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. “We’re aligned with our clients and our builders,” Farmer says. “We’re being sought out for it. The market has caught up with us a bit.”

Originally designed as a custom home for a young family, the plan shifted to three long-term rentals, each large enough for a family. “We think of Passive House from a sustainability standpoint but also in terms of density,” Farmer says. Because of the slope, the duplex garages are offset by six feet, which creates five staggered, finished levels. “There’s a lot of density without it feeling dense,” he adds.

The front house, a 1924 bungalow, was remodeled with help from Farmer’s wife, interior designer Adrienne Farmer, who led the materials and finish selections to align with sustainability standards—opting for reclaimed shiplap and formaldehyde-free cabinetry.

The duplex, comprising a 3,500-square-foot unit and a 1,600-square-foot unit, features wood-clad interiors and skylights, and is carefully integrated into the challenging site. “It was crumbling and unsafe,” Farmer says. “It’s almost twenty feet of grade change on a fifty-foot-wide lot—feels like you’re in San Francisco, not Texas.”

How they pulled it off: Passive House precision on a tricky lot

Mastering the mud: Building in Austin means dealing with clay soil, which can make for some muddy days. “We drove seventy-eight steel piles into the ground until friction stopped them—basically like a hairbrush in mud—then tied them together with concrete footers and floating foundations,” Farmer says. The excavation was further slowed when crews hit a natural spring. “We had to dam the spring long enough to pour the retaining walls.”
Refining the Passive House playbook: Farmer took lessons from his own Passive House for the process, keeping the form and framing simple, and “insulating like crazy,” he says. Passive House, a German-born building standard, is all about reducing energy through impermeable construction and efficient ventilation. AeroBarrier, a mist-based sealant applied during pressurization, took the home from “very airtight to extremely airtight,” says Farmer. (While the house already five times tighter than code, Passive House standards require 10.)
Performance on a budget: During design, $1 million was cut from the budget. The team spent just two-and-a-half percent extra—about $75,000—to hit Passive House standards, which include full stormwater reuse, heat pump systems, and net zero-ready infrastructure. “You can’t hear any outdoor noise,” Farmer notes. “The comfort and air quality are unmatched.” The project uses Marvin windows, a metal roof, Zip-R sheathing, and blown-in recycled newspaper cellulose insulation as well.

“Passive House seems like a solvable problem to me—that’s what attracted me to it,” Farmer says. “We’re in a good spot to use single-family homes like this as a sandbox for what’s possible.”