Even though the idea of altering an existing building is presently a well established practice within the context of adaptive reuse, when the building in question is a 'mnemonic building', of recognized heritage value, alterations are viewed with suspicion, even when change is a recognized necessity. This book fills in a blind spot in current architectural theory and practice, looking into a notion of conservation as a form of invention and imagination, offering the reader a counter-viewpoint to a predominant western understanding that preservation should be a 'still shot' from the past. Through a micro-historical study of a Renaissance concept of restoration, a theoretical framework to question the issue of conservation as a creative endeavor arises. It focuses on Tiberio Alfarano's 1571 ichnography of St. Peter's Basilica in the Vatican, into which a complex body of religious, political, architectural and cultural elements is woven. By merging past and present temple's plans, he created a track-drawing questioning the design pursued after Michelangelo's death (1564), opening the gaze towards other possible future imaginings. This book uncovers how the drawing was acted on by Carlo Maderno (1556-1629), who literally used it as physical substratum to for new design proposals, completing the renewal of the temple in 1626. Proposing a hybrid architectural-conservation approach, this study shows how these two practices can be merged in contemporary renovation. By creating hybrid drawings, the retrospective and prospective gaze of built conservation forms a continuous and contiguous reality, where a pre-existent condition engages with future design rejoining multiple temporalities within continuity of identity. This study might provide a paradigmatic and timely model to retune contemporary architectural sensibility when dealing with the dilemma between design and preservation when transforming a building of recognized significance.