One peculiarity of the construction sector is that each construction site represents a unique workplace. The specific characteristics of the site affect risk generation and its evolution. However, available risk assessment tools do not capture the specificities of construction sites that may affect risk, because they only focus on assessing identified risks from a predefined hierarchy of events. This paper proposes a new “site risk” concept that is defined as the risk associated to the whole construction site that is generated by having together different elements which individually affect risk. Potential risk synergies may exist and they only can be captured adopting the construction site as unit of analysis. In doing so, a new CONstruction Site Risk Assessment Tool (CONSRAT) is presented. This is done considering also both organizational structure and resources jointly with material conditions. The tool was used to assess 150 construction sites in order to obtain convergent and internal validity evidences. Another validated tool was used as external criterion: the Qualitative Occupational Safety Risk Assessment Model (QRAM). Results provide adequate validity evidences for both the internal structure and the expected relationships with the external criterion. CONSRAT design and complete instructions for its use are described. As a unique contribution, CONSRAT adopts a new site risk approach to assess the main live conditions, complexity factors and organizational structure characteristics which are related to construction site risk.