What key concept revolves around the idea of extending the use of existing structures?

The key concept that revolves around extending the use of existing structures is renovation. Renovation involves modifying, improving, or updating existing buildings to meet current needs or standards without completely tearing them down. This practice allows for the preservation of historical elements while bringing the structure up to date, enhancing its functionality and aesthetic appeal. Renovation can include various processes such as remodeling spaces, upgrading systems (like electrical and plumbing), and improving energy efficiency, thereby making efficient use of existing resources.

In contrast, the other choices reflect different concepts related to building processes. Demolition entails tearing down structures, reconstruction involves the building of new structures in place of the old or after destruction, and decommissioning refers to officially taking a building out of service, which does not apply to the extension of use.