Welcome to How They Pulled It Off, where we take a close look at one particularly challenging aspect of a home design and get the nitty-gritty details about how it became a reality. The largest certified Passive House project in the South isn’t where you’d expect. In Austin’s Castle Hill Historic District—a charming, walkable neighborhood of late 19th-century homes just steps from bars, restaurants, and downtown—an unassuming lot conceals a newly built 5,000-square-foot duplex and a freshly revived 100-year-old bungalow. For architect Trey Farmer of Forge Craft Architecture + Design, Passive House-building has become his calling card—a rigorous standard that emphasizes airtight construction, high-performance windows, and more moves that cut energy use by up to 90 percent. His own Austin home, a 1914 Craftsman, was one of the first Passive House Institute US (PHIUS)-certified projects in the region with a design that results in an average monthly energy bill of just $10.