When the Marriott Corp. decided to build a seven-story addition to its corporate offices in Bethesda it never thought it would have to go into the bridge building business to do it.

But the huge company found last week that building a bridge was only part of the price for expanding its headquarters in Montgomery County, where traffic congestion has become an unpleasant fact of life for thousands of commuters.

Before the Montgomery County Planning Board would approve the project, the firm also had to agree to encourage its employes to use public transportation.

Both moves were designed to bring the company into compliance with the county's tough Adequate Public Facilities (APF) ordinance, which regulates development according to the availability of roads schools and other public services.

"I guess it was somewhat unexpected, but we came to the conclusion that we should play a part in contributing to the welfare of this area," said Robert T. Souers, a company spokesman.

Ironically, Marriott was not legally required to comply with the law. It did so voluntarily. In the future, however, developers may not have a choice.

Planning Board Chairman Norman Christeller said this week he intends to recommend legislation to the County Council to extend the ordinance to cover such projects, which now fall outside the law.

"If there is going to be expanded development on a property that went through the APF at one level, then the new development doesn't have to go through it again," he said. "I consider it to be sort of a loophole."

The legislative effort is the latest in the county's running battle to get a grip on traffic congestion in fast-growing areas.

The APF ordinance was enacted by the County Council in the 1970s precisely to prevent development from outstripping the construction of new public facilities.

Last fall, in the face of growing traffic congestion in some parts of the county, the council amended the ordinance to tie development more closely to actual road construction.

"The council keeps asking us why there are areas where there is more traffic than we thought the APF would have allowed to occur," said Christeller. "This may be the one of the problems. It's a loophole in that sense."

In the Marriott case, the company proposed adding a 140,000-square-foot addition and a new garage to its 537,000-square-foot corporate headquarters.

The addition would house up to 500 new employes, while the garage would add 386 new parking spaces and boost overall parking from 1,835 to 2,211 autos.

"Because Marriott went through [the APF] at an earlier point in time, no limit was imposed on their property," said Douglas Alexander, coordinator of urban design at the planning board.

Marriott's headquarters sits on a tract bounded on the north by Democracy Boulevard, on the south and east by I-270 and on the west by Wildwood Shopping Center.

The property, known as the Davis tract, is just over 100 acres. Development is underway on at least two other sites there, and traffic congestion is already at critical levels, said Alexander.

"Recognizing the extreme problem we have in the Davis tract, the board realized that Marriott should do something. They were not legally required by law. But they were prepared in the end to negotiate something with us," he said.

Under the agreement, Marriott and two other developers will help pay for a bridge over I-270 linking Fernwood Road with the west end of WestLake Drive.

Each company's share of the cost is based on the number of auto trips that will be made over the bridge from their property.

The company has also agreed to set up a ride-sharing program and to hire a transportation coordinator to run it. The company will give employes information about bus routes and will offer subsidies on fares on a sliding scale for six months, Alexander said.