The board of directors of the Strathmore Hall Arts Center on Rockville Pike is expected to approve a plan soon for expansion of the Montgomery County-owned arts center into an arts park with gardens, an indoor concert hall and a museum -- the first such complex in the Washington area.

The expansion, which would take 10 to 15 years, would require an estimated $40 million in public and private funds, a spokesman said.

The 40-room, 72-year-old Strathmore Hall mansion, situated on 11 acres north of the Grosvenor Metro station, is used for art exhibits and rented for private social events.

Renovated by the county in 1983, it is operated by the private, nonprofit Strathmore Hall Arts Foundation.

Eliot Pfanstiehl, Strathmore Hall's managing director, said the park would be unique in the Washington area and help preserve a stretch of green space along the heavily traveled commercial corridor that has long been eyed by developers.

"Everybody has his own idea of what an arts park is," Pfanstiehl said, "but I don't know of anything else quite like this around here."

The mayor of nearby Garrett Park, Peggy Pratt, said she is concerned that additional development of Strathmore Hall could intensify traffic congestion in the area. But Pfanstiehl said Strathmore's location near a Metrorail stop would minimize traffic problems.

The board of directors of Strathmore is expected to approve the master plan at its May meeting, Pfanstiehl said. The plan would ultimately need county approval to take effect.

Drawn up by the Arlington consulting firm of Dewberry & Davis, the plan calls for development in two phases.

The first phase would add a 400-seat conservatory, bandstand and formal gardens.

The second calls for the acquisition of 10 adjoining acres and the construction of a museum, a 1,000-seat indoor concert hall and underground parking.

To pay for the expansion, "A private bequest would be best," Pfanstiehl said. "A corporate and business campaign to raise money also may be launched.

"And it is conceivable that the public sector -- local, municipal or state -- may help out, but we expect that it would be done in cooperation with private contributions."

Last year, the American Speech-Language-Hearing Association, which owns the property next to Strathmore Hall, proposed a rezoning of its land to allow commercial development and the construction of an office park at Rockville Pike and Strathmore Avenue. After a flurry of controversy about the proposal, the association withdrew its rezoning request.

"That idea didn't fly, so we went out and hired consultants to develop a master plan for Strathmore Hall Arts Center, in keeping with our own guidelines, and where we wanted to go with long-range development: a year 2000 plan," Pfanstiehl said.

The first recommendation was for the 400-seat conservatory, with a classical motif to match the mansion and a stage for indoor events, he said. The conservatory would be in a separate building connected to the mansion.

The grounds would be landscaped and an English sculpture garden and a terraced sculpture garden added.

The second recommendation hinges on the acquisition of 10 acres from the American Speech-Language-Hearing Association. This property would be the site of the museum, auditorium and parking.

The master plan was presented to four members of the Montgomery County Council last month. "We didn't request funds from them to build this, although we would like them to be partners in the development," Pfanstiehl said.

Neal Potter, one of the four council members who heard the presentation, called it "an attractive proposal," but he questioned whether Montgomery needs another 1,000-seat auditorium.

"We have a lot of high school auditoriums that are available for indoor concerts," Potter said.

As a result, he said, "We have to look at the cost versus the benefit, and I don't think we have enough data on usage and need that would fully justify the cost yet."

Pratt, the Garrett Park mayor, said that she also has reservations about the proposal for the auditorium, primarily because of parking and traffic.

"We generally support the Strathmore Hall idea, but we are generally concerned about any kind of development that will produce traffic on Strathmore Avenue," she said. "We will have to study what the traffic would be."