BASIX Simulation Method Checks NSW 2026: The 4 Things Councils and Certifiers Must Verify on NatHERS Certificates

Answer first: When a new NSW dwelling uses the BASIX Simulation method, councils and certifiers must verify four things: matching development details, a valid NatHERS certificate (template, QR code and assessor numbers), a NatHERS stamp on the design documents, and heating and cooling loads (MJ/m².year) that match between the NatHERS and BASIX certificates.

By: giantA Pty Ltd · Updated 15 October 2026

What is the DPHI guide for checking BASIX Simulation thermal performance?

The NSW Department of Planning, Housing and Infrastructure publishes BASIX Guide: Checking thermal performance for the Simulation method (May 2025; first published December 2024). It is written for councils and certifiers who must check that the Thermal Performance section of a BASIX certificate is valid when an accredited assessor has used the Simulation method. Accredited assessors can also use it to confirm they have prepared the required documents.

The guide states that four elements must be checked by the consent authority at approval stage and by the certifying authority during construction. If any element is inconsistent or invalid, the application should be returned to the proponent. The Planning Portal’s council or certifier page points councils and certifiers to this guide for how to check BASIX certificates.

This post walks those four checks for new dwellings. It is distinct from our guide on when NatHERS simulation is allowed for alterations over $50,000, and from posts on Planning Portal certificate validation, certificate validity, and May 2026 tool upgrades.

What is the BASIX Simulation method for new dwellings?

According to the guide, the Simulation method is one pathway to demonstrate compliance with the BASIX Thermal Performance standards for new dwellings. Simulations must be conducted by a Nationwide House Energy Rating Scheme (NatHERS) accredited assessor. Every assessor has a NatHERS assessor number that must appear on NatHERS and BASIX certificates, and assessors must follow the NatHERS assessor code of practice.

Simulations must use software approved by the NatHERS Administrator. Each simulation generates heating and cooling loads, measured in MJ/m².year, that must be entered into the BASIX Thermal Performance section. To meet the BASIX standards, the heating, cooling and total loads must be below the maximum loads calculated by BASIX for the relevant climate zone.

To complete the Simulation method, an applicant must:

• engage an accredited assessor who will simulate the dwelling with an approved software tool; and
• enter the calculated heating and cooling loads from the NatHERS certificate into BASIX.

The Portal’s Simulation method page and Thermal Performance section describe the same pathway. The BASIX Thermal Performance Protocol must be followed; the Portal currently lists the 7 June 2024 protocol for use with the current (V4) BASIX standards. Where the protocol and the NatHERS Technical Note differ, the protocol prevails.

You can tell Simulation has been used because the “(NatHERS) assessor details and thermal loads” section on page 2 of the BASIX certificate is populated. If Simulation has not been used, those fields show “n/a”.

How do Simulation checks differ for alterations and additions over $50,000?

The May 2025 DPHI verification guide is aimed at the Simulation pathway for new dwellings. The Portal’s Simulation method page states that Simulation is not available for alterations and additions except in special circumstances.

BASIX still applies to alterations and additions in NSW with an estimated development cost of $50,000 or more, and to pools and spas of 40,000 litres or more, according to the Portal’s alterations and additions page and the Starting a BASIX application pathway table. That threshold is $50,000 or more — not a lower figure.

Where Simulation is sought for alterations, the Portal’s using simulation for alterations and additions page sets a separate process: email [email protected], demonstrate heritage, structural or legal constraints (not design flexibility or aesthetics alone), and run existing, reference and proposed models. For when that pathway is allowed, see our alterations Simulation guide. The four-element checklist below is the new-dwellings verification sequence from the DPHI guide.

What are the 4 things councils and certifiers must verify?

The guide lists four elements. Both the consent authority (at DA or CDC approval) and the certifying authority (during construction, including construction certificate and occupation certificate stages) must check them.

Appendix A of the guide compiles these into a one-page review checklist. Soften any project-specific call: if something is unclear on the stamped set, return it to the proponent rather than inventing an interpretation.

How do you check development details (check 1)?

When Simulation is used, the development application must include both a BASIX certificate and a NatHERS certificate. Then check that the address or lot details on the plans, the NatHERS certificate and the BASIX certificate all match the details in the:

• development application or application for a complying development certificate;
• construction certificate; or
• occupation certificate.

In practice this is the first filter before anyone scans a QR code. A corner lot with two street names, a recently subdivided address, or a multi-dwelling project where one unit’s address was typed differently can trip the match even when the thermal model is sound. Align the property description before lodgement so the consent authority is not sent back to the applicant for an address fix alone.

Since 31 July 2026, DA and CDC lodgement also validates the BASIX reference number on the Planning Portal — a separate check from this Simulation guide. See our BASIX validation and ‘Review required’ guide.

How do you check the NatHERS certificate (check 2)?

A NatHERS certificate can only be issued by the software provider’s online generation system. The first page shows the NatHERS logo and follows the sample templates in the guide:

• Individual dwelling certificate — overview section first, then building features on following pages.
• Class 2 summary — first page of a certificate for a multiple-dwelling project such as an apartment building. It summarises all individual dwellings. Certificates for each individual dwelling must be attached to the Class 2 summary.

Each certificate displays a unique NatHERS certificate number and can only be generated by a NatHERS accredited assessor with a NatHERS assessor number. The first page of every NatHERS certificate and Class 2 summary must include a unique QR code that can be scanned to verify the certificate is valid.

The guide requires three items on every certificate:

1. Template — the certificate matches the individual-dwelling or Class 2 summary template.
2. QR code — scan it and confirm the linked certificate matches the one submitted.
3. Assessor details — the NatHERS assessor number and certificate number on the BASIX certificate match the NatHERS certificate.

A NatHERS certificate that fails any of those criteria is not to be accepted. For apartment projects, keep the Class 2 summary and the individual dwelling certificates together; the summary alone is not the full set. Our Class 2 BASIX and NatHERS checklist covers the multi-dwelling side in more depth.

How do you verify the NatHERS stamp on plans (check 3)?

For all dwellings, a unique NatHERS stamp must be added electronically to all design documentation relevant to the NatHERS certificate. At a minimum this includes, but is not limited to:

• site and floor plans;
• elevations and sections;
• materials documentation;
• window, skylight and door schedules;
• shadow drawings;
• electrical plans, including lighting and mechanical ventilation;
• insulation information in construction drawings where provided;
• any design changes; and
• supporting reports.

The stamp should not cover any information on the design documentation or the mark of any other practitioner. The guide’s check is:

1. verify the stamp details on the design documentation match the NatHERS certificate; and
2. scan the QR code on the stamp to confirm it links to the same certificate as the one submitted.

In our experience, the stamp check is where late design changes hurt most: a glazing schedule updated after the assessor’s final run, or an elevation revised without re-stamping, leaves the consent authority with mismatched sheets. Freeze the architectural set, complete the NatHERS run, stamp the full package, then lodge. Do not invent software product names or stamp prices — the guide only requires that the stamp comes from the accredited assessor’s certificate generation and that its details and QR match.

How do you check heating and cooling loads on the BASIX certificate (check 4)?

Heating and cooling loads from the Simulation method must appear in two places, and they must match:

• on the NatHERS certificate; and
• on the BASIX certificate itself.

Loads are expressed in MJ/m².year. The guide’s check differs by certificate type:

• BASIX single-dwelling certificates: heating and cooling loads on page 2 of the BASIX certificate must be the same as those on page 1 of the NatHERS certificate.
• BASIX multi-dwelling projects: heating and cooling loads for each individual dwelling in the Thermal Performance section of the BASIX certificate must be the same as listed on the NatHERS Class 2 summary.

Any mismatch means the assessment is not valid under the guide. Typical causes are a revised NatHERS run entered into an old BASIX project, a unit number swapped between dwellings, or a total load transcribed without the separate heating and cooling figures. Reconcile the numbers before the certificate is generated — and remember the May 2026 accuracy declaration requires confirmation that NatHERS assessor information is true and correct before generation.

At which approval stages do these Simulation checks apply?

The guide says the four elements must be checked by both the consent authority at the approval stage and by the certifying authority during construction. Mapped against the Portal’s BASIX and planning approvals path:

1. DA or CDC lodgement and assessment — consent authority checks the four elements as part of assessing the BASIX certificate information. Since mid-2026 the Portal also validates the BASIX reference number before submit.
2. Construction certificate — attach the BASIX certificate and ensure commitments (including thermal) are shown on the plans; the certifying authority inspects construction and should re-check Simulation documentation where it still applies.
3. Occupation certificate — attach the BASIX certificate; after final inspection and before the occupation certificate is issued, the certifying authority must issue a BASIX completion receipt confirming the project was built as per the certificate.

The guide does not publish fixed turnaround times for returning inconsistent Simulation packages. Treat mismatches as a return-to-proponent item rather than assuming a clearance window. Certificate validity for lodgement remains the three-month window described on the Portal — see our validity guide.

What step-by-step process should applicants follow before lodging?

Use this sequence so the four council/certifier checks pass first time:

1. Confirm Simulation applies. For new dwellings, choose Simulation when the design needs NatHERS modelling (complex fabric, multi-dwelling, materials outside DIY). For alterations over $50,000, do not assume Simulation is available — follow the Portal’s special-circumstances process or stay on the alterations pathway.
2. Engage a NatHERS accredited assessor and confirm their assessor number will appear on both certificates.
3. Freeze the architectural set — plans, elevations, sections, glazing schedules, insulation notes and services drawings that affect thermal performance.
4. Complete the simulation under the current BASIX Thermal Performance Protocol and obtain the NatHERS certificate (and Class 2 summary plus individual certificates for multi-dwelling).
5. Stamp every relevant sheet electronically; keep the stamp clear of other practitioners’ marks.
6. Enter the heating and cooling loads from the NatHERS certificate into BASIX and generate the BASIX certificate only when the numbers match.
7. Cross-check the four elements yourself using Appendix A of the DPHI guide before lodgement.
8. Lodge within three months of BASIX certificate generation, with matching property details on the Planning Portal form.

For the broader certificate process, see our step-by-step BASIX certificate guide. For renovation triggers, see when a renovation triggers BASIX and why you still need a certificate over $50k.

Where does giantA fit in?

Simulation packages fail when the NatHERS certificate, stamped drawings and BASIX loads describe three slightly different buildings. Our BASIX and NatHERS service coordinates assessments, certificates and plan mark-ups from our Parramatta studio so the four DPHI checks line up. For Sydney projects, see BASIX assessment in Sydney. If you need a project-specific review before DA or CDC lodgement, contact giantA or browse our services.

Frequently asked questions

What are the four Simulation method checks for councils and certifiers?

Development details (both certificates present and addresses matching), NatHERS certificate validity (template, QR code and assessor/certificate numbers), NatHERS stamp on design documents, and matching heating and cooling loads between the NatHERS and BASIX certificates.

Does the May 2025 Simulation guide apply to alterations over $50,000?

The guide is written for the Simulation method for new dwellings. Simulation for alterations and additions is only available in special circumstances under a separate Portal process. The $50,000 or more alterations threshold still triggers BASIX under the alterations pathway.

What is a NatHERS Class 2 summary?

It is the first page of a NatHERS certificate for a multiple-dwelling project such as an apartment building. It summarises all dwellings; certificates for each individual dwelling must be attached. Multi-dwelling BASIX thermal loads are checked against that summary.

Must heating and cooling loads match exactly between NatHERS and BASIX?

Yes. The guide says the calculations on both certificates must match for the assessment to be valid. Compare MJ/m².year figures for heating and cooling (and, for multi-dwelling, each dwelling’s loads against the Class 2 summary).

What documents need the NatHERS stamp?

All design documentation relevant to the NatHERS certificate, including at least site and floor plans, elevations and sections, materials documentation, window/skylight/door schedules, shadow drawings, electrical plans, insulation information where provided, design changes and supporting reports.

Can a council reject a NatHERS certificate if the QR code does not match?

Yes. The guide says a NatHERS certificate that fails the template, QR code or assessor-details checks is not to be accepted, and inconsistent or invalid elements should be returned to the proponent.

Preferred sources (verify current versions)

• DPHI — BASIX Guide: Checking thermal performance for the Simulation method (May 2025; first published December 2024, PDF)
• NSW Planning Portal — Simulation method
• NSW Planning Portal — Thermal Performance
• NSW Planning Portal — Council or Certifier
• NSW Planning Portal — BASIX and planning approvals
• NSW Planning Portal — Starting a BASIX application
• NSW Planning Portal — Alterations and additions
• NSW Planning Portal — Using simulation for alterations and additions
• NSW Planning Portal — BASIX Newsletter August 2026
• NSW Planning Portal — BASIX (development and assessment)