In 1911, 20 acres of Twin Oaks (see NW10) were sold to Ohio financier James Parmelee, who commissioned Charles Adams Platt to design an estate that included a gardener's cottage, carriage house, and extensive formal and informal gardens, large areas of which survive in neglected condition. When Ambassador Joseph Davies and his wife Marjorie Merriweather Post bought the estate in 1940, they renamed it Tregaron and added a wood dacha as part of their famous Russian art collection. The Washington International School is the present owner.

Tregaron's Neo-Georgian design owes more to English than to American eighteenth-century traditions. Its shallow portico of four monolithic limestone attenuated Corinthian columns encompasses three out of nine bays of its long facade. Platt's plain, even severe, basement walls, pilasters, entablature, and main door and bull's-eye window enframements are a subdued gray in contrast to the brick walls. The only sculpted elements are limestone panels that mark the division between the two stories behind the portico and the pediment's festooned circular window, both of which exemplify the refined elegance associated with the architect's decorative sensibilities. Platt's landscaping successfully masks an octagonal conservatory on the west and low service wing on the east to maintain the impression of a symmetrical and formal facade.