The private foundation formed to oversee the publicly owned mansion that once was home to Civil War Col. John S. Mosby now hopes to erect an additional building on the grounds for the Fauquier County Chamber of Commerce, a visitors center and restrooms.

These and other new costs detailed in a plan presented last week to the Warrenton Town Council would add $500,000 to the estimated price tag of a Mosby museum project intended to increase tourism.

When the town voted to purchase the historic property earlier this year for $460,000, there were no plans for an additional building at a cost of $275,000.

The new plan also raised the estimated cost of renovating the existing building.

David A. Norden, acting chairman of the John Singleton Mosby Foundation board and an at-large council member, said the new expenses will be necessary to create a "first-class" institution.

The foundation hopes the museum will be open for business in 2001.

"It is an ambitious plan," Norden said, "but eminently doable."

Norden said earlier cost estimates were only preliminary, not to be set in stone.

There is no formal agreement between the foundation and the Town Council, which is scheduled to debate a new lease for the foundation at a work session early next month.

Council member Samuel Tarr (Ward 4) said he was surprised at the foundation's revised estimates and cautioned that the new building might not fit in with the residential character of the neighborhood.

"I wouldn't want to build anything that would detract from the value of the house if it had to be resold," Tarr said, mentioning a possibility he has floated in case the foundation is unable to make the project work.

The foundation's plan anticipates $575,000 in federal and state grant money, $200,000 from charitable foundations and an additional $100,000 from corporations.

No grant money has been requested yet, Norden said, and donations in hand total about $40,000.

He predicted that the pace of fund-raising would quicken now that potential donors can see a more detailed plan.