Green buildings are known for yielding a better indoor habitat, saving energy and protecting the environment. However, they require greater investment than conventional buildings. Green buildings can be classified at different levels ranging through certified, silver, gold and platinum. The classification chosen is likely to affect project costs. Adopting a platinum level of green building specifications tends to incur the highest project costs. While with gold, silver and certified levels expenditure will be reduced accordingly. The actual degree of project cost differences also depends on additional factors, such as site location, design specifications, construction condition, material and equipment selection and LEED consultant expertise. Construction budgeting represents a crucial factor for project owners developing green buildings. This research presents a method for determining the minimum project costs of green building developments. The processes and steps to be completed in determining such minimum project costs are presented based on a case study of a learning centre building which received LEED platinum certification. It was found that choosing certified and silver levels has an indifferent effect on project costs, whereas expenditure increases exponentially with gold and platinum levels of certification.