The new buzz words of the 21st century—“organic,” “ecofriendly,” “sustainable”—have inundated today’s architectural vocabulary despite their indifference to definition. When you get down to it, whether a work of architecture is “green” is usually a shade of gray.

Architect Charlotte Skene Catling, principal of the firm Skene Catling de la Peña, shrugs off any hard-and-fast characterization of the environmental principles that guided her renovation of and addition to a 1902 building in the historic, 850-acre Hadspen estate in Somerset, England. Although demolishing the old building and replacing it would have been much less expensive, she and her client saw the value in maintaining the integrity of the timeworn masonry structure and its role in the estate as a whole. Catling gutted and renovated the building from roof shingles to reclaimed wood floorboards, adding an extension clad in sheets of glass and oak that houses circulation space between the first and second floors, as well as three bathrooms. An attached, 215-square-foot pool acts as a heat sink for a biomass power source in the summer.

But more than just keep the house’s size small (just over 2,000 square feet) and minimize energy use, Catling sought to keep the project local. The oak, with matching layers of float glass that clad the extension’s second floor, come from cords stored in sheds opposite the Dairy House. Catling hired regional workers who live less than 20 miles from the site: a local cabinetmaker, who constructed the extension; a glass laminator responsible for joining the extension’s layers of glass; and a stonemason who restored the brick facades and fashioned pathways and the pool from locally quarried slate. These moves represent an equally important side of sustainabilty—what her architecture-savvy client, Niall Hobhouse, calls “social sustainability.”

Hobhouse sought out Catling, an old friend, to help him redesign the dairy with the intention of renting the house out. However, once work began on renovating the building, which had been a working cheese-making facility until the 1960s, the client saw an ideal retreat for himself, friends, and family.

Hobhouse—whose commissions of a Robert Smithson folly on the grounds and a design competition to reimagine the estate’s beloved Hadspen Parabola garden have created controversy in the landscape design field—was intrigued by the question of how to insert modern architecture in old houses. In England, where many old buildings are “listed,” or landmarked, the issue is particularly fraught. Often houses are either restored to look like period pieces, or modern extensions overwhelm and undermine the old structure. For this project, Catling proposed something in between—she discreetly inserted the addition, using transparency to dematerialize its bulk.