The 2549-square-metre estate across the road from the exclusive private school was bought by wealthy eastern suburbs property developer Eduard Litver.

The English Manor-style residence was sold to the school under the hammer in 2013 for $15.5 million by the estate of the late arts philanthropist Claire Dan to be part of the school's expansion plans.

However, those plans were abandoned earlier this year and it was returned to the market in September with a guide of $18 million-plus with Elliott Placks, of Ray White Double Bay, and Bill Malouf, of LJ Hooker Double Bay.

Neither agent would comment when asked about the buyer's identity or sale price, except to confirm the property has sold.

Independent sources say Mr Litver was one of at least two buyers circling the property in recent weeks, and the exchange was finally approved by the school's board late on Wednesday.

When approached for comment, Mr Litver said he and his family plan to make it their permanent residence.

"It's very close to the school, which is appealing to us, and we are part of the Cranbrook community, so we plan to move into it," he said.

First, however, he said they have plans to either renovate the house or build a new home on the Victoria Road property.

Given the bull run on prestige sales in the four years Cranbrook owned the property the sale has netted a capital gain of more than $2.5 million.

Thanks to section 275 of the Duties Act the school would have been exempt from paying the $1,025,490 stamp duty on the purchase.

Mr Litver and his developer partner Allen Linz are best known for the redevelopment of the old Swiss Grand Hotel at Bondi into the Pacific Bondi Beach. The development not only claims some of Sydney's most expensive apartment sales and high-profile residents like heiress Ginia Rinehart and fund manager Will Vicars, but won the Urban Taskforce development of the year prize at last year's Development Excellence Awards.

Sundorne was originally part of the Cranbrook estate before it was subdivided in 1917 and the residence built in 1925 by solicitor Edward Percy Simpson as a wedding present for his son Edward Telford Simpson.

The late Sir Peter Abeles and his first wife Claire Dan became only the second owners of it when they bought it in 1958 for ??60,000. When the couple divorced in 1970 Dan took Sundorne as part-settlement. Dan died in 2012.