Legacy Building

ADUs — small, self-contained backyard homes with their own bed, bath, and kitchen — are getting attention as a way to add housing without new lots, and builders are finding they lend themselves well to energy-efficient construction. More cities and states now allow them "by right," and some offer help: Los Angeles piloted a $75,000 forgivable loan for ADUs used as affordable housing, and California even allows ADUs to be sold separately from the main house in some areas. Homeowners build them to house family members, generate rental income, or set up a place to age in place later.

Because ADUs are small, new-build projects, they're a good testing ground for high-efficiency techniques. Some builders are pushing toward Passive House standards (very tight, heavily insulated construction measured by ACH50, or air changes per hour under pressure testing). One modular ADU in Massachusetts hit an airtightness rating better than Passive House requires, plus a net-positive energy score, and cost $275,000 versus an estimated $400,000 for a site-built version. Builders are also using structural insulated panels (foam sandwiched between wood sheathing) to speed construction, though some avoid the foam for environmental reasons, and swapping concrete foundations for helical piles (screwed-in steel posts) or frost-protected shallow foundations to cut cost and carbon.

Homeowners in the featured case studies added ADUs for reasons like housing aging parents, earning rental income, or replacing a burned-down garage, with features like mini-split heat pumps, solar panels, and heat-recovery ventilation systems. Programs and rules vary by city and state, so what's available depends on where you live.