Adaptive reuse architecture increasingly confronts a critical tension: when does the grafting of contemporary elements onto historic structures produce innovative synthesis, and when does it create what critics term ‘Frankenstein architecture’ buildings perceived as fragmented, incoherent assemblages lacking cultural authenticity? Whilst the Frankenstein metaphor circulates widely in architectural discourse, particularly following Liliane Wong’s identification of ‘Frankenstein Syndrome’ in 2016, no peer-reviewed study has systematically operationalised it as an analytical framework. This paper addresses that gap by developing a transferable diagnostic tool that integrates Mary Shelley’s literary metaphor with the Vitruvian triad (firmitas, utilitas, venustas), extended with Clarke and Kuipers’ proposed fourth virtue of dignitas (respect for heritage values). Through comparative analysis of three paradigmatic cases the Royal Ontario Museum Crystal (Libeskind, 2007), Port House Antwerp (Zaha Hadid Architects, 2016), and Elbphilharmonie Hamburg (Herzog & de Meuron, 2017) the study reveals a spectrum from parasitic failure to mutualistic success. The framework demonstrates that architectural ‘monstrosity’ emerges not from stylistic contrast per se, but from failures in structural integration (firmitas), functional coherence (utilitas), aesthetic synthesis (venustas), and heritage sensitivity (dignitas). The paper contributes to adaptive reuse theory by providing architects and scholars a systematic method for evaluating the phenomenological experience of architectural assemblage.