Engineering design and technological modelling have been argued as valid premises from which to increase authenticity, relevance and create bridges between the STEM disciplines while maintaining subject integrity. Previous research indicates that projects which emulate how engineers work has the potential of both integrating STEM disciplines and being authentic. At the same time, earlier research also cautions that few integrated STEM projects consider students’ interests and their everyday contexts. The aim of this study is to investigate the implementation of an integrated STEM project in the Technology Programme at a Swedish upper secondary school. The studied STEM project involves students’ designs for improving their physical school environment in terms of well-being, feasibility, and sustainability. Data collection consisted of participatory observations, as well as teacher and student interviews. The results are presented in terms of three themes, namely (1) cooperation and real-life application are fundamental for authentic learning; (2) using models and modelling for communicating design ideas are central to authentic technology and engineering; and (3) integration of STEM content and methods do not draw on all four disciplines. It is concluded that there might be easily accessible pathways to promote integrated STEM and authenticity, such as utilizing the school environment as a starting point. However, formally implementing authentic practices remain a challenge even though a majority of teachers are enthusiastic about real-world relevance in design projects. Integrated STEM in the design project mostly included technology and engineering content, and aspects of science and mathematics albeit to a lower degree, which made simultaneous integration of all STEM disciplines a challenging task.