WILLIAMSBURG, Va. — The old mahogany furniture is shrouded in white dust covers, and the espaliered gardens overlooking the James River have gone to seed. Colonial Williamsburg is selling Carter’s Grove, an imposing 18th-century Georgian mansion and one of the most renowned plantations in Virginia.

Colin Campbell, Williamsburg’s chairman and president, said he had tried to interest other preservation groups in the property, with no luck. And so the 400-acre riverfront residence, closed because of declining attendance and shifting priorities, will be available for private purchase at a price local agents estimate could be well over $20 million. “Perhaps in January,” Mr. Campbell said. “We don’t want to linger.”

Although it will be protected by easements to prohibit subdivision, there will be no requirement that Carter’s Grove be open to the public.

The sale by Williamsburg, the country’s biggest and most prestigious living history museum, has riveted preservationists’ attention on the plight of hundreds of other house museums across the country that have either closed or are struggling to stay open in the face of dwindling interest, diminished staff and lack of endowment dollars.