Which of the following can be covered under a builders risk policy?

A builders risk policy is specifically designed to provide coverage for buildings under construction and includes various types of construction activities. The correct choice, which addresses additions to existing buildings, highlights an important aspect of builders risk coverage. This policy can indeed insure structures that are being renovated or expanded, as these activities involve construction risks similar to those associated with entirely new projects.

The rationale behind this coverage is that both new construction and substantial modifications to existing structures entail risks such as theft, vandalism, and certain types of damage during the building process. As a result, extensions and additions are included under this policy's umbrella, ensuring that these projects have appropriate insurance protection.

While the other options present different scenarios, they do not encapsulate the broad application of builders risk insurance as effectively as the option about additions. For instance, limiting coverage to only new construction or applying certain cost thresholds for renovations does not reflect the policy's full intent and applicability in the field of construction insurance. Thus, the option focusing on additions accurately captures the essence of what builders risk can protect.