In September 2009 I completed a deep energy retrofit ("A Cost-Effective Energy Retrofit," 8/10) on a small house in Point Reyes Station, Calif., some 50 miles north of San Francisco. The owners were so pleased with the results they asked if I would build a new 750-square-foot rental unit in the back of their property, designed to meet the demanding Passive House energy-use standard. (Because affordable rental housing in California is in such short supply, the state encourages the construction of such second units - though they're limited to a maximum footprint of 750 square feet.)I began work in February...