In the standard design process of a building, total building simulation using building simulation software is encouraged to be incorporated into the design process as early as possible. However, this paper demonstrates that building simulation can be used as late in the process as the early construction phase of a building project, though usually with increasing cost of building modifications. Using building utility budget as an indicator, the options presented through use of building simulation tools can justify the change to the design or construction, by showing a reduction in the expected operational costs over the lifetime of the building.