The introduction of simulation into medium-scale building services and engineering firms can provide design teams with access to new information about the performance implications of design decisions. This new information can disrupt "business as usual" and lead to radical changes in how design teams approach projects and in the resulting design. This paper reports on the experiences of consulting firm Rybka Scotland and the Scottish Energy Systems Group in the redevelopment of a historic structure. The work began as focused assessments on daylighting issues within perimeter atria and adjacent office accommodation. Early access to simulation allowed the design team to identify new opportunities for marketing the project as a "green" building. As the design evolved, further assessments were carried out and this lead to a decision to make use of the new Carbon Emissions Calculation Method (Scottish Part J and English Part L) provisions of the UK Building Code. In support of this effort, the design team altered their working practices, performance issues were treated explicitly and within the context of integrated assessments rather than isolated component designs. The paper explores the impact of simulation on the design process from the point of view of the services engineer, the lighting consultant and simulation staff. It discusses the evolution of simulation models, procedures developed during the project, methodologies used to assess the equivalence of novel design elements with code compliant constructions and reviews the methodology used to meet the provisions of the new Carbon Emissions Method. The paper concludes with a discussion of future applications of simulation to the new code provisions.