What must not adversely affect the performance of all structures?

The correct answer emphasizes that additions, alterations, and repairs must all be performed in a way that does not adversely affect the performance of structures. This principle is grounded in the fundamental objectives of the International Residential Code, which seeks to ensure the safety, health, and welfare of the public through sound construction practices.

When adding to, altering, or repairing a structure, it is crucial that these actions do not compromise the integrity, stability, or safety of the existing building. Any construction activity—whether expanding the footprint of the building, modifying existing structural components, or fixing damage—has the potential to impact the building's intended performance. Therefore, it's essential that these activities adhere to the same safety and performance standards outlined in the code.

By ensuring that all such activities are carried out properly, it mitigates risks associated with structural failures, which can lead to hazards for occupants and other nearby structures. This comprehensive approach—considering additions, alterations, and repairs—ensures a holistic view of how changes can impact the overall performance and safety of residential structures.