The District of Columbia's historic landmarks commission rejected yesterday plans to erect 180 town houses on the stately Tregaron estate in Northwest Washington and told the developer to cut back his proposal if he wants future approval.

Moments after the action, a spokesman for the developer said plans already are on the drawing boards to reduce the project to about 135 units. The landmarks panel's action and the developer's announcement were immediately hailed as a victory for residents of the adjacent Cleveland Park and Woodley Park neighborhoods, who have fought for years to maintain the wooded 20-acre estate as an island of tranquility in the midst of the city.

Tregaron, once the home of the late lawyer and diplomat Joseph E. Davies at 3001 Macomb St. NW, is the largest remaining property owned, undeveloped tract in Northwest Washington. It has been the subject of turbulent legal disputes among the Davies heirs and neighborhood controversies sparked by a series of development plans.

Last year, after a bitter squabble among the heirs, a D.C. Superior Court judge ordered the sale of the property for $4.6 million. A six-acre section containing the 69-year-old neo-Georgian mansion went to the Washington International School. The remaining 14 acres was acquired by the Tregaron Development Corp., which had hoped to erect 180 town houses selling for an estimated $200,000 each.

Since the Tregaron mansion and its grounds were designated a historic landmark in 1978, the development project was faced with a extra bureaucratic hurdle -- review by the D.C. Joint Committee on Landmarks, the panel that refused its approval yesterday by a unanimous vote.

"The proposed plan," the landmarks committee report declared, "will dramatically and negatively alter the curvilinear roadway with its stone walls and bridges which winds through the estate . . . The committee recommends that the applicant restudy the overall concept for the purpose of reducing the density of development . . . ."

Parodoxically, the lawyer for the development corporation, E. David Harrison, emerged from the meeting room voicing praise for the committee action and promising to abide by its recomendations. "We think this is an excellent step forward" for both sides, he said, noting that "we're down to about 135 units" in replanning the project.

On the specific item that raised neighborhood objections, Harrison said the original so-called "causeway" leading from Klingle Road into the estate will be kept and restored and not covered over to form a structure shown on the rejected plans as a "teahouse."

Jacques DePuy, lawyer for Friends of Tregaron, a neighborhood group opposed to the plans, said the landmarks commission "has taken the right approach" in suggesting a scaled-down development.

Charles Szordai, a member of the Cleveland Park-Woodley Advisory Neighborhood Commission, called the decision a victory for citizens. The commission had opposed the 180-unit plan.

On another controversial matter, the landmarks commitee postponed action on George Washington University's plan to erect a high-rise office building in the 2000 block of I Street NW that entails what critics describe as the partial demolition of Red Lion Row, a group of 19th century town houses. tThe committee formed a special panel to seek a compromise during the next month.