BASIX for Alterations and Additions in NSW

BASIX can apply to renovations, extensions and other alterations and additions to existing homes in NSW. Many homeowners assume BASIX only applies to new homes, but the NSW Planning Portal explains that BASIX applies to alterations and additions valued at $50,000 and over, and to projects that involve a pool or spa with a total volume greater than 40,000 litres.

For alterations and additions, the BASIX assessment needs to understand what already exists, what is being changed and what new work is proposed. This can make renovation assessments more nuanced than a simple new home project, because the certificate may need to distinguish between existing building fabric, altered construction and new construction.

When does BASIX apply to alterations and additions?

BASIX applies to all alterations and additions in NSW that are valued at $50,000 and over. It also applies where the work involves the installation of a pool or spa with a total volume greater than 40,000 litres. This means a renovation, extension, second storey addition or major internal and external alteration may need a BASIX Certificate if it reaches the relevant threshold.

The key question is not only whether the home already exists. The key question is whether the proposed work triggers BASIX. If the project does trigger BASIX, the certificate should be prepared before lodgement and should match the plans and specifications submitted for approval.

Does every renovation need BASIX?

Not every renovation needs BASIX. A smaller renovation below the relevant cost threshold may not require a BASIX Certificate unless another trigger applies, such as a qualifying pool or spa. However, the project team should still confirm the requirement with the approval pathway, council, certifier or planning adviser before lodgement.

It is also important to distinguish between building approval requirements and good performance planning. A project may not need BASIX but may still benefit from early energy performance advice, especially where the renovation affects glazing, insulation, ventilation, hot water, roof colour, shading or future comfort.

Why alterations and additions can be more complex than new homes

A new home assessment usually starts with a complete proposed design. An alteration or addition starts with an existing building, then adds or changes parts of it. This means the assessor may need to understand which areas are existing, which areas are being altered and which areas are entirely new construction.

This distinction matters because not every part of the existing home is necessarily treated the same way. If the work only affects certain walls, ceilings, roofs, floors, windows or systems, the BASIX assessment should reflect the actual scope rather than assuming the entire house is new.

What counts as new or altered construction?

For renovation projects, the BASIX assessment may need to identify new or altered construction. NSW Planning provides guidance for new or altered construction, including how to treat ceilings and roofs where insulation already exists in one part but not the other. This helps prevent the certificate from requiring insulation where insulation already exists, while still assessing the work that is actually being changed.

For example, if the work involves entirely new ceilings with roof above, that may need to be entered as new construction. If the work alters existing walls, windows or roof areas, the assessor may need to understand the existing construction and the proposed upgrade. Clear renovation drawings are essential for this step.

What BASIX assesses in alterations and additions

BASIX for alterations and additions can still relate to water, energy use and thermal performance. The exact inputs depend on the scope of work, but the assessment may include changed fixtures, new or altered windows, new insulation, hot water systems, rainwater tanks, solar panels, pool or spa details and other commitments connected to the proposed works.

The assessment may need to review:

• The estimated development cost and BASIX trigger.
• Which rooms, walls, roofs, ceilings and floors are new or altered.
• New or altered windows and glazed doors.
• Insulation in new or altered construction.
• Roof colour, roof insulation and ceiling insulation where relevant.
• Hot water, heating, cooling and ventilation systems.
• Rainwater tanks, water fixtures and alternative water uses.
• Solar PV or other energy commitments.
• Pools or spas included in the works.

Can alterations and additions use simulation?

Yes. NSW Planning explains that for some alterations and additions, using the simulation method can provide greater design flexibility. It can be useful where practical or legal constraints prevent full compliance with the standard BASIX requirements for alterations and additions.

Examples listed by NSW Planning include heritage considerations, structural limitations and strata restrictions in unit buildings. In these cases, simulation can help balance the performance of different new building elements rather than relying only on a simpler standard pathway.

Common BASIX issues in renovation projects

Renovations often create BASIX issues because the existing building may not be fully documented. Older homes may not have clear construction details, window schedules, insulation information or accurate existing drawings. This can make it harder to separate existing conditions from proposed changes.

Common issues include:

• Unclear distinction between existing, new and altered construction.
• Missing existing dwelling information.
• Window changes not clearly shown on elevations.
• Insulation assumptions not documented.
• Hot water or heating and cooling systems changing during design.
• Rainwater tank commitments that do not fit the existing site.
• Existing solar PV being assumed without checking how it can be used in BASIX.
• Plans changing after the BASIX Certificate is issued.

Secondary dwellings and converted spaces

Some secondary dwelling projects, such as granny flats, are created by converting existing habitable space within the primary dwelling. NSW Planning notes that the BASIX tool can be used to generate a BASIX Certificate for a secondary dwelling where that dwelling is created by converting existing habitable space within the primary dwelling.

This is another example of why project classification matters. A new detached secondary dwelling, a garage conversion, an internal conversion and a larger alteration may not all be treated the same way. The assessor needs to understand the actual project scope before selecting the correct BASIX pathway.

What documents are needed?

For alterations and additions, the assessor needs enough information to understand both the existing home and the proposed works. If the drawings only show the new work but not how it connects to the existing home, the assessment may need more clarification.

Useful documents and details include:

• Project address and estimated development cost.
• Existing and proposed floor plans.
• Demolition plans, if available.
• Elevations and sections showing new and altered work.
• Roof plan and roof form changes.
• Window schedule or clear glazing information.
• Construction and insulation details for new or altered walls, roofs, ceilings and floors.
• Hot water, rainwater, solar, heating, cooling and ventilation details.
• Pool or spa details, if included.
• Any heritage, strata, structural or site constraints that affect compliance options.

Can a renovation BASIX Certificate be amended?

Yes. If the renovation design changes after the BASIX Certificate is issued, the certificate may need to be checked or amended. This can happen if window sizes change, the extension footprint changes, roof form changes, insulation changes, hot water systems are updated or rainwater and solar commitments are revised.

Renovation projects often evolve during design or construction, so it is important to check BASIX before relying on an older certificate. If the certificate no longer matches the current plans, it may create approval or certification issues later.

How Certified Energy can help

Certified Energy prepares BASIX Certificates for alterations and additions across NSW. Our team can review the proposed works, confirm whether BASIX is likely to apply, identify what information is missing and prepare the certificate once the project is ready for assessment.

Where the project involves complex renovation constraints, changed glazing, new insulation, existing solar, pools, spas or thermal performance issues, we can help coordinate the BASIX pathway with the drawings and approval documentation before lodgement.

Related resources

• Complete Guide to BASIX Certificates in NSW
• BASIX Certificate Service
• Can a BASIX Certificate Be Amended?
• Common BASIX Mistakes That Delay Approval
• BASIX vs Home Energy Rating: What Is the Difference?

Frequently asked questions

When does BASIX apply to alterations and additions in NSW?

BASIX applies to alterations and additions in NSW when the estimated development cost is $50,000 or more, and also where the work involves a swimming pool or spa with a total volume greater than 40,000 litres.

Does a renovation always need a BASIX Certificate?

Not every renovation needs a BASIX Certificate. BASIX generally applies to alterations and additions valued at $50,000 or more, or projects involving a pool or spa above the relevant volume threshold. The approval pathway and project scope should be checked before lodgement.

What information is needed for a BASIX alterations and additions certificate?

An assessor usually needs the project address, current plans, existing and proposed floor plans, elevations, sections, roof plans, window details, construction and insulation details, hot water, rainwater, solar, pool or spa details and a clear description of the new or altered work.

Can BASIX for alterations and additions use simulation?

Yes. NSW Planning explains that some alterations and additions may use the simulation method where practical or legal constraints make full compliance with standard BASIX requirements difficult, such as heritage, structural limitations or strata restrictions.