In New South Wales, alterations and additions valued at $50,000 or more generally need a BASIX certificate — and so do pool/spa works over 40,000 litres. Most renovations clear the standard alterations & additions pathway on the NSW Planning Portal. When heritage, structural, or strata constraints block a full Deemed-to-Satisfy path, you may seek permission to use the simulation method, with NatHERS modelling against existing and reference cases. This guide explains when that path is available, how the three-model process works, and what documentation councils and certifiers expect.
Primary sources: Alterations and additions (NSW Planning Portal); Using simulation for alterations and additions; Simulation method. Last checked September 2026. Always confirm live Portal guidance for your project.
When does a NSW renovation trigger BASIX?
According to the NSW Planning Portal, BASIX applies to all alterations and additions valued at $50,000 and over, and/or involving a pool and/or spa with total volume greater than 40,000 litres. Do not rely on outdated $25,000 figures circulating online — the Portal threshold for alterations is $50,000.
The alterations & additions assessment differs from new dwellings: there are no set percentage reductions for energy or water. Instead the tool pushes efficiency improvements (glazing, insulation, design and materials), requires at least 40% of new or altered light fixtures to use high-efficiency globes, promotes lower-emission hot water, and requires new taps, toilets and showerheads to meet a minimum 3 WELS stars.
If works are valued at $50,000+ but are not for use as a dwelling (or capable of being used as one), check with the local council whether BASIX applies.
| Scenario | BASIX trigger (Portal) | Typical pathway |
|---|---|---|
| Alterations/additions ≥ $50,000 | Yes | Alterations & additions tool on Planning Portal |
| Pool/spa > 40,000 L (serving one dwelling) | Yes | Alterations tool / pool commitments |
| Works under $50,000, no large pool/spa | Usually no BASIX certificate for alterations | Confirm scope; other approvals may still apply |
| Heritage / structural / strata constraints block DTS commitments | Still BASIX if threshold met | May request simulation permission |
What is the BASIX simulation method — and when is it available for alterations?
For new dwellings, the simulation method lets an accredited NatHERS assessor model heating and cooling loads in approved software, then enter those loads into BASIX. Loads must sit below the maximums BASIX calculates for the climate zone.
For alterations and additions, the Portal is clear: simulation is not the default. It is reserved for practical or legal constraints that prevent full compliance with standard BASIX alterations requirements — for example heritage considerations, structural limitations, or strata restrictions in unit buildings. It is not available for design flexibility alone, or for purely aesthetic reasons.
If you want simulation for alterations, email basix@planning.nsw.gov.au with subject “BASIX – simulation for alterations and additions” and explain why the project cannot fully comply. Proceed with modelling only after the BASIX team advises that simulation is allowed.
How does the three-model NatHERS process work for renovations?
Once permission is granted, alterations simulation is more involved than a new dwelling. You must run three separate simulations:
- Existing — model the dwelling “as is”, documenting all assumptions about the thermal performance of the existing fabric.
- Reference — model the existing dwelling plus the alterations/additions, with all new work complying with the relevant BASIX Construction and Glazing commitments.
- Proposed — model existing plus alterations with the proposed performance of new elements. Only change fabric performance where new work is occurring (for example added insulation or a replaced window).
Pass criteria (Portal):
- Proposed total loads ≤ existing total loads; and
- Proposed total loads ≤ reference total loads.
In other words, the renovation must not worsen overall thermal performance, and must perform at least as well as if new work had followed the standard BASIX Construction and Glazing commitments.
What documentation do you need after the models pass?
If thermal loads meet the criteria, collect:
- Simulation outputs for existing, reference, and proposed models from Approved Software
- A signed record of assumptions about the existing building fabric
- Supporting evidence (for example a council letter, engineer’s report, or building/strata manager letter)
- Plans and specifications for the proposed work
- Photos of the existing dwelling
Output notes from the Portal:
- Existing and reference: interim/diagnosis reports and building data reports from the simulation software and/or the software provider’s online system
- Proposed: the NatHERS universal certificate issued from the online generation system by the software provider
How do NatHERS and the BASIX Thermal Performance Protocol fit together?
The BASIX Thermal Performance Protocol governs simulation assessments: who may accredit assessors, which software is approved, and how simulations must be run. It refers extensively to NatHERS technical notes. Where the Protocol and NatHERS notes differ, the Protocol prevails. Use the current Protocol version aligned with your BASIX standards (the Portal lists the 7 June 2024 Protocol for current V4 standards).
Engage an accredited assessor early when simulation is likely — heritage and strata projects often need existing-fabric evidence that takes longer than a standard alterations certificate.
Practical sequencing for designers, builders, and homeowners
- Confirm the cost threshold. Estimated development cost on the DA/CDC application is what consent authorities and certifiers use (EP&A Regulation 2021 reg 6A), unless they decide the estimate is not genuine or accurate.
- Try the standard alterations tool first. Many projects pass with glazing, insulation, lighting, hot water, and WELS commitments without simulation.
- Document constraints. If heritage, structure, or strata block DTS commitments, gather evidence before emailing the BASIX team.
- Wait for written permission before commissioning the three-model NatHERS package.
- Lodge the BASIX certificate with the DA or CDC package once commitments (or approved simulation outcomes) are locked.
| Path | Best for | NatHERS role |
|---|---|---|
| Standard alterations & additions tool | Most ≥ $50k renos without hard constraints | Usually not required |
| Simulation (with permission) | Heritage / structural / strata constraints | Three models + NatHERS universal certificate for proposed |
| New dwelling simulation | New houses / multi-dwellings | Single Assessor Certificate loads into BASIX |
Common mistakes that delay BASIX on renovation DAs
Three issues show up repeatedly on NSW alterations packages:
- Wrong cost threshold. Teams still cite $25,000 from older secondary articles. Consent authorities use the estimated development cost on the application against the Portal’s $50,000 alterations trigger (and may reassess if the estimate looks inaccurate).
- Simulation without permission. Lodging NatHERS outputs for an alterations project before the BASIX team agrees wastes fees and programme time. Email first; model second.
- Thin existing-fabric evidence. The existing and reference models stand or fall on documented assumptions. Photos, strata letters, and engineer notes should be collected before the assessor finalises models — not after a fail.
Internal links for related giantA guidance: BASIX for alterations and additions ($50k threshold) and BASIX ventilation changes 2026.
Checklist before you lodge
- Estimated cost ≥ $50,000 or pool/spa > 40,000 L confirmed
- Standard alterations tool attempted (or clearly blocked by heritage/structure/strata)
- If simulating: written BASIX team advice on file
- Three models complete with pass criteria met
- NatHERS universal certificate (proposed) + diagnosis/building data reports (existing/reference)
- Signed fabric assumptions, plans, photos, supporting evidence
- BASIX certificate generated on the Planning Portal and attached to DA/CDC
How giantA helps with BASIX and NatHERS on NSW renovations
giantA Pty Ltd supports residential alterations and additions across NSW with design documentation coordinated to Planning Portal BASIX commitments. Where simulation may be needed, we work with accredited NatHERS assessors, document existing fabric assumptions, and keep DA/CDC packages aligned with Portal requirements — without inventing thresholds or fees. For project advice, contact giantA via gianta.com.au.
Byline: Franz Phan, Senior Building Designer — giantA Pty Ltd (NSW).
Frequently asked questions
Is the BASIX alterations threshold still $25,000?
No. The NSW Planning Portal states BASIX applies to alterations and additions valued at $50,000 and over (and/or pools/spas over 40,000 litres). Use the Portal figure, not older secondary summaries.
Can I choose NatHERS simulation for design flexibility on a renovation?
No. For alterations, simulation is only for specific heritage, structural, or legal/strata constraints after the BASIX team agrees. Aesthetic preference or design flexibility alone is not enough.
Do I need three NatHERS models for every renovation?
Only if you are approved to use simulation for alterations. Most renovations use the standard alterations & additions tool without the three-model process.
What certificate do I lodge for the proposed simulation model?
The Portal requires the NatHERS universal certificate from the software provider’s online generation system for the proposed model, plus diagnosis/building data outputs for existing and reference models.
Where do I apply for simulation permission?
Email basix@planning.nsw.gov.au with subject “BASIX – simulation for alterations and additions” and explain why full compliance is not possible.
Does embodied emissions modelling apply to alterations?
NSW Planning has clarified that alterations needing a BASIX certificate do not have to calculate embodied emissions of materials — that obligation is for new residential development. Confirm current Sustainable Buildings SEPP settings on the Portal for your lodgement date.