Transitioning to a circular built environment can reduce the environmental impacts, resource consumption and waste generation emanating from buildings. However, there are many options to design circular building components, and limited knowledge on which options lead to the best environmental performance. Few guidelines exist and they build on conventional environmental performance assessments that focus on single life cycles, whereas the circular economy (CE) focuses on a sequence of multiple use- and life cycles. In this article, environmental design guidelines for circular building components were developed in five steps. First, examples of circular variants of a building structure were synthesized. Second, the environmental performance of these variants was compared with a business-as-usual variant through Life Cycle Assessments (LCA) and Material Flow Analysis (MFA) respectively. Circular parameters of these variants were tested using a scenario-specific approach. Third, from 24 LCAs and MFAs, a scorecard, rules-of-thumb and nine environmental design guidelines for designing circular building components were developed that provide guidance on which circular pathways and variants lead to the best environmental performance. For components with a long functional–technical lifespan, the following are promoted: resource efficiency, longer use through adaptable design, low-impact biomaterials and facilitating multiple cycles after and of use. Fourth, the design guidelines were evaluated by 49 experts from academia, industry and government in seven expert sessions. Further research is needed to validate the generalizability of the design guidelines. However, this research makes an important step in supporting the development of circular building components and, subsequently, the transition to a circular built environment.

Purpose: The global capacity of offshore wind energy is rapidly expanding, with the North Sea leading this growth. However, this region also hosts some of the world’s oldest offshore wind farms, requiring decommissioning in the coming years. Apart from their benefits for energy generation, constructing and decommissioning offshore wind farms physically affect the marine ecosystem. This study aims to develop characterization factors to quantify impacts on marine benthic biodiversity, assessing changes in species richness and accounting for both positive and negative effects. Method: The study utilizes data on species richness at 17 artificial offshore structures and reference seabed sites in the North Sea. Polynomial models are developed to express the species richness on the structure as a function of structure age. The study considers two construction scenarios and three decommissioning scenarios and compares species richness on the artificial structure to the reference seabed. Additionally, it investigates species composition changes in different taxonomic groups and alien species. The richness models are used to develop characterization factors for biodiversity impacts, expressed as the time-integrated potentially disappeared fraction of species. Results and discussion: The developed characterization factors quantify potential loss or gain of species across taxonomic groups, enhancing the representation of biodiversity in life cycle assessment. While species richness on offshore structures generally increases over time, the net biodiversity impacts depend on the seabed type on which the structure is constructed. The characterization factors indicate that for sandy seabed, species richness on the structure exceeds that of the reference seabed after ~13 years. However, over a 25-year lifetime, construction on sandy seabed generally results in a net species loss due to declining species richness after the peak at a 13-year lifetime. Construction on hard seabed supports a net gain of species. Furthermore, characterization factors indicate that partial decommissioning will preserve 80–99% of the species richness on turbine structures. The net effect on ecosystem functioning is yet unclear, depending on, e.g., recolonization opportunities, reference state, and interaction with other marine activities. Conclusion: The developed characterization factors quantify both positive and negative biodiversity impacts from habitat changes associated with offshore wind farm construction and decommissioning in the North Sea, thus providing a basis for understanding and managing the ecological consequences of offshore wind farm projects. The findings indicate that the construction and decommissioning activities will cause changes in the total species richness and the richness of alien species and shifts in richness between taxonomic groups.