Can sustainability and adaptability be integrated in a single decision tool for designing future buildings? Indeed, it is not possible to know what lies ahead for future buildings but, using current research on sustainability and the impact on natural resources and climate, it is possible to forecast the connection between built environment activity and sustainability. This paper demonstrates that the assessment of future adaptation in newly designed building is achievable by using the adaptSTAR model. This new design-rating tool, based on detailed analysis of 12 award-winning adaptive reuse projects in Australia, will assist designers in making decisions to achieve optimum efficiency and useful life from their creations. This paper reports on the second and last stages of the research used in the development of the rating tool. The use of the adaptSTAR model can empower designers of buildings to make critical decisions that contribute to improving longevity and future reuse, thereby ensuring that future building adaptive reuse will foster sustainable built environments. In order to validate the tool, results are compared to Langston's ARP model. It is found that both models are strongly correlated. Furthermore, when improvements to the case studies are proposed, lower ARP scores are computed since the buildings are expected to become obsolete later in their life cycle than a design reflecting poorer performance. It can be observed that the improved designs for each case study out-perform the as-built designs, as indicated by the higher adaptSTAR scores, while the relationship between adaptSTAR and ARP is strengthened.