The Virginia State Corporation Commission should approve a private firm's plans to extend the Dulles Toll Road from the airport to Leesburg, key commission staff members said yesterday.

In testimony filed for a June 27 SCC hearing, key agency staff members urged that the commissioners issue the certificate the Toll Road Corp. of Virginia needs before it can start work. The company still would have to obtain various permits, complete financing and acquire right of way. The SCC will set toll rates and regulate the operation of the highway.

The staff recommendation represents a shift in its position and is the most recent event that has raised the morale of Toll Road officials. The for-profit company has struggled with government and private sector obstacles in its unusual effort to extend the heavily traveled highway 15 miles beyond Route 28 to Leesburg.

The staff said uncertainties remain concerning the company's ability to build the link, particularly since -- unlike a government -- the company cannot take land for right of way. However, the staff says, the risk of committing the venture to private hands has been reduced in the two months since it publicly asked if the Virginia Department of Transportation could do a better job.

The Toll Road Corp.'s estimated construction cost and completion time have increased but remain reasonable, according to staff testimony. The company's profit margin is proposed to run as high as 20 percent on some investments.

Toll Road Corp. Chairman Ralph L. Stanley said yesterday he is pleased that the commission staff has found his application to be "in the public interest."

The development comes after the signing of a technical agreement between the company and the transportation department and the withdrawal of a breach-of-contract lawsuit and SCC protest filed by ousted toll road company president John R. Reilly.

Meanwhile, officials of several government agencies have been meeting to try to resolve the Metropolitan Washington Airports Authority's refusal to give Stanley's company permission to use Dulles property for the extension.

The authority has said that permission is dependent upon whether the state earmarks a substantial part of the profits from current toll road for mass transit in the Dulles corridor. Other right-of-way issues near Leesburg also are unsettled.

Stanley, a onetime Republican partisan and Reagan administration official, credits Democratic Gov. L. Douglas Wilder and Wilder's new transportation secretary, John G. Milliken, with bringing a sense of urgency in Richmond toward the concept of private road building. "They've made this happen, no question," said Stanley.