insurance increase threatened to close the route.

In 1980, there were 13 commuter buses running from northern St. Mary's County and the Waldorf area to the District. Today, three Gold Line buses provide the only service with fares between $40 and $45 a week.