The private company planning to build a 15-mile extension of the Dulles Toll Road between Dulles International Airport and Leesburg says it has acquired all the privately owned right of way for the highway.

Ralph L. Stanley, president of the Toll Road Corp. of Virginia, said the final signatures were obtained yesterday after about a year of negotiations with several property owners. The road extension is targeted for completion in 1993.

Stanley's firm has set up a trust to buy about 120 acres, or one-fifth of the privately owned land needed for the toll road and interchanges, but he and other parties to the deal would not disclose the cost. Stanley said that some private landowners in the highway's path contributed money to the trust fund, but that his firm provided most of it.

The rest of the right of way was donated by landowners hoping to increase the value of their property with frontage on the road, which is designated as a major growth corridor by Loudoun County.

The Metropolitan Washington Airports Authority, which owns Dulles and National airports, has yet to give the Toll Road Corp. of Virginia several miles of right of way north of the Dulles runways. The authority wants Virginia officials to designate some of the profits from their existing toll road for mass transit between Tysons Corner and Dulles.

Once all right-of-way matters are resolved, the firm will try to arrange financing for the project, which has state approval but is awaiting permits from the Army Corps of Engineers because the road impinges on wetlands.

The toll road extension would have an initial one-way toll of $1.50. The project is the first private toll highway authorized in Virginia since 1816.