As part of a major expansion program in the Washington area, Marriott Corp. celebrated yesterday the completion of the roof of its new 236-room tower at the Key Bridge Hotel.

The new structure -- which will give the hotel 561 rooms -- will make the facility the largest of the five Marriott hotels in the Washington area and the largest hotel in Northern Virginia.

Before an audience of company and regional officials and local corporate executives, J. W. Marriott Jr., the company's president, linked the hotel's growth to the boom in Northern Virginia.

Marriott emphasized the importance of the company's expansion plans here, a program that includes the recent opening of the 307-room Bethesda Marriott and the construction of new hotels in Tysons Corner, in the downtown West End neighborhood and along the Pennsylvania Avenue corridor between 13th and 14th Streets NW.

In addition, Marriott said in an interview that the company "is still negotiating" with investors about plans to build a second Crystal City Marriott.

Speaking to more than 100 guests at a reception, Marriott said the new tower at Key Bridge will give the company more than 1,900 rooms in Washington and with the completion of the new projects, the total will reach 3,500 rooms.

Expansion of the Key Bridge Hotel includes the addition of a 4,600-square-foot ballroom and an indoor-outdoor pool and a health club, with memberships available to the public.

The tower and most of the new facilities will be ready for full use on Dec. 15, with work on the construction of new and remodeled restaurant facilities scheduled for completion next year.

"We couldn't have done it without the free enterprise system and the enlightened leadership of the Arlington County Board," Marriott said.

The company's largest project in the city is the construction of the 781-room Pennsylvania Avenue hotel, a part of a $110 million project planned by the company and the Quadrangle Development Corp. as a part of the city's redevelopment of that historic stretch between the White House and the Capitol.

The complex will include the hotel, a three-level shopping mall and 450,000 square feet of office space. Construction is expected to begin next year and could be completed by 1983.