Under what condition can an authority having jurisdiction apply code retroactively?

The correct answer is that an authority having jurisdiction can apply code retroactively when a distinct hazard to life or adjacent property is present. This means that if there is a situation where continued use of an existing installation poses a significant risk to safety or property, the authority can mandate compliance with current codes and standards.

Applying codes retroactively in such situations serves to protect public safety and ensure that any dangers are addressed promptly. The presence of a distinct hazard indicates that the existing conditions are not acceptable under current safety standards, requiring immediate action to mitigate any risks.

In contrast, the other choices do not adequately justify the need for retroactive application of codes. Minor repairs might not warrant a full application of current codes since they do not typically involve major safety concerns. Similarly, applying codes retroactively simply because it does not cause inconveniences may overlook critical safety issues that need addressing. Lastly, modernization alone does not present an immediate danger; therefore, it does not provide sufficient grounds for retroactive code enforcement unless it directly impacts safety.