Sustainable Conservation are the processes of change through which the components of the inherited ecosystem from the past retain their value for present and future generations. As such, the value assessment is critical to recognise the values of heritage, not only by its aesthetical and historical values, but also by its contribution to a more sustainable future. Despite recent policies and standards highlighting the role of heritage for sustainability and encouraging urban conservation, sustainable conservation is not yet the most common practice. The behavioural dimension is intrinsic to the decision-making process; however, studies analysing designers’ decision behaviours regarding sustainability in built heritage are seldom found in recent literature. This research aims to increase the understanding of the gap in the implementation of best practices of sustainable conservation of built heritage, and to achieve solutions for behavioural change. It applies methods from psychology to analyse designers’ decisions behaviours, by eliciting common beliefs, challenges, and opportunities in the implementation of conservation intentions towards heritage buildings. The results demonstrate that design decisions result from conscious and unconscious processes, some of them socially driven, while others result from individual attitudes. Targeting the primary belief in the study population on the (in)compatibility between sustainability and heritage conservation, a building passport for sustainable conservation was developed aiming at raising awareness in the value of built heritage to sustainability. The results of this research can support the redesign of heritage buildings and demonstrate the importance of considering behavioural factors in the development of future sustainable conservation policies and tools.