How can 3000 square meters be added to this finite and symmetrical 19th century building with height restrictions while leaving its garden intact— one of the museum’s most important yet underutilized assets? The extension takes advantage of available surfaces of the museum and its grounds. The new layer of program is conceived as a natural process modeled on the accretion of rock strata in geology. This new stratum of program wraps three contiguous surfaces of the Staedel: the roof of the Garden Wing, its south façade, and the garden itself. The Late 20th century Collection is consolidated into a new gallery built on top of the Garden Wing and the Corporate Galleries are partially submerged in the footprint of the existing garden, its undulating green roof providing a new surface for the public. The galleries are connected via the Circulation Façade at the museum’s south face, a glazed vertical site reflecting the intertwining of culture and leisure characteristic of the 21st century art museum. A fast stair provides direct connections between galleries while a slow stair provides respite— housing a café, lounges, and unprogrammed spaces of rest. A new electro-chromatic glass face puts the museum’s 19th century south façade, now full of human activity, on display much the way an archeological artifact is displayed in a museum vitrine. The Staedel Museum and its new expansion are given a double orientation like the opposing faces of Janus, looking to the past and future respectively.