The commissioning process can be applied to existing buildings that have never been commissioned to restore them to optimal performance. Retro-commissioning is a systematic, documented process that identifies low-cost operational and maintenance improvements in existing buildings and brings the buildings up to the design intentions of its current usage.

Retro-commissioning typically focuses on energy-using equipment such as mechanical equipment, lighting and related controls and usually optimizes existing system performance, rather than relying on major equipment replacement, typically resulting in improved indoor air quality, comfort, controls, energy and resource efficiency.

Retro-commissioning typically includes an audit of the entire building including a study of past utility bills, interviews with facility personnel. Then diagnostic monitoring and functional tests of building systems are executed and analyzed. Building systems are retested and remonitored to fine-tune improvements. This process helps find and repair operational problems. The identification of more complex problems are presented to the owner as well. A final report, recommissioning plan and schedule are then given to the owner.

Ongoing commissioning, also referred to as "retro-commissioning follow-up," is continual retro-commissioning focusing on the persistence of completed improvements. Ongoing commissioning involves regularly scheduled sessions with the building occupants along with operation and maintenance personnel. This process incorporates monitoring and analysis of building performance data provided by permanently installed metering equipment to verify building performance, the satisfaction of the facilities management and staff, and the extent of actual savings.