When must a builder notify the designer according to the Building Code Act?

The obligation for a builder to notify the designer specifically arises when site conditions affect compliance with the building code. This requirement is crucial because changes in site conditions—such as soil quality, drainage, or any unforeseen environmental factors—can significantly impact the design and the compliance of the construction project with the relevant building codes.

By informing the designer about these conditions, the builder ensures that any necessary modifications to the design can be made to adhere to the regulations, thereby maintaining safety and compliance standards. This proactive communication helps in addressing potential issues early in the construction process, which can help avoid costly changes or safety hazards later on.

The other options, while relevant to construction processes, do not specifically pertain to the builder's requirement to notify the designer about compliance issues related to site conditions. Permits are typically related to initial stages of construction, and notifying the designer at the completion of construction does not address the ongoing compliance that needs monitoring during the build. Similarly, multiple contractors' involvement does not inherently necessitate direct communication with the designer about site condition issues unless those conditions impact compliance.