Thessaloniki’s postWWII city center and its main element, the multistorey housing building, aka polykatoikia, address emerging challenges of a future authenticity evaluation. The aspects of the information sources considered in authenticity judgments spread from XL to S scale. The transformation of the artistic, historic, social, and scientific dimensions is challenging the postwar city identity and its ‘Ordinary Everyday Modernism’ buildings parameters of authenticity. Thessaloniki’s peculiarity is that 2/3 of the city center has been designated Historical Site since 1994 [XL scale], containing both listed and non-individually listed buildings. The city's building blocks [L scale] are constituted by a diverse assemblage of built and internal free space. The built part consists of various eras constructions exhibiting diverse architectural styles, techniques, and other internal and external factors. The polykatoikia [M scale], integrates the contemporary practices concerning changes such as the middle-class lifestyle, the short-term rentals opportunities and the energy consumption reduction EU norms. The polykatoikia’s distinctive ownership model which divides the building into numerous individual ownerships [S scale], like flats and stores, enters a new era that affects significant aspects like the building’s uses and functions from a bottom-up view. The paper is based on the author's postdoctoral research (Alexiadou, 2024a); recent literature and case studies analysis of OEM buildings interventions support the objectives.