According to the International Standards for the Professional Practice of Internal Auditing (Standards) set by the Institute of Internal Auditors (IIA), internal audit functions are allowed flexibility in including consulting engagements in their annual plans. Standard 2010 - Planning states that "the chief audit executive must establish a risk-based plan to determine the priorities of the internal audit activity, consistent with the organization's goals." This risk-based plan should consider the potential value-add of consulting engagements. Consulting engagements that are expected to add significant value or align with strategic objectives are often included, but not all requests need to be automatically included unless they meet these criteria.
Reference:
Institute of Internal Auditors (IIA), International Standards for the Professional Practice of Internal Auditing (Standards), Standard 2010 - Planning.