General informationThis is not building or energy advice. BASIX is the NSW Government’s sustainability tool for homes in NSW; its rules and the tool itself are on the NSW Planning Portal.
Which part of the tool
BASIX has separate sections for a single dwelling and for multiple dwellings, and the Portal’s examples sort a duplex between them by how the land and services are held.
Single dwelling
For a new single dwelling, typically a Class 1 building. The Portal says this can cover one home in a conjoined or semi-detached house, where that home sits on its own land title and shares no common property or services.
Multi-dwelling
For a project with more than one dwelling, or with common property or services shared with other development. The Portal’s examples include two or more semi-detached houses, whether or not they share common property or services, and a dual occupancy whose two homes sit on a single parcel of land and share common property, services or both.
The practical difference is the thermal assessment. The Portal says the do-it-yourself method for thermal comfort cannot be used for multiple dwellings: an accredited assessor must complete that section using NatHERS-accredited software.
One certificate, or two
For new dwellings, the Portal says the applicant must submit one BASIX certificate covering all the dwellings with the development application. The council, as consent authority, may allow the project to be split into separate certificates, but the applicant has to ask; if the council refuses, the applicant may ask it to discuss the matter with the department.
Of the cases the Portal lists for a split, two can fit a duplex:
- a development on land with common property, shared services or shared facilities, such as a strata-titled dual occupancy, as long as the common areas and shared services can be divided between the parts each certificate covers;
- a staged development, built in separate stages over time.
The Portal gives the reasons a split can help: it allows part consent for a staged development, means only the certificates for changed dwellings need revising later, and avoids complications if one part is sold on or built by a different builder.
Where the certificate goes, stage by stage
- While the design is changing. The Portal suggests using the BASIX report function until the design is final and ready to lodge, to avoid additional fees.
- Generating the certificate. It can be printed only once the project meets the BASIX targets, every mandatory field is complete and the fee is paid. Since May 2026 the user must also declare that the information entered is accurate.
- Three months to lodge. A certificate is valid for 3 months before it is submitted to the council or certifier. Once lodged, it is valid for the maximum life of the development application; one not lodged in time must be generated again, with new fees.
- With the application. The certificate goes in with the development application or the complying development certificate application. The Portal now asks for the certificate’s reference number in the application form and checks it before the application can be submitted.
- If the design changes. The certificate must be revised and resubmitted. There is no charge to revise a certificate for the same property and project if it was lodged or is still valid, and the commitments it lists must be noted on the plans.
- Construction certificate. The BASIX certificate is attached to the construction certificate application, with every BASIX commitment shown on the plans. A certifying authority inspects the work.
- Completion receipt. After the final inspection and before the occupation certificate, the certifying authority must issue a BASIX completion receipt confirming the project was built as the certificate says.
What the tool measures
From the design details entered, the tool calculates a water score and an energy score, where higher is better, and a heating load and a cooling load for thermal comfort, where lower is better. The scores compare the design with a similar benchmark dwelling from before BASIX began in 2004: a water score of 40% means an estimated 40% less water than the benchmark house.
The standards themselves are in Schedule 1 of the State Environmental Planning Policy (Sustainable Buildings) 2022: a water standard set by location, an energy standard set by location and type of development, and thermal standards made up of a maximum heating load, a cooling load cap and a maximum combined load. The tool also has a materials index that measures the embodied emissions of the construction materials.
On 1 October 2023 the thermal standard for new homes rose to 7 stars on NatHERS, and the increase applied everywhere with two exceptions: homes in climate zones 9, 10 and 11, and apartment buildings of 5 storeys or fewer. The department publishes an energy standards and thermal loads calculator and water standard maps for working out the figures for a particular location.
The tool changed in May 2026
On 1 May 2026 the Portal introduced a set of BASIX tool updates. Those that touch a duplex:
- Multi-dwelling projects can ask to have up to 1,000 dwellings on a single certificate, in any mix of single dwellings, duplexes, townhouses or apartments.
- The standards version for new homes moved from v4.03 to v4.04, and the Portal says projects already in progress are not affected.
- Exhaust from bathroom, kitchen and laundry ventilation can no longer be directed into the roof space.
- Certificates and reports for new homes now show the thermal performance method on the first page.
The department says that from 1 October 2023 the BASIX standards line up with the National Construction Code 2022 and complement it. For the building code’s rules on the wall between the two homes, see the party wall page.