In its communication from March 2020 about "A new Circular Economy Action Plan", the European Commission stipulated that in order to achieve its ambition, Europe would need to accelerate the transition towards a regenerative growth model. The central idea of this model is to give back to the planet more than is taken and to advance towards keeping the resource consumption within the planetary boundaries. This requires reducing the consumption footprint and doubling the circular material use rate in the coming decade. This political understanding of a Circular Economy is to a large extent in line with Empa's scientific view on the issue – a view that is based on an integrative, cascading, resource-based approach aimed at an environmentally sustainable and socially beneficial economy.

For our society, this can be translated into the objectives of using the materials and products in a maximum number of cycles and recycling them with a minimum waste and pollution. The objectives are of the highest relevance for the RFA "Sustainable Built Environment", but also for further RFAs such as "Resources and Pollutants". Two of the key issues that need to be tackled are the identification of the sustainable resource base and a determination of how this base is then used best within our society.

For the area of a sustainable built environment, the following points need to be considered:

• Building materials are produced in amounts exceeding all other fields of applications.
• The construction phase of a building is determining in large part its actual operational phase (such as energy consumption or its flexibility in use).
• Buildings, and with this the related materials stocks, remain operational over decades, creating new types of potential urban mines that need to be identified and communicated in a comprehensive manner in order to allow an efficient re-use in another cycle.

The application of suitable methods and indicators at the various interfaces between products, systems and related technologies, in particular measuring their potential for more circularity and their positioning in view of the planetary boundaries, will allow us to make a step towards a more sustainable society.