NatHERS Existing Homes
A NatHERS existing home assessment helps explain how an established dwelling performs as it stands today.
Unlike a new home NatHERS assessment, which is usually based on proposed design documentation, an existing home assessment responds to the real dwelling. It looks at the current construction, installed systems, available documentation and observable performance issues.
For homeowners, designers, builders and property professionals, the process becomes much easier to understand when it is broken into stages: property details, site data collection, assessment input, modelling, review and reporting.
Quick Answer
A NatHERS existing home assessment usually starts with basic property details and any available documentation. The assessor or data collector then gathers information about the dwelling, including construction, layout, insulation, glazing, shading, appliances and energy systems.
That information is entered into an approved assessment pathway so the home’s energy performance can be modelled and reviewed. The final output may include a Home Energy Rating Certificate and supporting information about comfort, energy use and possible upgrade opportunities.
The process is designed for homes that already exist. It is not simply a new home NatHERS assessment copied onto an older dwelling. Existing homes often have missing plans, unknown insulation, previous renovations and installed systems that need to be carefully recorded or reasonably assessed.
Existing homes are more complex than proposed designs because the building has already been constructed, lived in, altered and maintained over time. The assessment process needs to work with real conditions rather than ideal design intent.
Some homes have complete drawings and construction details. Others have very little documentation. Some have been renovated several times, with insulation, glazing, appliances or heating systems changed along the way. This makes data collection and review an important part of the assessment.
A clear process helps homeowners understand what is being assessed, why the information is needed and how the final rating can support better decisions.
The first stage is usually a review of the property and the purpose of the assessment. This helps confirm whether a NatHERS Existing Homes assessment is the right pathway.
Useful starting information may include:
The reason for the assessment matters. A homeowner planning upgrades may need different advice from a property team preparing for future disclosure, portfolio review or program participation.
If plans, specifications or renovation records are available, they can help the assessment process. They may show wall construction, glazing changes, extension areas or parts of the home that are difficult to verify visually.
However, many existing homes do not have complete documentation. This does not necessarily prevent an assessment, but it does affect how information is collected, recorded and reviewed.
A good assessment process should identify what is known, what can be observed and where assumptions may be required. This is one reason existing home assessment is different from new home NatHERS, where the assessor usually works from design documentation before construction.
On-site data collection is the part of the process that makes an existing home assessment practical and grounded. The dwelling needs to be recorded as it actually exists, not only as it may have been designed many years earlier.
The site data collection process may record:
This stage may involve a trained assessor or, depending on the delivery model, a trained data collector working with an assessor. The important principle is that the information needs to be collected consistently enough to support a reliable rating.
Once the property data has been collected, it needs to be entered into the relevant assessment process. This is where the dwelling information becomes structured enough to support modelling and rating.
For existing homes, this step needs careful handling because not every input will be known with the same level of certainty. Some information may be measured. Some may be documented. Some may need to be based on approved defaults or reasonable assumptions, depending on the assessment rules.
The quality of this stage depends on how clearly the data has been collected and how well the assessor understands existing-home construction, energy systems and assessment requirements.
The assessment data is then used to calculate the home’s energy performance. This may include thermal performance, comfort-related outcomes and whole-of-home energy considerations such as major appliances, solar and batteries.
This stage connects the observed dwelling to a performance rating. It helps translate building features into a clearer understanding of how the home is likely to behave in different seasons and conditions.
For a deeper explanation of how these assessment pathways differ from new home compliance, see NatHERS Existing Homes vs New Home NatHERS Assessments.
Existing homes often raise questions during review. An assessor may need to clarify whether an extension was insulated, whether a window has been replaced, whether a system is still in use or whether documentation matches the actual dwelling.
This review stage is important because a rating should not simply be generated from incomplete or poorly understood information. Where uncertainty exists, it should be handled consistently and carefully within the assessment rules.
For homeowners, this may mean answering follow-up questions, providing photos, confirming system details or supplying any missing information that becomes relevant during the review.
The final stage is reporting. Depending on the assessment pathway, this may include a Home Energy Rating Certificate and supporting information about the dwelling’s performance.
A useful report should help the property owner understand more than a single rating. It should explain what the rating means in practical terms and how it may relate to comfort, energy use, upgrade planning or disclosure readiness.
The report can become a decision-making tool. It may help a homeowner decide whether to improve insulation, replace heating and cooling, address draughts, upgrade hot water, review solar and battery options or stage renovation works more carefully.
A homeowner does not need to have perfect records before requesting an assessment. However, the process is easier when basic information is available.
Useful information may include:
Even partial information can be helpful. The assessor can then identify what is already known and what needs to be confirmed through data collection or review.
A NatHERS existing home assessment can help turn general energy concerns into a more structured pathway.
It may support decisions about:
For a broader definition of this type of rating, see What Is a Home Energy Rating for Existing Homes?
Timing can vary depending on the property, documentation, access, assessment pathway and whether follow-up information is required.
A simple home with clear access and available documentation may be more straightforward. A larger or heavily altered dwelling may take longer because more information needs to be checked and entered carefully.
The best first step is to send the property details, available plans and the reason for the assessment so the correct pathway and likely process can be reviewed.
One misunderstanding is that the assessment is only a visual inspection. Visual observation matters, but the process also involves structured data collection, input, modelling, review and reporting.
Another misunderstanding is that a homeowner must already know every construction detail. In many existing homes, some details are unknown. The process is designed to work with a combination of measured, documented, observed and assessed information.
A third misunderstanding is that the rating automatically tells the homeowner to buy one specific product. A good assessment should support better decision-making, not simply push a single upgrade.
Finally, an existing home assessment should not be confused with new home compliance. For that distinction, see NatHERS Existing Homes vs New Home NatHERS Assessments.
For homeowners, the assessment can provide a clearer sequence for upgrades rather than a disconnected list of energy ideas.
For architects and designers, it can provide a stronger understanding of the existing dwelling before renovation decisions are locked in.
For builders, it may help identify where energy performance improvements need to be integrated into the work rather than added later.
For property professionals, it can help explain comfort and energy performance in a more structured way as disclosure pathways continue to develop.
For consultants, it helps separate real built performance assessment from proposed design compliance.
A NatHERS existing home assessment usually begins with property details and available documentation, followed by on-site data collection, assessment input, modelling, review and reporting. The purpose is to assess the real performance of a dwelling that has already been built.
A NatHERS existing home assessment generally requires on-site data collection so the dwelling’s construction, systems, appliances and other relevant features can be recorded. Depending on the delivery model, this may be completed by the assessor or by a trained data collector working with the assessor.
The information may include dwelling layout, construction type, insulation, windows, shading, orientation, heating and cooling systems, hot water, lighting, appliances, solar, batteries and observable comfort or performance issues.
No. A new home NatHERS assessment usually assesses a proposed design before construction. A NatHERS existing home assessment assesses a dwelling that already exists and focuses on current performance, documentation gaps and improvement opportunities.
The final output may include a Home Energy Rating Certificate and information about the dwelling’s energy performance, comfort and possible upgrade opportunities. The exact reporting format depends on the assessment pathway and scheme requirements.
Useful information includes the property address, available plans, renovation history, insulation details, heating and cooling system information, hot water system details, solar or battery information and any known comfort issues.
Assessment Pathway Review
Send the property details, available plans and the reason for the assessment so the assessment pathway can be reviewed.