A BASIX assessment for a new home is not approached in exactly the same way as an assessment for an alteration, extension or second-storey addition. Both project types may need to demonstrate residential water, energy and thermal performance, but the starting point and assessment boundary can be different.
A new home can generally be assessed as one complete proposed dwelling. An alteration or addition begins with an existing building and requires the plans and assessment to distinguish between what is retained, removed, changed and newly constructed.
Understanding that distinction early helps architects, designers, builders and homeowners prepare the right information and avoid conflicting assumptions between the BASIX Certificate and the approval drawings. For a broader overview of the framework, see What Is BASIX? →
In Brief
What Is the Difference Between BASIX for a New Home and an Addition?
A new home is generally assessed as a complete proposed dwelling, with the new building envelope, glazing, water measures, energy systems and thermal-performance inputs coordinated as one design. An alteration or addition begins with an existing home and requires the project documentation to distinguish between retained, demolished, altered and newly constructed elements. The BASIX framework can be relevant to both, but the assessment scope, available design flexibility and required information may differ.
New Homes and Alterations at a Glance
| Consideration | New Home | Alteration or Addition |
|---|---|---|
| Starting point | A complete proposed dwelling. | An existing dwelling combined with proposed work. |
| Assessment boundary | The full new design can generally be coordinated as one building. | The relevant relationship between retained, altered and new elements must be understood. |
| Design flexibility | Orientation, glazing, shading and construction can often be coordinated from the beginning. | Existing orientation, room layout and construction may limit available options. |
| Building fabric | Walls, roofs, floors, windows and insulation are all newly specified. | Existing and proposed constructions may need to be clearly distinguished. |
| Glazing | The complete new window and door schedule can be assessed together. | Plans may need to identify retained, removed, altered and new glazing. |
| Services | New hot-water, heating, cooling and water systems can be coordinated as part of the complete design. | Some existing systems may remain while others are replaced, extended or newly introduced. |
| Common documentation risk | Specifications change after the assessment without the certificate being updated. | The drawings do not clearly identify which parts of the home are existing and which are proposed. |
What Remains the Same?
Although the assessment starting point differs, the BASIX documentation still needs to describe the proposed development consistently. The Certificate, architectural drawings, schedules and consultant information should not rely on conflicting assumptions.
For both new homes and alterations or additions, coordination may include:
- window and glazed-door sizes, orientation and performance
- wall, roof and floor construction
- insulation levels and installation locations
- shading devices and external obstructions
- hot-water, heating and cooling selections
- rainwater collection and nominated reuse
- pools, spas and other project-specific measures where relevant
If these elements change after the assessment has been completed, the BASIX documentation may need to be reviewed. This applies whether the project is a completely new dwelling or a smaller residential addition.
The Main Distinction
The Assessment Must Describe the Right Project Boundary
For a new home, the proposal can usually be documented as one complete new building. For an alteration or addition, the assessment must work with an existing home and the particular scope of proposed work. The difference is not simply the number of new rooms. It is the way the proposed development interacts with retained building fabric, existing services and the approval drawings.
How BASIX Is Approached for a New Home
A new home begins with a complete proposed design. The external envelope, internal zoning, windows, roof, floors and building services can generally be assessed as one coordinated residential project.
This can provide greater opportunity to consider thermal performance early. Orientation, room planning, window area, shading and insulation can be adjusted before they become fixed within the documentation.
A new-home assessment may need coordinated information covering:
- the complete dwelling geometry
- all external wall, roof and floor constructions
- the complete window and glazed-door schedule
- eaves, awnings, balconies and other shading
- insulation and thermal-bridging measures
- water, hot-water and energy-service selections
- landscape and rainwater information where relevant
The main advantage is design freedom. The corresponding risk is that a large number of related specifications need to remain consistent through design development, approval and construction.
Read the complete guide to BASIX for new homes in NSW →
How BASIX Is Approached for Alterations and Additions
An alteration or addition begins with a home that is already built. The proposed work may include a rear extension, additional storey, reconfigured internal area, new windows, changed roof form or a combination of these elements.
The documentation must make the project scope understandable. Drawings should clearly distinguish existing construction from demolition, altered work and new construction. Where these categories are unclear, the assessment can be based on assumptions that do not match the intended building work.
An alterations-and-additions assessment may need to consider:
- which rooms and building elements will remain
- which walls, roofs, floors and openings will be removed
- the construction of new walls, roofs and floors
- retained, altered and new glazing
- the connection between the existing building and the addition
- existing services that will remain in use
- new or upgraded water and energy systems
The existing building can also limit design choices. The orientation is fixed, retained rooms may have existing glazing and the new work may need to connect to older construction that was built under a different standard.
This does not necessarily mean an addition is more difficult than a new home. It means that the existing conditions and assessment boundary must be documented more carefully.
Read the complete guide to BASIX for alterations and additions →
What Information Is Needed for Each Project Type?
New Home
Typical Design Information
- site plan and orientation
- floor plans, elevations and sections
- complete window and door schedule
- wall, roof and floor constructions
- insulation specifications
- shading and external obstruction information
- hot-water, heating and cooling systems
- rainwater, landscape and pool details where relevant
Alteration or Addition
Typical Project Information
- existing and proposed floor plans
- demolition plans where relevant
- clear existing and proposed annotations
- retained, removed and new windows
- existing and proposed construction details
- details of connections between old and new work
- existing services that will remain
- new or modified water and energy systems
Why Thermal Performance Can Be Different
For a new home, the building envelope can generally be designed as a coordinated whole. Window placement, room orientation, shading, insulation and construction can be considered together from the earliest stages.
An alteration or addition must work around existing conditions. The original home may have fixed orientation, retained single glazing, limited insulation or a room arrangement that cannot be substantially changed.
The proposed work can also affect the existing home by:
- changing solar access to retained rooms
- removing or enclosing existing external walls
- altering natural ventilation paths
- introducing larger areas of new glazing
- creating new roof exposure on an upper-floor addition
- changing room zoning and internal connections
- creating junctions between old and new insulation systems
For this reason, an extension should not be considered as an isolated room without regard to the retained dwelling. The relevant interaction between the existing and proposed building fabric needs to be understood within the applicable assessment pathway.
Common BASIX Assumptions That Cause Confusion
“A renovation is assessed exactly like a new home.”
Not necessarily. An alteration or addition begins with an existing building, so the scope, available information and design constraints may differ from a completely new dwelling.
“Only the new room matters.”
This is too broad an assumption. The assessment needs to describe the proposed development and the relevant relationship between retained, altered and newly constructed elements.
“Existing construction can simply be guessed.”
Existing details should be established as clearly as the project requires. Unsupported assumptions about walls, roofs, floors or glazing can create inconsistencies between the assessment and the actual building.
“The project type is obvious from the title block.”
A project described as a renovation may include substantial demolition and new work, while an addition may retain most of the existing building. The actual scope shown on the plans is more useful than a general project label.
“BASIX can wait until the drawings are complete.”
Late assessment can reveal that glazing, shading, insulation or services need to change. Early review gives the design team more opportunity to resolve performance requirements without disrupting completed documentation.
Which Guide Matches Your Project?
New Residential Design
BASIX for New Homes in NSW
Explore the assessment approach, documentation and design considerations for a completely new dwelling.
Existing Home Project
BASIX for Alterations and Additions
Understand how retained, altered and newly constructed parts of an existing home may affect the assessment.
Framework Overview
What Is BASIX?
Read the broader introduction to the NSW residential sustainability assessment framework.
Frequently Asked Questions
BASIX for New Homes and Alterations
Is BASIX different for a new home and an extension?
The same BASIX framework may be relevant, but the project starting point and assessment scope can differ. A new home is generally a complete proposed dwelling, while an extension must be considered in relation to the existing home and the proposed work.
Does an addition require the whole existing house to be assessed?
The relevant scope depends on the proposal and the applicable assessment pathway. It should not be assumed that only one new room matters or that every existing element must be treated as new. The plans need to clearly describe how the proposed work relates to the retained dwelling.
Are existing windows included in a BASIX assessment?
Existing windows may be relevant where they form part of the assessed building area or are affected by the proposed work. Drawings should distinguish windows that are retained, removed, altered or newly installed.
Does a second-storey addition need a different approach?
A second-storey addition can introduce new roof exposure, walls, glazing, stairs and connections to retained rooms. The assessment and plans need to describe these proposed elements and their relationship with the existing ground floor.
What plans are needed for a BASIX renovation assessment?
The assessor will generally need coordinated existing and proposed plans, elevations, sections and construction information. A demolition plan and clear existing-versus-new annotations can be especially useful for larger renovations and additions.
Can one BASIX Certificate cover retained and new work?
The BASIX documentation should correspond with the proposed development shown on the approval drawings. Where retained and new work form part of the same proposal, the assessment must describe the relevant project scope consistently.
Should BASIX be completed before a DA or CDC is lodged?
The BASIX assessment should be coordinated before the approval documentation is finalised and lodged. This allows any required changes to glazing, shading, insulation or systems to be incorporated into the drawings rather than discovered after submission.
Related Knowledge
Continue Exploring BASIX in NSW
BASIX Project Review
Confirm the Right BASIX Approach for Your Residential Project
Send the available plans and project information for an initial review. Certified Energy can help identify whether the proposal should be assessed as a new dwelling, alteration or addition and what documentation is likely to be required.
Send Your Plans for Review →
