The Commissioning Authority (CxA) should be brought onto the project team early in the Schematic Design (SD) phase, as recommended by the LEED rating system1. This allows the CxA to participate in the design process and provide input on the owner's project requirements, the basis of design, and the commissioning plan. The CxA can also help to identify and resolve potential issues before they become costly problems during construction or operation.
Reference:
LEED v4 for Building Design and Construction, p. 36
LEED Green Associate Candidate Handbook, p. 11
Commissioning Authority Guide
Commissioning Authority | WBDG - Whole Building Design Guide