This document discusses existing buildings' potential impact on mitigating climate change and achieving net zero carbon targets. It asks questions about where building owners should start with retrofits, why improving the building envelope is key, and how realistic achieving net zero is for old buildings. The document notes that existing buildings need to reduce energy use to around 20 kBtu/sf/yr to meet targets, compared to 85-100 kBtu/sf/yr for typical existing buildings, and discusses key performance indicators for measuring progress to net zero, including energy use intensity and greenhouse gas emissions. It aims to address whether the 2050 net zero carbon targets can realistically be achieved.