NSW Residential Compliance Guide
BASIX design optimisation is the process of identifying which project inputs are limiting compliance and refining them without making unnecessary changes to the wider residential design.
A project that does not initially achieve its required BASIX outcome does not necessarily need a complete redesign. In many cases, the result can be improved through a targeted review of the assumptions, specifications or design elements affecting the relevant assessment component.
The important first step is diagnosis. Water, energy and thermal comfort respond to different inputs, and a change that improves one part of the assessment may have little effect on another. Repeatedly upgrading unrelated products can increase project cost without resolving the actual constraint.
This guide focuses specifically on how BASIX outcomes can be refined through a structured design process. For the wider assessment, certification and NSW submission pathway, visit the BASIX Knowledge Hub.
In Brief
What Is BASIX Design Optimisation?
Diagnose
Identify whether the limiting issue sits within water, energy, thermal comfort or the accuracy and coordination of the project information.
Prioritise
Test the changes most likely to improve the result while protecting cost, buildability, planning outcomes and architectural intent.
Coordinate
Update the assessment, drawings, schedules and specifications so the final commitments describe one consistent project.
Optimisation is not a universal product checklist. The appropriate response depends on the particular BASIX component, climate, dwelling type, modelling pathway and design constraint affecting the project.
Assessment Structure
BASIX Does Not Have One Single Result to Optimise
BASIX assesses several related but technically distinct aspects of a residential project. A design can perform strongly in one component while requiring further work in another.
The optimisation process should therefore begin by identifying the exact result that needs attention:
- Water: the relationship between demand, fixtures, landscape use, rainwater capture and reuse commitments;
- Energy: the systems and services affecting operational energy and greenhouse-gas performance;
- Thermal comfort: modelled heating and cooling performance arising from the building design and construction; and
- Materials reporting: the construction information used to calculate and record embodied emissions.
These components should remain connected at project level but should not be treated as interchangeable. Increasing a rainwater tank will not repair a thermal comfort result, and upgrading insulation will not resolve an unsuitable hot-water system selection.
Diagnostic Process
How Is a BASIX Design Optimised?
Confirm the project information. Check that plans, schedules, dwelling data and selected systems have been entered accurately before changing the design.
Locate the limiting component. Establish whether the shortfall relates to water, energy, heating performance, cooling performance or several connected issues.
Identify the high-impact inputs. Review which assumptions or design elements are materially influencing that result rather than changing every specification.
Test proportionate alternatives. Compare realistic adjustments against design impact, construction cost, product availability and documentation consequences.
Record the final solution. Update the BASIX assessment and ensure the resulting commitments are reflected consistently in the project documentation.
First Review
Check the Inputs Before Changing the Architecture
An unexpected result may sometimes arise from incomplete, outdated or inconsistent project information rather than from an inherently unsuitable design.
Before recommending physical changes, the assessment should be checked against the current documents. Relevant questions can include:
- Are the floor areas, ceiling heights and dwelling configuration current?
- Have window dimensions, orientation and shading been represented correctly?
- Do the modelled construction systems match the architectural sections and schedules?
- Are the nominated hot-water, heating, cooling and ventilation systems still proposed?
- Are rainwater catchments, tank sizes and reuse connections accurately documented?
- Has an earlier design revision remained within the assessment after the plans changed?
Correcting an unsupported assumption is not a design optimisation measure. It is an accuracy requirement. The assessment should always begin from the project that is genuinely intended to be approved and built.
Thermal Comfort
What Can Be Reviewed When Thermal Performance Is Limiting?
Building Geometry
Orientation and Room Exposure
The arrangement of rooms, walls and windows affects when solar gains and external temperatures influence each part of the dwelling.
Transparent Envelope
Window Size and Performance
Glazing area, frame type, U-value, solar heat gain characteristics and elevation can influence heating and cooling in different ways.
Opaque Envelope
Insulation and Construction
Roof, ceiling, wall and floor systems should be reviewed against the specific heat-flow problem identified by the thermal model.
Solar Control
Shading and Exposure
Eaves, balconies, screens, recesses and surrounding obstructions can alter useful winter sun and unwanted summer heat gain.
Air Movement
Openings and Penetrations
Operable openings, ventilation pathways and recognised penetrations can affect the modelled behaviour of the dwelling.
Room-Level Review
Target the Sensitive Zones
The most effective adjustment may relate to one exposed room or elevation rather than requiring an upgrade across the complete dwelling.
A strong thermal optimisation process distinguishes between heating and cooling. A change that reduces heat loss may not resolve summer overheating, while aggressive solar control can remove useful winter gains. The modelled problem should determine the response.
Proportionate Response
The Most Expensive Upgrade Is Not Automatically the Best One
It is common to assume that the strongest available glazing, highest insulation value or largest shading device will produce the best thermal outcome. In practice, the effect of any measure depends on where and why it is applied.
For example, a selected window may be creating unwanted solar gain, excessive heat loss or both. Those conditions do not necessarily call for the same glass characteristics. Similarly, additional insulation may provide limited improvement where the dominant issue is a large exposed glazing area or unsuitable summer shading.
Targeted optimisation may involve:
- upgrading only the windows materially affecting the result;
- adjusting one high-impact opening while retaining the wider elevation;
- refining shading on the most exposed orientation;
- changing a construction layer where it addresses the identified heating or cooling load; or
- reviewing a sensitive room before applying project-wide upgrades.
The preferred solution should achieve the required performance while remaining available, documentable, buildable and proportionate to the project.
Operational Energy
What Can Be Reviewed When the Energy Result Is Limiting?
The BASIX energy component responds to the systems and services nominated for the development. The appropriate optimisation measure depends on the dwelling type, available infrastructure and the selections genuinely intended for installation.
Relevant inputs may include:
- the domestic hot-water system and its energy source;
- heating and cooling system types and efficiencies;
- ventilation and exhaust arrangements;
- lighting assumptions and fixed energy commitments;
- renewable-energy systems where they form part of the proposal;
- common-area systems in relevant multi-dwelling developments.
A system should not be selected only because it improves the assessment. The nominated commitment needs to suit the site, dwelling, electrical or gas infrastructure, owner requirements and construction scope.
Thermal comfort and energy should also remain conceptually separate. A more efficient air-conditioning system may improve the energy component, but it does not repair poor modelled building-fabric performance within the thermal comfort assessment.
Water Performance
What Can Be Reviewed When the Water Result Is Limiting?
Water optimisation involves reviewing the relationship between expected demand, fixture efficiency, landscape requirements, alternative water supply and the areas connected to that supply.
Fixture efficiency. Review the nominated taps, showers, toilets and other applicable fixtures against realistic available products.
Rainwater storage. Confirm that the proposed capacity is appropriate for the connected catchment and documented reuse commitments.
Connected roof area. A larger tank cannot receive runoff from roof areas that are not physically connected to it.
Reuse demand. Toilet flushing, laundry connections and landscape irrigation should only be nominated where they can be delivered.
Landscape and pool inputs. Relevant areas, water demand and project features should remain consistent with the current site plan.
The strongest water response is usually one that can be clearly traced through the hydraulic concept, roof plan, landscape information and BASIX commitments rather than relying on an oversized or poorly connected system.
Embodied Emissions Reporting
How Does the BASIX Materials Index Fit into Optimisation?
Current BASIX applications can include construction-material information used to calculate and record embodied emissions. This reporting component should not be casually described as another interchangeable water, energy or thermal comfort score.
Material selections can still have wider project implications. A change to wall, floor or roof construction may affect thermal modelling, embodied-emissions reporting, structural design, detailing, cost and procurement at the same time.
Where a construction system is changed during optimisation, the revised assembly should be coordinated across each relevant assessment and consultant document rather than updated in BASIX alone.
Decision Hierarchy
Which Changes Should Be Tested First?
Priority One
Correct the Information
Resolve outdated plans, incorrect inputs and inconsistencies before treating the result as a design failure.
Priority Two
Refine Specifications
Test credible fixture, system, glazing or construction selections that do not materially alter the architecture.
Priority Three
Target High-Impact Geometry
Review the specific window, shade, room, roof catchment or landscape element driving the result.
Priority Four
Consider Broader Redesign
Revisit the wider layout or building form only where proportionate targeted measures cannot resolve the project constraint.
Architectural Coordination
Can BASIX Be Optimised Without Losing the Design Intent?
Often, yes. Effective optimisation does not begin by removing every large window, changing the complete façade or replacing the architectural concept with a generic compliance solution.
The assessor should identify which aspects of the design materially affect the result and communicate the available trade-offs. The architect, designer, builder or owner can then determine which option best fits the project priorities.
For example, retaining an important view may justify improved performance in selected windows, additional external shading or refinement elsewhere in the envelope. A constrained roof catchment may require a more realistic water strategy rather than an arbitrarily larger tank.
Optimisation works best as an informed design conversation. The assessment identifies the technical consequence; the project team decides how that consequence should be resolved within the broader architecture.
Project Timing
When Should BASIX Optimisation Begin?
The most useful time to review BASIX performance is when the design is sufficiently developed to provide reliable plans but still flexible enough for targeted changes.
Earlier review is particularly valuable for projects involving:
- large or highly exposed glazing areas;
- constrained solar orientation;
- complex roof forms or construction systems;
- duplexes, townhouses or multi-dwelling configurations;
- significant alterations to an existing home;
- limited roof catchment or landscape area; or
- systems and specifications that have not yet been selected.
Late-stage optimisation remains possible, but the available options may be narrower. Changes can also affect drawings, consultant coordination, procurement, cost planning and approval documentation that has already progressed.
After Refinement
The Final Assessment Must Match the Final Design
An optimisation measure only becomes a valid project solution when it is incorporated into the design and documentation.
Depending on the change, coordination may be required across:
- architectural plans, elevations and sections;
- window, door and glazing schedules;
- insulation and construction specifications;
- hydraulic, electrical and mechanical information;
- landscape and roof-catchment information;
- supplier selections and builder documentation; and
- the BASIX Certificate and any associated NatHERS documentation.
A compliant result based on a specification that is missing from the drawings or unavailable during construction creates a documentation problem rather than a resolved compliance pathway.
Common Mistakes
What Can Make BASIX Optimisation Less Effective?
Changing the Wrong Component
Upgrading energy systems will not resolve a thermal comfort shortfall, and additional insulation will not improve the water score.
Applying Blanket Upgrades
Project-wide upgrades may add cost where only one elevation, room, system or fixture selection is materially limiting performance.
Using Unavailable Products
A nominal specification is not a practical solution if suitable products cannot be sourced within the project budget or construction programme.
Ignoring Design Trade-Offs
A measure that improves one seasonal condition may adversely affect another if heating and cooling consequences are not considered together.
Optimising an Old Plan Set
Assessment work based on superseded layouts or schedules can create commitments that no longer correspond with the development.
Failing to Update Documents
A successful modelling change must be carried through to the drawings, schedules and specifications used for approval and construction.
Technical Boundary
What Does BASIX Optimisation Not Replace?
BASIX optimisation does not replace architectural design, structural engineering, hydraulic design, mechanical design, condensation analysis, product procurement or cost planning.
It also does not establish that the highest numerical result is the most appropriate project outcome. The objective is to achieve the applicable requirements through a credible and coordinated design—not to maximise every input regardless of cost or practical consequence.
Where a proposed adjustment affects another consultant’s scope, the project team should confirm that the revised solution remains technically suitable before it is adopted as a BASIX commitment.
Frequently Asked Questions
BASIX Design Optimisation FAQs
What is BASIX design optimisation?
BASIX design optimisation is a structured review of the project inputs affecting water, energy or thermal comfort. It identifies the limiting component, tests proportionate adjustments and coordinates the selected solution with the design documentation.
Does BASIX optimisation always require a redesign?
No. Some projects can be resolved through corrected information or targeted specification changes. Broader changes to windows, shading, layout or other geometry are considered where lower-impact measures do not address the identified constraint.
What affects a BASIX thermal comfort result?
Relevant factors can include climate, orientation, room layout, glazing, shading, insulation, construction systems, thermal mass, ventilation and recognised penetrations. Their relative influence varies between projects.
Is double glazing always the best BASIX optimisation measure?
No. Double glazing may assist some projects, but the appropriate response depends on whether the issue relates to heat loss, solar gain, glass area, orientation, shading or another part of the building design.
Can system selections improve the BASIX energy result?
Yes. Hot-water, heating, cooling, ventilation and renewable-energy selections may affect the energy component. The nominated systems should remain practical, available and consistent with the intended development.
When should BASIX optimisation happen?
It should ideally occur while the plans are developed enough to assess accurately but before the architecture, systems and construction specifications are fully locked in.
Do optimisation changes need to appear on the plans?
Yes, where the change affects the documented development. The BASIX Certificate, NatHERS documentation, architectural drawings, schedules and specifications should describe one consistent project.
Related Guidance
Continue Through the BASIX Knowledge Pathway
BASIX Project Review
Does the Current BASIX Design Need Refinement?
Certified Energy can review the available plans, assessment inputs and current project result to identify the limiting BASIX component and test practical options for improvement.
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