When might it be necessary to prepare the foundation outside a building?

Preparing the foundation outside a building is necessary when gravel or crushed stone is required for support. This choice indicates the use of a specific type of foundational material that provides stability and drainage for the overall structure. Gravel and crushed stone help to distribute the weight of the building evenly and allow for water drainage, preventing moisture buildup that could affect the integrity of the foundation. Such materials are often used as a base layer before pouring concrete, especially in areas prone to heavy rainfall or where soil conditions demand enhanced drainage solutions.

In contrast, the other options deal with specific scaffolding methods or materials that may not directly relate to the foundational requirements necessary outside a building. For instance, metal frames may be used in scaffolding, but they do not dictate the need for foundation preparation outside. Leveling with bricks is more about constructing walls than providing foundational support. Similarly, having concrete already available does not inherently necessitate preparing the foundation outside; in fact, a foundation may need preparation regardless of the concrete's presence to ensure stability and compliance with structural guidelines.