Drawing on the theory of monument preservation and restoration science after 1900, this paper argues that recent alterations to historical buildings can possess significant relevance for their status as heritage sites. Rather than focusing solely on a presumed “original state,” the text advocates for a process-oriented view of the monument as a historical document, where transformations are seen as contributing to, rather than detracting from, its cultural value. This theoretical perspective is illustrated through three exemplary case studies of Austrian post-war modernism: the specific interventions by Karl Odorizzi, Carl Pruscha, and Hermann Czech demonstrate how architectural evolution creates new layers of meaning. The contribution describes the design quality of these modifications and makes a strong case for the preservation and recognition of such interventions. It concludes that these works must be regarded as indispensable records of 20th-century architectural history, deserving of the same protection as the older fabric they inhabit.