There is a demand for lean construction in Europe; even though lean construction is still an emerging field and there is growing interest, there are no regulations on this topic. The main objective of this research is to regulate this role when in a project and to define and develop a building agent structure, according to the Building Standards Act (LOE by its acronym in Spanish), to be able to incorporate it into the Spanish law, protecting it from civil liabilities. In Spain, there is jurisprudence in civil jurisdiction based on the LOE to acquit or convict building agents, who are defined in the courts as “constructive managers” or similar. For this reason, courts could establish in the future several liabilities for the lean construction specialist and other agents of the project, depending on their actions and based on the implementation of the lean project delivery system, the target value design and the integrated project delivery. Conversely, it is possible that the level of action of the lean construction specialist may comprise design management, construction management and contract management. Accordingly, one or more building agents should be appropriately incorporated into the LOE according to their functions and responsibilities and based on the levels of action of the lean construction specialist. The creation of the following agents is proposed: design manager, construction manager and contract manager, definitions that are developed in this study. These agents are loosely defined, because any project manager, building information modeling manager or similar may act as one or as more-than-one of them. Finally, the creation of the lean construction manager is also proposed, as the agent who takes on the role of the design manager, construction manager and contract manager, but focused on the lean production principles.