BASIX for Granny Flats and Secondary Dwellings

A granny flat or secondary dwelling in NSW will often need a BASIX Certificate as part of the residential approval process. This is because BASIX applies to NSW residential development and assesses water, energy use and thermal performance. For a secondary dwelling, the BASIX pathway depends on the type of project, how the dwelling is created and whether the approval is being lodged as a Development Application or Complying Development Certificate.

The NSW Planning Portal specifically asks users to indicate whether a project includes a secondary dwelling, such as a granny flat. Making the correct project type selection matters because it can affect the BASIX assessment pathway and the information required before the certificate can be issued.

What is a secondary dwelling?

A secondary dwelling is commonly known as a granny flat. It is a smaller dwelling located on the same lot as a principal dwelling. It may be detached in the backyard, attached to the main house or created by converting part of the existing home into a separate dwelling space.

From a BASIX perspective, the important question is how the project is classified and what part of the existing property is changing. A new detached secondary dwelling may be assessed differently from a conversion of existing habitable space. The correct pathway should be confirmed before the certificate is prepared.

Does a granny flat need a BASIX Certificate?

In many cases, yes. A granny flat or secondary dwelling will commonly need a BASIX Certificate because it is part of NSW residential development. BASIX is used to assess the sustainability performance of residential projects, including water, energy and thermal performance commitments.

The exact requirement depends on the scope of the project. If a secondary dwelling is being created by converting existing habitable space within the primary dwelling, the BASIX alterations and additions pathway may be relevant. If a new secondary dwelling is being built, the project may need to be entered in a way that reflects the new residential dwelling scope.

Why the BASIX project type matters

When starting a BASIX application, the project type selection is important because it determines the assessment pathway applied by the BASIX tool. The NSW Planning Portal notes that users must select the options that most closely describe the proposed development. This can determine whether the project is treated as a single dwelling, multi dwelling or alterations and additions assessment.

For a granny flat, this means the assessor needs to understand whether the secondary dwelling is part of a new build, a separate dwelling on an existing lot, an addition to the main home or a conversion of existing space. Selecting the wrong pathway can create confusion and may lead to the wrong information being requested or the wrong assessment assumptions being applied.

Granny flat BASIX for DA and CDC approvals

A secondary dwelling in NSW may be approved through a Development Application or Complying Development Certificate pathway, depending on the site, planning controls and project details. NSW Planning describes the approval process for secondary dwellings as getting approval for all works through either a development application or complying development certificate before building the granny flat.

For BASIX, the practical requirement is the same: the certificate should be prepared before lodgement and should match the drawings and specifications submitted with the application. If the plans change before lodgement, or if the approval pathway changes from CDC to DA, the BASIX Certificate may need to be checked or updated.

What BASIX looks at for a granny flat

For a granny flat or secondary dwelling, BASIX can include water, energy and thermal performance requirements. The certificate may include commitments relating to fixtures, hot water, rainwater, ventilation, heating and cooling, insulation, glazing, shading and other building fabric details.

The assessor may need to review:

• The size and layout of the secondary dwelling.
• The orientation of the building and windows.
• Window sizes, frame type, glass type and shading.
• Wall, roof, ceiling and floor construction.
• Insulation levels.
• Hot water, heating, cooling and ventilation systems.
• Water fixtures and rainwater tank commitments.
• Solar panels, if proposed or relied on in the assessment.

Does a granny flat need NatHERS as well?

Some granny flat or secondary dwelling projects may need NatHERS thermal comfort modelling as part of the BASIX pathway. NatHERS is used to estimate the heating and cooling loads of the dwelling and can support the thermal comfort section of BASIX when the simulation method is used.

This can be especially relevant where the granny flat has large windows, difficult orientation, limited shading, lightweight construction or design features that make thermal comfort harder to achieve. If NatHERS modelling is required, the assessor will need enough construction, glazing and insulation detail to model the dwelling accurately.

What documents are needed for a granny flat BASIX Certificate?

The required documents are similar to other BASIX residential assessments. The assessor needs enough information to understand the design, construction and systems proposed for the secondary dwelling. If the granny flat is attached to or created within the existing home, the assessor may also need to understand which parts of the existing dwelling are affected.

Useful documents and details include:

• Project address and site plan.
• Floor plans showing the granny flat layout and floor area.
• Elevations showing windows, doors, wall heights and external materials.
• Sections showing ceiling heights, roof form and construction details.
• Roof plan, especially if solar panels or roof shading are relevant.
• Window schedule or glazing details, if available.
• Wall, roof, floor and insulation specifications.
• Hot water, heating, cooling and ventilation details.
• Rainwater tank, water fixture or solar information, if known.
• Any council, certifier or planner requirements already received.

Common BASIX issues for granny flats

Granny flats can look simple, but they can still create BASIX issues. Because the dwelling is often small and located on an already developed site, the design may have less flexibility for orientation, window placement, shading, rainwater tank location or solar panel placement.

Common issues include:

• Limited space for rainwater tanks or outdoor services.
• Large glazing areas on small elevations.
• Poor solar orientation because the main dwelling or site layout limits placement.
• Overshadowing from the main house, fences or neighbouring buildings.
• Unclear construction details for walls, roof and floor systems.
• Late changes to window sizes, hot water systems or insulation.
• Confusion about whether the secondary dwelling is part of an alteration, addition or new dwelling assessment pathway.

Can a granny flat BASIX Certificate delay approval?

Yes, it can delay approval if the BASIX Certificate is missing, the project type is unclear, the documents are incomplete or the certificate does not match the plans. This is especially important for CDC projects, where the approval pathway is often expected to move quickly once the documents are ready.

The best way to avoid delay is to confirm the BASIX pathway early, provide complete plans and resolve major commitments before lodgement. If the granny flat design changes after the certificate is issued, the BASIX Certificate should be checked before the updated plans are submitted or built.

How Certified Energy can help

Certified Energy prepares BASIX Certificates for granny flats and secondary dwellings across NSW. Our team can review your available plans, confirm the likely assessment pathway and identify what information is needed before the certificate can be prepared.

If your project also requires NatHERS thermal comfort modelling, we can coordinate the BASIX and NatHERS components together so the certificate, drawings and performance assumptions remain aligned before lodgement.

Related BASIX resources

• Complete Guide to BASIX Certificates in NSW
• BASIX Certificate Service
• BASIX DA vs CDC: What Changes?
• What Documents Do You Need for a BASIX Certificate?
• NatHERS Assessments

Frequently asked questions

Does a granny flat need a BASIX Certificate in NSW?

A granny flat or secondary dwelling will commonly need a BASIX Certificate when it is assessed as residential development in NSW. The pathway depends on how the secondary dwelling is created and what approval pathway applies.

Is a secondary dwelling treated as a new dwelling for BASIX?

A secondary dwelling can be assessed through BASIX as part of the relevant residential project type. The NSW Planning Portal asks users to indicate whether a project includes a secondary dwelling, such as a granny flat, because this can affect the assessment pathway.

Can a granny flat be approved through DA or CDC?

Yes. A granny flat or secondary dwelling in NSW may be approved through a Development Application or Complying Development Certificate pathway, depending on the site, planning controls and project details.

What documents are needed for a granny flat BASIX Certificate?

An assessor usually needs the project address, site plan, floor plans, elevations, sections, orientation, window details, construction information and any available hot water, rainwater, solar, heating, cooling or ventilation details.