What is a green building? A large amount of definitions and green rating tools prove that an exact definition is still a point of discussion. To research the differences between green rating tools, four different buildings are assessed with: EPN, BREEAM, LEED, GreenCalc+ and EcoQuantum. These tools are national and international commonly used to assess (green) buildings. A residential-, office-, sports- and school building are assessed. Each of these buildings consisted of a general and a green variant. The general variant represents the current state. And in the green variant measures are taken to make the building more energy efficient and sustainable. The assessments show large differences between the evaluation results. The same building does not fulfil the requirements of a green building according to one tool, while another tool rates the building very “green”. The improvement between the general and green variant also show large differences. Based on the same input values, the office building is only 12% improved according to EPN and 45% according to LEED. The differences are mainly caused by the environmental aspects taken into account. EPN considers only the energy performance. GreenCalc+ takes also material and water aspects into account, while LEED and BREEAM assess an even larger amount of building aspects. Even if the type and amount of building aspects correspond, differentiation in appreciation of these aspects causes other end results.