The NatHERS Assessment Process: A Step-by-Step Guide
A complete walkthrough of the NatHERS assessment process, from engaging an assessor to receiving your star rating report and making improvements.
Overview
The NatHERS assessment process evaluates the thermal performance of your home's design to determine its star rating. Whether you are building a new home or undertaking a major renovation, understanding each step helps you prepare properly and get the best possible result. Here is a detailed walkthrough of what to expect.
Step 1: Engage an Assessor Early
The ideal time to engage a NatHERS assessor is during the design development stage, before your plans are finalised. Engaging early allows the assessor to provide input on design decisions that significantly affect the star rating, such as window placement, insulation specifications, and building orientation.
If you wait until plans are already submitted to council, making changes to improve the rating can be costly and time-consuming. Most experienced assessors recommend being involved from the preliminary design stage.
Step 2: Provide Your Plans and Specifications
Your assessor will need the following documents to perform the assessment:
- Architectural floor plans — showing room layouts, dimensions, window and door positions
- Elevations — showing external wall heights, roof pitch, eave widths, and window sizes
- Site plan — showing the building's orientation (north point), setbacks, and any significant neighbouring buildings or vegetation
- Specifications — including wall construction type (brick veneer, lightweight, reverse brick veneer), insulation R-values for ceiling, walls and floor, and glazing details (single, double, or triple glazed; frame material; any coatings)
- Section drawings — showing ceiling heights, sub-floor details, and construction build-ups
The more complete and accurate the information you provide, the more accurate the assessment will be. Missing or unclear details may require the assessor to make assumptions, which could affect the result.
Step 3: The Assessor Models Your Home
Using one of the four NatHERS-accredited software tools (FirstRate5, AccuRate, BERS Pro, or Hero), the assessor creates a detailed digital model of your home. This process involves:
- Drawing the floor plan in the software, room by room
- Setting the construction type for each wall, ceiling, and floor element
- Inputting insulation R-values and positions within the construction build-up
- Specifying each window and door — size, orientation, frame material, glazing type, and any external shading
- Adding eaves, pergolas, and other shading structures
- Setting the correct NatHERS climate zone based on the site address
The software then runs a simulation using hourly climate data for the full year, calculating how much energy would be needed to maintain comfortable temperatures (typically 20 degrees in winter and 25 degrees in summer) in every room of the home.
Step 4: Understanding the Results
The assessment produces a report that includes:
- Overall star rating — the headline figure from 0 to 10 stars
- Heating load — energy required for heating, in MJ/m2/year
- Cooling load — energy required for cooling, in MJ/m2/year
- Total conditioned floor area — the area of the home included in the assessment
- Room-by-room breakdown — showing which rooms are performing well and which are dragging down the overall rating
The room-by-room breakdown is particularly valuable. It identifies the weakest-performing areas of the home, allowing targeted improvements rather than broad (and often expensive) upgrades across the entire building.
Step 5: Iterative Improvement
If the initial assessment does not achieve your target star rating, the assessor can model changes to identify the most cost-effective improvements. Common adjustments include:
- Upgrading insulation in specific areas (for example, increasing wall insulation from R2.0 to R2.5)
- Changing window glazing from single to double glazed, or adding low-E coatings
- Adding or extending eaves to shade north-facing windows in summer
- Adjusting window sizes — sometimes reducing a large south-facing window or adding a north-facing window can improve the rating
- Adding ceiling fans to reduce cooling loads in appropriate climate zones
A good assessor will quantify the star rating improvement and approximate cost of each change, helping you make informed decisions about where to invest. Read our guide on improving your home energy rating for more specific strategies.
Step 6: Final Report and Compliance Certificate
Once the design achieves the required star rating (minimum 7 stars under NCC 2022), the assessor issues the final NatHERS certificate. This document is submitted to your building surveyor or council as part of the building permit application.
The certificate includes the final star rating, the assessed plans, and the specifications used in the model. It is important that the home is built in accordance with these specifications — if you change insulation, glazing, or construction type during building, the actual performance may differ from the assessed rating.
Timeline and What to Expect
A typical NatHERS assessment takes 3 to 7 business days from when the assessor receives complete plans and specifications. Complex homes or those requiring multiple rounds of design iteration may take longer.
Most assessors include one to two rounds of design revisions in their quoted fee. Additional revisions are usually charged at an hourly rate or a fixed fee per revision. To keep costs down and timelines tight, provide complete and accurate information upfront and discuss your target star rating with the assessor before they begin modelling.