Most conversations about net-zero buildings focus on new construction. The harder problem is the existing housing stock. The U.S. Department of Energy's Advanced Building Construction Initiative supports research into industrialised approaches to this challenge, including prefabricated envelope panels, integrated mechanical packages, and digital scan-to-model workflows intended to reduce on-site time. The Energiesprong model, developed in the Netherlands and expanded to other markets, shows one version of this approach: factory-built façade and roof systems designed to fit specific building typologies, pre-integrated with mechanical systems, and installed in days rather than weeks. The appeal is repeatability: if a retrofit package can be standardised for common housing types, it may be possible to deliver it at scale and cost. The practical barriers are significant. Dimensional variation in existing structures complicates factory production. Moisture management at the junction between old fabric and new panels requires careful design. Contractor capacity for a new installation method takes time to develop. Financing at household level remains complicated in most markets, and "net-zero annual energy" claims that include rooftop solar are not the same as improving the thermal performance of the building fabric. DOE's programme page does not publish individual project costs or timelines, and demonstration projects should not be generalised to the wider housing stock without evidence of replication at scale. Editorial caution: Separate site energy from source energy from net-zero annual energy claims. Clarify whether a solar array is part of the retrofit package. Do not extrapolate demonstration installation times to average contractor performance.