For some alterations and additions, using the simulation method can give greater design flexibility. It allows you to balance the performance of different new building elements.
This method can be useful when practical or legal constraints prevent full compliance with the BASIX requirements for alterations and additions, such as:
- Heritage considerations
- Structural limitations
- Strata restrictions in unit buildings.
If you want to use the simulation method for alterations and additions, follow the process outlined below.
Application process
Email [email protected] with subject “BASIX – simulation for alterations and additions”, and outline why you believe that your project cannot fully comply with the BASIX requirements.
Note: Simulation for alterations and additions is not available for:
- design flexibility in the absence of specific heritage, structural, or legal restrictions; or
- purely aesthetic reasons.
If the BASIX team advises that you can use simulation for alterations and additions, follow the modelling process below.
Modelling process
The modelling process for alterations and additions is much more involved than for a new dwelling. You will need to run 3 separate simulations:
- Existing: model the existing dwelling ‘as is’, and document all assumptions about the thermal performance of the existing building fabric.
- Reference: model the entire existing dwelling ‘as is’ and also model the alterations and additions, ensuring all new work complies with the relevant BASIX Construction and Glazing commitments.
- Proposed: model the entire existing dwelling and the alterations and additions, but with the proposed performance for the new building elements. Only change the performance of the parts of the existing building fabric where new work is being done, such as installing additional insulation or replacing a window.
You need to demonstrate that the alterations and additions do not reduce the overall thermal performance of the dwelling, and that the proposal performs at least as well as if it had been in accordance with the BASIX Construction and Glazing commitments. That is:
- the total loads calculated for the proposed model must be less than or equal to the total loads calculated for the existing model; and
- the total loads calculated for the proposed model must be less than or equal to the total loads calculated for the reference model.
Documentation requirements
If the thermal loads meet the criteria described in the Modelling Process section, you will need to collect the following documentation:
- the simulation output generated by Approved Software for the existing, reference, and proposed models
- a record of all assumptions made about the building fabric of the existing dwelling, signed by the applicant
- supporting evidence such as a letter from council, an engineer's report or a letter from the building or strata manager
- plans and specifications for the proposed work
- photos of the existing dwelling.
Notes on the required simulation outputs:
For the existing and reference models, it is the interim/diagnosis reports and building data reports from the simulation software and/or the online system from the software provider.
For the proposed model, it is the NatHERS universal certificate issued from the online generation system by the software provider.