According to English Heritage (which oversees historic buildings for the British government), Woodchester House, a Georgian mansion built in 1746 and located on 30 acres of Gloucestershire countryside, is architecturally untouchable and unchangeable. For both the architect, London- and Paris-based Robert Grace, and the client, a financier and author, that was a problem.

The new 18-foot-high structure may relate to the house functionally, but hardly stylistically. “It’s totally of the ‘now,’” says Grace. Two slender reinforced concrete columns support a concrete roof, while expanses of glass diminish the sense of mass. Triple-glazed units enclosing the space are almost nonexistent, especially where they meet at a corner facing the garden: With a push of a button they glide back on an invisible motorized track. Oak plank floors unite the various parts of the retreat, which includes an entry, bathroom, laundry, and living area with a fireplace carved in a stone wall.