There are roughly 20 million buildings in use in Germany alone – millions of properties that highlight so much potential to achieve our climate protection goals. The question is, where do we begin? Is there something every individual can do – in practical terms – to use or operate the buildings they own or occupy more sustainably? This is where the DGNB System for Buildings in Use comes in. We use nine criteria to focus the mind on all factors with a bearing on sustainability. In a series of blog posts, we describe why it makes sense for everyone with a stake in buildings to think more about these topics. In our first post, we look at the three criteria of buildings in use, which are relevant for the environment.