When you acquire an existing building that needs renovations, you inherit more than its contents. You also inherit the building’s carbon footprint—the sum of all the greenhouse gas emissions from the extraction of raw building materials to their current state—and you decide the building’s future carbon footprint through its renovation, operation, and ultimate demolition. Yet most embodied carbon tools assume a clean site and brand-new structure. What environmental impacts from the past do we own? How significant are the impacts we create when we renovate with a seismic retrofit? This case study examines UC Law San Francisco’s 28-story 100 McAllister and introduces a practical "carbon inheritance" framework for architects and engineers working on major renovations and seismic retrofits. 100 McAllister is a 1930 landmark hotel being seismically upgraded while also transformed into modern student housing and shared academic space as part of the UC Law SF Academic Village. Expanding on research by Vaclav Hasik, KieranTimberlake, and Architecture 2030, this session: • Walks through a clear, visual model of the life cycle of a renovated building, distinguishing "sunk" impacts from those that the current design team controls • Shows how the project team used BIM quantities and structural embodied carbon benchmarks to compare the seismic retrofit pathway against a representative new building, without designing a full hypothetical replacement • Shares the outcome of that comparison for structure, and how it sits alongside Perkins & Will’s work on energy and envelope upgrades • Offers simple diagrams and talking points architects can use with clients to keep existing structures in play as serious decarbonization options, even in high-seismic settings This session is ideal for architects, structural engineers, and institutional clients who are wrestling with "renovate vs. rebuild" and want a conceptually clear, visually driven way to make those decisions more climate cognizant