Historic sites develop over time and rarely reflect the characteristics of a single period. Architectural approaches, technologies and materials evolve and change, providing variety and richness in context but following traditional morphology, typology, and land use. However, a misunderstanding of the concepts of authenticity and integrity tend to disregard this architectural richness and create a stylistic uniformity defined by the period that is considered more valuable and/or dominant. Although they may be compatible with the context, 20th century layers, reflecting different approaches, are usually erased in this process: Examples dated to the first half of the century are generally more traditional in design, not imitating but continuing vernacular practices. Beginning from the 1930s, new building technologies, materials, and architectural elements are integrated, creating new but compatible designs. New building regulations in the 1960s and 1970s disregarding the historical quality of sites, on the other hand, enabled the production of incompatible examples whereas there also were infill designs reflecting period approaches, following or contradicting site characteristics. Today all of these buildings have become authentic components of the historic sites, and their evaluation is necessary before any decisions are made to remove and replace them with pseudo-historic designs. This paper aims to discuss examples from urban and rural sites in Turkey to propose a basis for the evaluation and preservation of 20th century layers.