Montgomery County Executive Douglas M. Duncan sought to reassure council members yesterday that the county had contained the creeping cost of building a landmark concert hall, but he would not rule out the possibility that the price could go higher.

Duncan (D), during a breakfast meeting with the County Council, said that the estimated cost of building the Strathmore Hall Arts Center in North Bethesda has risen from $68 million to $88.9 million--a 31 percent increase. He said the site and building design have proved more challenging than anticipated by several consultants hired to study the issue.

"I have told them [the project managers] that we can't go back for more money after this," Duncan said, defending the project. "The only caution I would add is for inflation. When it opens four years from now, who knows what could happen."

Strathmore Hall is the latest in a line of county public works projects that, taken together, will cost at least $50 million more to complete than originally proposed. The price of renovating the historic Silver Theatre has more than doubled, for example, bringing the overall cost increase of the Silver Spring redevelopment to more than $30 million. Other projects, including the Clarksburg jail and the conference center, have been scaled back partly to keep costs down.

In the case of Strathmore, county officials originally proposed spending $50 million to build a 2,000-seat concert hall on the same property as a historic Montgomery mansion on Rockville Pike. That figure emerged from a study conducted by the same architectural firm that designed the award-winning Seiji Ozawa Hall in Tanglewood, Mass.

Strathmore officials explained yesterday that the 11-acre site's rolling topography has required more engineering work, including the construction of retaining walls. The project also will feature items not on the original plans, such as a pedestrian bridge to the Grosvenor Metro parking garage and a limestone-and-glass facade. And designers say they have only now determined that a below-ground floor must be built to house the air conditioning system so its whir won't interfere with the music.

Robert K. Kendal, Montgomery's budget director, said the inaccurate estimates can be partly explained by the county's inexperience in building such complicated projects. The booming economy, he said, also has pushed up prices for building materials and labor that has resulted in higher costs.

"We do that process well in some cases and not so well in other cases," Kendal said. "Especially with unusual projects, we are not as precise at any given moment in time [as] when we work with more common projects."

But council members have started to question how Duncan and his staff develop the initial estimates that, in many cases, are used to sell the projects to the public and elected officials.

"We can't keep chasing moving targets on every large project," said Council President Michael L. Subin (D-At Large).

The state is sharing the cost of Strathmore with the county, and Duncan is scheduled to meet with Gov. Parris N. Glendening (D) tomorrow to discuss the project along with the rest of the county's capital request for next year. The governor has included $14 million for the project in his budget proposal for next year, though Duncan is unsure how Glendening and other state officials will react to the increased costs.

"We'll work with them, but we're only going to pay a certain amount of money," said state Sen. Barbara A. Hoffman (D-Baltimore), chairman of the Budget and Taxation Committee and a supporter of the Strathmore project. "I couldn't tell you today where that line will be."

The council has approved $6 million in planning money for the Strathmore project. But members still must approve construction funds. Despite the increased cost, most seemed inclined to stand by a project Duncan calls "a once-in-a-lifetime opportunity."

"I can't believe we didn't have a better estimate about what this was going to cost," said council member Nancy Dacek (R-Upcounty). "The fact they didn't know the air conditioning had to go underground--that just seems so basic. It's very disconcerting."