Yes. A BASIX assessment can begin before every drawing, specification and product selection is finalised. In many cases, starting before the approval set is completely locked provides useful time to identify thermal-performance issues, clarify missing information and coordinate practical design responses.
However, there is an important distinction between beginning a preliminary BASIX review and issuing the final BASIX Certificate. An early review can work with developing information. The final Certificate must be based on a sufficiently resolved design and must correspond with the plans and specifications being submitted for approval.
There is no universal rule that plans must be a particular percentage complete. Readiness depends on whether the available documents describe the building geometry, glazing, construction and relevant systems accurately enough for the assessment stage being undertaken.
In Brief
A preliminary BASIX assessment can begin once the proposed dwelling has reliable geometry, orientation, room layouts, glazing locations and an identifiable construction approach. Some product and system selections may remain provisional. The final BASIX Certificate should only be issued when the inputs relied upon by the assessment are coordinated with the plans intended for DA or CDC lodgement.
The phrase “starting BASIX” can describe two different stages. Separating them helps the design team understand what can be assessed early and what must be confirmed before certification.
| Stage | What It Can Do | What It Requires |
|---|---|---|
| Preliminary review | Identify the likely assessment pathway, test the developing design and highlight information or performance issues that still need to be resolved. | Reliable concept or developed-design plans with enough information to understand the proposed dwelling. |
| Final Certificate | Document the commitments that form part of the proposed development and support the relevant approval submission. | Coordinated plans, confirmed assessment inputs and sufficient detail for the Certificate to match the project being lodged. |
The most useful time to start is generally when the design is developed enough to produce meaningful assessment results, but early enough for those results to influence the documentation.
This is usually after the fundamental form of the dwelling has been established. The building should have a recognisable footprint, orientation, floor layout, elevations and window arrangement. Major construction systems should also be identifiable, even where some detailed products remain under consideration.
Early Concept
An early review may identify the likely BASIX and thermal-assessment pathway, but results remain sensitive to changes in geometry, glazing and construction.
Developed Design
The dwelling can be assessed with meaningful accuracy while there is still time to coordinate glazing, shading, insulation and system commitments.
Approval Set
Before the Certificate is relied upon for lodgement, the final assessment inputs should be checked against the drawings and specifications being submitted.
Not every specification needs to be final, but the information that defines the building and its performance should be reliable enough to assess.
The following items should generally be established before a detailed BASIX or NatHERS assessment begins:
Where these elements are still changing substantially, the assessment may still provide early guidance, but it should be treated as preliminary rather than as a final certification result.
Design Readiness
Some project information can remain provisional during an early assessment, provided that realistic assumptions are used and the details are confirmed before final certification.
Depending on the project, this can include:
The assessment should not rely on optimistic placeholder selections that the project is unlikely to deliver. Any provisional commitment still needs to be practical, achievable and suitable for inclusion in the final design.
Yes. Where the BASIX thermal-performance pathway uses NatHERS simulation, the thermal model can often begin during developed design.
The model needs enough information to represent the proposed building accurately. Geometry, orientation, zones, glazing, shading and construction all influence the result. If these inputs remain highly fluid, the model may need substantial updating as the design develops.
Early modelling is particularly useful where the project includes extensive glazing, complex geometry, split levels, steel framing, unusual construction systems or architectural constraints that may make the thermal result more sensitive.
The final NatHERS documentation should be coordinated with the final BASIX Certificate and the plans intended for lodgement. It should not remain based on an earlier design iteration once the architectural drawings have changed.
Starting at developed-design stage allows BASIX to operate as part of design coordination rather than as a final administrative check.
Glazing, shading, insulation or systems can be reviewed while the design team still has practical options available.
Resolving commitments before the documentation is complete can reduce changes to window schedules, elevations, specifications and consultant drawings.
Where a design is not meeting an applicable requirement, the assessor may be able to test a combination of responses rather than defaulting to a late product upgrade.
Water, energy and construction commitments can be checked against the intended project before they become part of the approval documentation.
The architect, designer, assessor and other consultants can incorporate the agreed commitments before the submission package is assembled.
A concept may be too early for detailed assessment when the information defining the building is still expected to change substantially.
Examples include:
At this stage, an assessor may still provide high-level guidance or identify likely information requirements. A detailed result should not be treated as stable until the underlying design is stable enough to support it.
Final Coordination
The final Certificate should be generated when the project has enough coordinated information to describe the development being lodged for approval.
Before issue, the project team should confirm that the assessment reflects the current architectural set, including the building form, glazing, shading, construction and any water or energy measures relied upon by the result.
The aim is not to wait until every construction-stage detail is known. It is to ensure that the Certificate is not issued against obsolete concept plans or assumptions that conflict with the approval documentation.
Design development during the assessment is normal. Updated information can be incorporated while the Certificate is being prepared.
The assessor should receive the updated drawings where changes affect building geometry, glazing, shading, construction, insulation or nominated systems. The assessment can then be checked before the final documentation is generated.
Once a Certificate has been issued or lodged, the question becomes whether the existing Certificate still matches the project or needs to be revised. That later-stage amendment process is addressed separately.
Read when a BASIX Certificate may need to be amended →
A project is usually ready for an initial BASIX review when most of the following questions can be answered.
Is the site and project address confirmed?
Do the plans show true north and the proposed building orientation?
Are the footprint, storeys and room layouts reasonably stable?
Are elevations or equivalent glazing details available?
Are the principal wall, roof and floor systems identifiable?
Is there still time to coordinate changes before the approval set is locked?
The full document requirements depend on the project type and assessment method. A separate checklist is available for teams preparing the complete assessment package.
See what documents are needed for a BASIX Certificate →
Frequently Asked Questions
Yes. A preliminary review can begin while the plans are being developed, provided there is enough reliable information to understand the proposed dwelling. The final Certificate should be coordinated with the completed approval-stage design.
There is no universal percentage that determines whether plans are ready. What matters is whether the available information is sufficiently stable and detailed for the assessment being undertaken.
Concept plans may support early advice, but they are not always detailed enough for final certification. The final BASIX Certificate needs to reflect the development and commitments being submitted for approval.
Not necessarily. Some selections can remain performance-based or provisional during the assessment. The final commitments must still be realistic and should be confirmed before the Certificate is relied upon for lodgement.
The supplier may not need to be selected, but the assessment requires appropriate glazing performance values. The windows ultimately specified should satisfy the performance assumptions used in the thermal model.
BASIX can often begin before the structural documentation is complete. However, the intended framing, floor, wall and roof systems should be understood because changes to these systems can affect thermal-performance inputs.
The updated plans should be provided to the assessor. Relevant changes can then be incorporated before the final Certificate and supporting documentation are issued.
Waiting until every drawing is complete can reduce the opportunity to resolve performance issues efficiently. A better approach is usually to begin once the main design is stable and finalise the Certificate once the approval documentation has been coordinated.
Related Knowledge
What Documents Do You Need for a BASIX Certificate? →
Can a BASIX Certificate Be Amended? →
Assessment note: The level of design information required depends on the project type and thermal-performance pathway. Preliminary advice should not be treated as a final Certificate unless the assessment has been checked against the current approval documentation.
For regulatory information, refer to the NSW Planning Portal guidance on BASIX and planning approvals.
Last reviewed: July 2026.
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