In 1912, she acquired 554 acres (224 ha) of land in Hillsborough,[21] on which she intended to build a house and garden that would excite "the wonder and admiration of America" and reflect her many refined and cultivated interests.

Harriett Carolan commissioned plans for the chateau from the Parisian architect Ernest Sanson, at the time France's foremost designer of prestigious private houses.

In his original, ambitious design for Carolands, Duchêne planned miles of roadways leading across extensive grounds, landscaped with thousands of shrubs and trees, accented by fountains and statuary.

His plans included a dry moat around two sides of the house, discreetly located to provide light and air, and access, to the service spaces in the basement, while not blocking views of the gardens from the principal rooms on the main floor.

In his design, Sanson incorporated three 18th-century period rooms that Carolan had purchased in Paris with the advice of the famous antique dealer Boni de Castellane.

In 1914, Willis Polk began grading the great terraces planned by Duchêne, sending progress photographs to the owner and her architects.

The house as completed had ninety-eight rooms, including nine bedrooms and baths for the owners and their guests, each with an antechamber to guarantee quiet and privacy.

[3] In 1947, Life Magazine published an article about a charity event held at the house,[26] the first opportunity for San Francisco-area residents to see its interior.

[4] Countess Lillian Remillard Dandini purchased Carolands Chateau in 1950,[8] saving it from demolition by speculators interested in developing the land, and uninterested in the house's architectural significance.

[27] The countess willed the house and the remaining 5.83 acres (23,600 m2) to the Town of Hillsborough to be used as a French and Italian musical, artistic and literary center,[28] but was unable to include an endowment.

[29] In 1985, David Allen Raley, a security guard, lured two high school students onto the property where he sexually assaulted and stabbed them, leaving them for dead in a ravine near San Jose.

[33] In 1986, Michael DeDomenico, an heir to the family controlling Rice-a-Roni and Ghirardelli, bought Carolands,[12] and that year developers commissioned an Environmental Impact Report in support of a proposal to further subdivide the land and build additional houses.

In 1991, the Hillsborough Designer Showhouse was held at Carolands, attracting 68,000 visitors, each paying $20 admission, netting more than $1 million for the sponsoring charity and reviving interest in the house.

[37] By valuing the mansion so highly and by obtaining tax-exempt status, the Johnsons collected more than $38 million in tax savings from the estate over five years.