The property, originally part of a larger estate, "Twin Oaks", was bought in 1888 by Gardiner Greene Hubbard, founder of the National Geographic Society, and named "The Causeway".
 His daughter Mabel married Alexander Graham Bell and inherited the property, which she sold to James Parmelee, a Cleveland financier.
Parmelee hired Charles Adams Platt to design a country house for the property.
Platt employed Ellen Biddle Shipman as landscape architect for the project.

After Parmelee's death, the estate was purchased in 1940 by Joseph E. Davies and his wife.
Davies was the first chairman of the Federal Trade Commission; American ambassador to the Soviet Union (1936-1938); ambassador to Belgium and Luxembourg (1938-1939); a lawyer (1937 Law Firm:  Davies, Richberg, Beebe, Busick and Richardson); and special advisor to President Harry Truman and Secretary of State James Byrnes, with rank of ambassador at the Potsdam Conference.
 Davies renamed the place "Tregaron" (the town of Saint Caron) after the town where his father, Edward Davies, was born in Ceredigion, Wales.

After Davies's death in 1958, the Washington International School purchased 6 acres (24,000 m²) of the 20 acres (81,000 m²), and the Tregaron Limited Partnership, an Israeli corporation, purchased the remaining 14 acres (57,000 m²).
Attempts to develop the space were opposed by the "Friends of Tregaron", a community group.
In 2006, an agreement was reached whereby 13 acres (53,000 m²) of the estate are to be conserved as open green space and managed by Tregaron Conservatory.
The property, originally part of a larger estate, "Twin Oaks", was bought in 1888 by Gardiner Greene Hubbard, founder of the National Geographic Society, and named "The Causeway".  His daughter Mabel married Alexander Graham Bell and inherited the property, which she sold to James Parmelee, a Cleveland financier. Parmelee hired Charles Adams Platt to design a country house for the property. Platt employed Ellen Biddle Shipman as landscape architect for the project. 
After Parmelee's death, the estate was purchased in 1940 by Joseph E. Davies and his wife. Davies was the first chairman of the Federal Trade Commission; American ambassador to the Soviet Union (1936-1938); ambassador to Belgium and Luxembourg (1938-1939); a lawyer (1937 Law Firm:  Davies, Richberg, Beebe, Busick and Richardson); and special advisor to President Harry Truman and Secretary of State James Byrnes, with rank of ambassador at the Potsdam Conference.  Davies renamed the place "Tregaron" (the town of Saint Caron) after the town where his father, Edward Davies, was born in Ceredigion, Wales.
After Davies's death in 1958, the Washington International School purchased 6 acres (24,000 m²) of the 20 acres (81,000 m²), and the Tregaron Limited Partnership, an Israeli corporation, purchased the remaining 14 acres (57,000 m²). Attempts to develop the space were opposed by the "Friends of Tregaron", a community group. In 2006, an agreement was reached whereby 13 acres (53,000 m²) of the estate are to be conserved as open green space and managed by Tregaron Conservatory.
Tregaron Estate