Owned and occupied by Patriquin Architects, the 1816 “King’s Block” building is the oldest remaining Federalist-style commercial building in southern Connecticut. In order to extend the lifespan of the 200-year-old masonry building, minimize our energy impact, and improve interior environmental quality, we are pursuing a PHIUS-certified deep energy retrofit that will make the building “Zero Energy Ready.” This retrofit requires three key strategies: 1) an entirely new “exterior” envelope on the inside of existing brick walls, roof, and basement ceiling, 2) replacing the existing gas-fired appliances with all-electric, high-efficiency systems, and 3) installing strategic areas of on-site photovoltaics to help offset our evolving commercial electrical load. Currently under construction, the project provides many lessons-learned for future historic masonry retrofits. Regulatory challenges included designing a retrofit that was entirely on the building’s interior in order to meet strict Local Historic District Commission requirements. Technical challenges included large scale (an insulated, moisture-prone basement that was ultimately excluded from the conditioned envelope) and detail scale (how to design exterior masonry walls to be a vapor-open rainscreen). Construction has highlighted the need to address structural or material damage prior to closing up the envelope tightly, and many challenges with air-sealing in an old building.