Recent research on flexible office designs have shown that open‐plan and/or flex offices may not have the expected effects in terms of employees’ productivity, well‐being, job satisfaction, organizational commitment, and retention. In this article, we propose to consider that the feeling of de‐humanization may explain such dark side of office designs. Adopting a mixed methods approach, we administrated a quantitative survey to 534 employees working in a variety of office designs, and conducted 12 semi‐structured interviews among the respondents to the survey in order to investigate how they experienced their office designs, notably in terms of de‐humanization. Results showed that the three specific office designs under study (i.e. cell, open‐plan, and flex offices) are associated with different levels of de‐humanization and that this feeling of de‐humanization mediates their impact on employees’ job satisfaction, affective organizational commitment, extra‐role performance, psychological strains, and turnover intentions. Interviews’ analysis reveals three main mechanisms in the development of the feeling of de‐humanization in such office designs: a triple feeling of dispossession (of space, voice and professional mastery), a feeling of abandon and an injunction to adopt a modern behaviour.