What Is the Difference Between Commissioning and Construction?

Most engineers starting out are told commissioning begins at handover. It does not.

That misunderstanding makes the first few months on site harder than they need to be, because you arrive expecting a clean sequence – construction finishes, then commissioning starts – and the reality on a live project looks nothing like that.

Understanding the fundamental difference between commissioning and construction early will help influence how you work alongside a construction team for the rest of your career.

The Short Answer – How They Relate

Construction is the process of building and installing physical systems.

Commissioning is the independent quality assurance process that runs before – alongside – and beyond – construction to verify those systems are designed, installed, and operating as the owner intended.

Commissioning is not a phase that starts after construction ends; per ASHRAE Guideline 0-2019, it begins at pre-design and continues through handover and into the warranty period of the building and its systems.

So, is commissioning part of construction?

The short answer is no – and yes, depending on how you frame the question.

Commissioning is a distinct discipline with its own scope, its own team, and its own deliverables. It operates within the construction programme, but it does not answer to it. Commissioning does not happen after construction is finished; it starts before, then runs in parallel, and continues past the point where construction has closed out.

For a broader understanding of commissioning – read our article ‘What is commissioning in construction? A complete guide’

What Construction Delivers – and What It Does Not

Construction delivers the physical work. The General Contractor [GC] is responsible for building to specification: installing ductwork, setting plant, pulling cable, pouring concrete, and coordinating the trades to turn an MEP design into a built systems and assets.

Their scope is defined by the contract documentation, drawings and project specifications. When the system is installed and the GC signs it off, their primary contractual obligation is met – though the GC typically remains responsible through to the Defects Liability Period [DLP], attending site to close out issues that emerge in the months following practical completion.

Construction does not deliver independent verification of what has been installed and if it is working. The GC is not – and should not be – the final check on whether what they built performs as the owner intended. A GC / Designer cannot objectively commission their own work and expect the same focus and rigour as an independent party doing it. The GC builds to specification. Whether the specification itself was correctly interpreted, and whether the installed system performs against the Owner’s Project Requirements [OPR], is what needs to be proved.

Construction also does not typically own the Basis of Design [BOD] tracking, the O&M training sign-off, or the post-occupancy verification activities. Those belong to commissioning.

What Commissioning Adds That Construction Cannot

Commissioning brings independent verification to a construction project – an objective check that the systems are not just installed, but performing as designed and as the owner intended. This is the core purpose of commissioning and the reason it cannot be self-delivered by the contractor.

In practical terms, commissioning adds the following project value:

• OPR ownership and tracking. The Owner’s Project Requirements [OPR] document defines what the building must do from the owner’s perspective. Commissioning traces every project decision back to those requirements from pre-design through occupancy. No other party owns this continuous, big-picture alignment.
• Design intent verification. During design, the Commissioning Authority [CxA] reviews drawings and specifications against the OPR. Catching errors during the design phase prevents costly site variations later [the widely cited $1:$10:$100 cost escalation principle applies directly].
• Independent quality assurance. Rather than relying solely on the installing contractor to objectively check their own work, commissioning introduces an independent party to validate that the final integrated building actually functions.
• Warranty-period support. Commissioning does not end at practical completion. It continues into the warranty period to catch issues that only emerge under real seasonal and operational conditions.

What happens without commissioning is well documented: systems pass installation checks but fail under load, energy performance falls short of design targets, and the owner accepts a building that does not do what they paid for.

The WBDG states it plainly: commissioning is “not a design or construction function – it verifies their results.” That is the cleanest distinction available.

When Commissioning Starts – Earlier Than Most People Think…

This is where the misconception causes the most issues. Commissioning does not start during construction or at handover. It does not start when the plant is energised. The commissioning process begins at pre-design [before the designer is employed] and runs continuously through every project phase, to after everyone has left the project.

A high level time line looks like this:

Construction, by contrast, starts at the construction phase and ends at practical completion. Commissioning begins long before the first brick is laid and continues long after the GC has left site.

Commissioning vs Construction Management – They are not the Same Role

This is a question that causes genuine confusion, particularly for engineers coming from a project management background.

Commissioning and construction management roles have fundamentally different responsibilities, and their independence from each other is what gives each role its value.

The table below sets out the distinction clearly. Who is responsible for commissioning is a good question to ask – and it is not the Construction Manager.

ASHRAE Standard 202-2018 notes the moment commissioning is absorbed into the contractor’s scope, its objectivity is compromised and its value diminishes significantly.

Note – Design and Build contracts: In Design & Build [D&B] procurement, the GC may appoint the commissioning team directly as part of their supply chain. Where this occurs, the independence of the commissioning process relies on clear contractual separation and, ideally, owner-side oversight of the commissioning scope and deliverables. The principles in this article reflect best practice commissioning as defined by ASHRAE Guideline 0-2019, ASHRAE Standard 202-2018, and BSRIA BG 11.

In a D&B arrangement, the requirement for a competent, qualified Commissioning Authority does not change – only the contractual route through which they are engaged.

What the Commissioning Engineer Does That the Contractor Does Not

On site, the commissioning engineer [CxE] operates in a different lane to the contractor – same project, different scope, different accountability.

What the CxE does that the contractor does not:

• Pre-functional checks and certification. Before any system is tested, the CxE verifies installation against the design drawings and certifies the system as ready-for-test. This is a formal gating milestone, not a duplicate of the GC’s snag list. It is a systematic check that equipment is correctly positioned, connected, and accessible before power is turned on.
• Functional performance testing & BMS validation. The CxE witnesses and validates that systems perform their designed sequence of operations across all modes. On modern commercial projects, this heavily involves Building Management System [BMS] integration—verifying that control sequences, setpoints, alarms, and interlocks actually drive the mechanical equipment as intended. On larger projects, the CxE applies agreed sampling strategies to validate this efficiently. This is distinct from testing vs commissioning – static testing confirms an installation; functional performance testing confirms performance under real conditions.
• O&M training coordination. The CxE is actively responsible for planning and coordinating structured training for the facilities management team. They schedule sessions, confirm subcontractor attendance, witness the delivery, and formally sign off on completion. This is not a quick handover chat; it is a structured, documented process.
• Independent issues log management. The CxE maintains the commissioning issues log independently of the GC’s punch list. These are different documents with different owners. The CxE tracks performance and integration issues until they are definitively closed, not just informally resolved.
• Site-level OPR and BOD traceability. Every physical test, result, and deviation on site is rigorously documented and referenced back to the Basis of Design by the CxE, forming the foundation of the final commissioning report.

What This Means on Your First Project

When you arrive on site as a junior CxE, the construction team will be running to a programme you do not control. The GC is managing subcontractors, resolving RFIs, and driving installation completions. The Construction Manager is tracking cost and programme. Your scope intersects with all of them – but you report through the commissioning structure, not through the construction chain.

You will be asked, regularly, to sign off things that are not ready. The GC wants to move on. The CM wants the programme to hold. Your job is to verify – and if a system is not ready, the pre-functional check or functional performance test does not proceed, regardless of programme pressure. That push back is not you trying to cause problems, it is what the owner is paying for and has requested.

In practice, the best thing to do is understand who holds each deliverable, build direct working relationships with the MEP trade supervisors [they know the installed systems better than anyone on the GC’s management team], and communicate issues through the correct channel. A good CxE does not slow projects down; they catch the problems earlier than if noone was looking.

For a structured view of what this an engineers career path looks like beyond the first project, how to become a commissioning engineer covers it in detail.

And for the broader picture of where the role leads, this commissioning career path article sets out the stages from junior CxE to Cx Director.

Where to Go Next

This article covers how commissioning and construction differ in scope, timeline, and responsibility. The related P1 spoke articles go deeper on specific aspects of the commissioning process:

• What Is Commissioning in Construction? A Complete Guide – the full commissioning definition and process overview
• What Is the Purpose of Commissioning? – why commissioning exists and what it delivers for the owner
• Who Is Responsible for Commissioning? – roles, responsibilities, and accountability structures
• Types of Commissioning in Construction – new build, existing buildings, retro-commissioning, and more
• Testing vs Commissioning – The Difference – why static testing and functional performance testing are not the same thing
• How to Become a Commissioning Engineer – qualifications, experience, and the route into the role
• Commissioning Engineer Career Path – the stages from junior CxE to CxM and beyond