The project to turn a graceful Montgomery County mansion into a state-of-the-art concert hall is running far over budget, according to a revised cost estimate that is bringing new scrutiny from council members who have yet to approve construction funding.

The Strathmore Hall Arts Center renovation is supposed to transform the landmark on Rockville Pike into a 2,000-seat concert hall that could serve as the summer home for the Baltimore Symphony Orchestra. Among the project's most ardent supporters has been County Executive Douglas M. Duncan (D), who has called the hall a key to bridging the regional divide between the Washington suburbs and Baltimore.

But Duncan is scheduled to present new cost estimates today that revise the price from $68 million to $88.9 million while recommending that the completion date be pushed back by at least six months. The new figure is the latest in a series of rising cost estimates--the projection was $50 million when it was proposed two years ago--that have all come before a single spade of dirt has been turned.

The latest increase "is going to raise a number of questions," said County Council President Michael L. Subin (D-At Large), who along with other council members will receive a briefing from Duncan this morning. "These are folks who normally have answers, but whether the council accepts them and appropriates the money I don't know."

Montgomery officials attribute the cost increase to the complexity of the project's design and to the hilly 11-acre site. The initial estimate was based on a conceptual design that did not fully account for the challenges posed by the property, which will require more grading and paving than expected to provide adequate access to Rockville Pike. The building design also has been changed to enhance acoustics, enlarge the lobbies and improve the facade with limestone and glass.

The higher price comes as the Duncan and the council begin considering budget priorities for the coming fiscal year. The council has approved $6 million toward the project's design--half of the total planning cost--but has not signed off on any money for construction. The project is being done by the team that built the award-winning Seiji Ozawa Hall at Tanglewood in Lenox, Mass. They have recommended extending the construction schedule from a projected finish in the fall of 2003 to the spring of 2004.

The higher cost estimate also arrives two days before Duncan makes his annual visit to Annapolis to present the county's capital wish list to Gov. Parris N. Glendening (D). Because Strathmore Hall is designed to hold concerts with regional appeal, the state government previously had agreed to share the cost of the renovation with the county.

Glendening, who has had a rocky relationship with Duncan, has already included $14 million for the project in his proposed budget for next year. But Duncan said he did not know how the governor would react to the new figures.

"It's a great project and something that will become the cultural centerpiece in Montgomery County for decades to come," Duncan said. "It's worth doing right. We'll see what the council says and we'll see what the governor says. Hopefully, he'll be receptive."

CAPTION: The Strathmore Hall Arts Center plan is to transform the landmark on Rockville Pike into a 2,000-seat concert hall.