When are used materials not permitted according to fire code regulations?

The correct answer highlights that used materials are not permitted when the fire separation distance is less than the specified amount. This is rooted in fire safety regulations that prioritize maintaining adequate distances between structures to mitigate the risk of fire spreading. Fire separation distances are determined based on various factors, including building materials, occupancy types, and overall fire resistance features.

When the specified fire separation distance is not met, it increases the risk of fire exposure to adjacent buildings, thereby justifying the prohibition on using previously used materials, which may not provide the level of safety required in high-risk situations. In these cases, new materials that meet current safety standards are mandated to ensure the integrity and safety of the structure in case of a fire.

The other choices do not encapsulate the core reason for the regulation. For instance, while multi-family dwellings and un-sprinklered walls could suggest increased risk factors, they do not universally dictate the use of materials as per fire regulations. Similarly, all residential construction would not apply since specific guidelines are often in place for different building types and their contexts.