NatHERS for Renovations: When You Need an Assessment

Do Renovations Require a NatHERS Assessment?

Not every renovation triggers the need for a NatHERS assessment, but many do. The key factor is whether the renovation affects the building envelope — the external walls, roof, floor, windows, and doors that separate conditioned (heated or cooled) spaces from the outside.

As a general rule, a NatHERS assessment is required when:

• You are adding an extension that increases the floor area of conditioned spaces
• You are undertaking a major renovation that involves significant changes to the building envelope (such as replacing a wall, re-roofing, or reconfiguring the floor plan)
• You are converting an unconditioned space (such as a garage or attic) into a habitable room

Minor renovations such as kitchen or bathroom fitouts, painting, or replacing like-for-like fixtures generally do not trigger NatHERS requirements, provided the building envelope is not significantly altered.

State and Local Variations

The specific triggers for a NatHERS assessment during renovation vary between states, territories, and even individual councils. Some important variations include:

• New South Wales — BASIX (Building Sustainability Index) applies to alterations and additions over $50,000 in value. BASIX includes energy efficiency targets that may be assessed through NatHERS software.
• Victoria — The Victorian Building Authority requires energy efficiency compliance for most building works involving new or altered habitable rooms.
• Queensland — Energy efficiency provisions apply to new building work, including extensions and substantial renovations.

Always check with your local council or building surveyor to confirm whether your specific renovation requires a NatHERS assessment. Getting clarity on this early avoids surprises during the permit process.

Whole-Home vs Partial Assessment

One of the unique challenges of renovation assessments is deciding whether to assess the entire home (existing plus new work) or only the new or altered portion.

The approach depends on the scope of work and local requirements:

• Extension only — in many jurisdictions, only the extension needs to meet the current NatHERS standard (7 stars under NCC 2022). The existing portion of the home is not required to be upgraded.
• Whole-home assessment — required in some circumstances, particularly where the renovation is extensive enough that the entire building is effectively being rebuilt. This approach can be more challenging because the existing structure may have poor thermal performance that drags down the overall rating.
• Deemed-to-Satisfy (DTS) — for smaller renovations, it may be possible to demonstrate compliance through prescriptive measures (specific insulation levels, glazing types) without a full NatHERS modelling exercise.

Challenges of Assessing Existing Homes

Assessing renovations presents unique challenges compared to new builds:

• Unknown construction details — older homes may have no records of insulation type or R-value, wall cavity width, or sub-floor construction. The assessor may need to make reasonable assumptions or recommend a site inspection.
• Existing thermal weaknesses — single-glazed windows, uninsulated walls, and poor orientation cannot always be changed without significant expense.
• Heritage considerations — heritage-listed or character homes may have restrictions on external changes such as window replacement or the addition of eaves.
• Cost constraints — renovators typically work within tighter budgets than new-build clients, so the assessor needs to identify the most cost-effective improvements.

Cost-Effective Improvement Strategies for Renovations

When improving the energy rating of an existing home, focus on the changes that deliver the greatest thermal improvement for the lowest cost:

Quick Wins (Low Cost, High Impact)

• Add or top up ceiling insulation — if the existing insulation is thin or degraded, adding a new layer on top is one of the cheapest and most effective upgrades
• Draught seal doors and windows — foam strips, rubber seals, and door sweeps can significantly reduce air leakage for very little cost
• Install ceiling fans — particularly effective in warmer climates, reducing the cooling load credited in NatHERS software
• Add external shading — awnings, shade sails, or deciduous plantings can reduce cooling loads on west and north-facing windows

Medium Investment

• Upgrade windows to double glazing — either replace the full window or install secondary glazing panels over existing frames
• Add wall insulation — possible in some construction types by injecting foam insulation into existing wall cavities
• Install reflective sarking — a reflective membrane under the roof or behind wall cladding that reduces radiant heat transfer

Major Investment

• Re-roof with insulated panels — if the roof is being replaced anyway, consider insulated roof panels for superior performance
• Replace external cladding — an opportunity to add a full insulation layer during the re-cladding process

Working with Your Assessor on Renovations

When engaging an assessor for a renovation project, provide as much information as possible about the existing construction. Photographs of the roof space (showing insulation), sub-floor area, and wall cavities are extremely helpful. Your assessor can then model the existing home accurately and identify which upgrades will deliver the best return.

Many assessors offer a preliminary assessment of the existing home before the renovation design is finalised, allowing you to set realistic targets and budget accordingly. This early involvement is particularly valuable for renovation projects where the existing building performance is unknown. Start your search for a qualified assessor using our assessor directory.