Existing Buildings and the NCC Performance Pathway: What Engineers Actually Certify
The National Construction Code gives practitioners two routes to compliance: a Deemed-to-Satisfy solution, or a Performance Solution. For new buildings on clear sites, the DtS path is usually straightforward. For existing buildings, it rarely is. Older floor-to-ceiling heights, non-standard structural grids, fire-compartment boundaries that no longer align with the proposed use, and materials that predate current standards all create gaps between what the building is and what the code expects. The performance pathway exists precisely to bridge those gaps, but it requires engineering input that many project teams underestimate until a certifier puts the application on hold.
What the Performance Pathway Actually Requires
A Performance Solution under the NCC is not a waiver. It is a documented demonstration that a proposed design achieves the Performance Requirements of the code, even if it does not follow the prescriptive DtS provisions. The NCC 2022 framework, carried into the 2025 edition, requires that a Performance Solution be prepared by a person with the competency to assess it, and that the assessment method be one of four recognised approaches: evidence of suitability, expert judgement, comparison with a DtS solution, or quantitative analysis.
For structural matters, fire resistance, or building services, that competency requirement almost always points to a registered professional engineer. In Queensland, that means an RPEQ. In New South Wales and Victoria, the equivalent registration frameworks apply. The certifier cannot simply accept a Performance Solution on the basis that it looks reasonable. The code requires the solution to be documented, the assessment method to be identified, and the professional who prepared it to be qualified to do so.
This is where architects and developers sometimes encounter friction. A well-reasoned design narrative is not a Performance Solution in the NCC sense. Neither is a structural report that identifies defects without addressing code compliance. The documentation needs to be structured to the code's requirements, and it needs to be signed by someone whose registration supports the scope of the assessment.
Where Existing Buildings Create Specific Problems
Change of use is the most common trigger. A warehouse converting to residential, a heritage office building becoming a school, a ground-floor retail tenancy being subdivided into food premises: each of these shifts the applicable Performance Requirements, sometimes substantially. The building's existing structure may be adequate for the new loads, or it may not. The floor system may have been designed for 3 kPa and the new use requires 5 kPa. The facade may not meet the fire-resistance level now required for the new classification.
Adaptive reuse projects carry an additional layer of complexity because the documentation for the original structure is often incomplete or absent. Buildings constructed before the mid-1980s frequently lack as-built drawings that reflect what was actually built. Reinforcement layouts, connection details, and material grades are unknown until someone investigates. A certifier cannot approve a Performance Solution for structural adequacy if the engineer preparing it has had to assume the structure's properties rather than measure them.
This is not a paperwork problem. It is a technical one. The NCC's Performance Requirements for structural reliability reference AS/NZS 1170 and the suite of material standards. An engineer asserting compliance needs to know what the structure is made of, how it is connected, and what its current condition is. That knowledge comes from investigation: non-destructive testing, material sampling, NATA-certified laboratory analysis, and condition assessment. Without that foundation, a Performance Solution for structural matters is an opinion, not an assessment.
Form 15 and Where It Sits in the Sequence
In Queensland, Form 15 is the instrument through which a licensed professional certifies that the design or inspection work they have carried out complies with the relevant codes and standards. It is issued by the engineer or other professional, not by the certifier. The certifier relies on it as part of the documentation package that supports the building approval or completion certificate.
For existing-building projects involving a Performance Solution, Form 15 typically sits near the end of the engineering sequence but before the certifier can close the file. The sequence generally runs as follows.
First, the project team identifies the gaps between the existing building and the Performance Requirements triggered by the proposed change. This is often done during a feasibility or pre-application phase, and it shapes the scope of the engineering investigation.
Second, the engineer investigates the existing structure to the depth required to support a Performance Solution. For structural matters, this means quantifying the extent and severity of any deficiencies, not just identifying that deficiencies exist. A report that lists cracks, spalling, or section loss without measuring them does not give the certifier or the engineer enough to work with.
Third, the engineer prepares the Performance Solution documentation. This sets out the Performance Requirements being addressed, the assessment method used, the data relied upon, and the conclusion. If the structure meets the requirements, the documentation says so and explains why. If it does not, the documentation identifies what remediation is needed and what the residual risk is.
Fourth, once any required remediation has been designed and, where the certifier requires it, carried out or inspected, the engineer issues Form 15. The form certifies that the design or the inspected work complies with the applicable standards. It is a professional declaration, and it carries the engineer's RPEQ registration number.
The certifier then has what they need to close the file. Without Form 15, the performance solution documentation is incomplete regardless of how thorough the underlying engineering work is.
Common Misunderstandings That Delay Projects
The most frequent source of delay is the assumption that a structural report prepared for one purpose will satisfy another. A report prepared for a body corporate to assess defects is not a Performance Solution. A report prepared for a vendor's due diligence is not a Form 15 certification. Each document has a specific scope, a specific audience, and a specific standard of evidence. Repurposing one for another purpose almost always creates gaps that the certifier will identify.
A second source of delay is the sequence in which engineering is engaged. Performance Solution documentation needs to be prepared before the certifier can assess the application, not after. Engaging an engineer to produce Form 15 at the end of a project, after design decisions have been locked in, sometimes reveals that the design does not actually meet the Performance Requirements it was assumed to meet. Retrofitting the engineering to the design is more expensive than integrating the engineering into the design process from the start.
A third issue arises with heritage buildings, where the performance pathway intersects with conservation obligations. The NCC contains specific provisions for existing buildings, and the Queensland Development Code and heritage authority requirements add further layers. An engineer working on a heritage adaptive reuse project needs to understand both the structural performance requirements and the conservation constraints. The two are not always compatible, and the resolution requires documented professional judgement, not a template.
What a Certifier Needs to Close the File
From an engineer's perspective, the certifier needs three things to close a performance solution file for an existing building.
First, a Performance Solution document that is structured to the NCC framework: Performance Requirements identified, assessment method stated, data sources referenced, and conclusion clearly stated. The document should be self-contained enough that a reviewer who was not involved in the project can follow the reasoning.
Second, Form 15 issued by an RPEQ whose registration scope covers the matters being certified. A civil engineer certifying fire-resistance compliance, or a mechanical engineer certifying structural adequacy, will create a problem. The registration needs to match the scope.
Third, where the Performance Solution depends on the building's existing condition, the investigation data that supports the engineer's conclusions. This does not need to be attached to Form 15, but it needs to exist and be available if the certifier or a reviewing authority asks for it.
Projects that provide all three at the point of application move through assessment without delay. Projects that provide partial documentation, or that provide the right documentation in the wrong format, create review cycles that cost time and money.
Getting the Sequence Right
The performance pathway for existing buildings is not more difficult than the DtS path. It is different. It requires earlier engineering engagement, more thorough investigation of the existing structure, and documentation that is explicitly structured to the NCC's framework rather than to a general engineering report format.
For architects and developers, the practical implication is that engineering scope needs to be defined at the start of the project, not after design development. The investigation scope drives the Performance Solution, which drives Form 15, which closes the certifier's file. Compressing or skipping steps in that sequence does not save time; it creates rework.
For certifiers, the implication is that a Performance Solution for an existing building should be accompanied by evidence that the engineer has actually investigated the building, not just reviewed drawings. Where that evidence is absent, the Performance Solution rests on assumptions rather than data, and the certifier's reliance on it is correspondingly weaker.
TRSC works with architects, certifiers, and developers on exactly this sequence: investigation, Performance Solution preparation, and Form 15 certification for Queensland projects. If you are working on an existing-building project where the NCC compliance pathway is unclear, the place to start is a conversation about what the certifier will actually need. More information is available at [https://trsc.au](https://trsc.au).