A Guide to BASIX Requirements for NSW Homes

A new home, granny flat or major extension can look straightforward on a sketch, then become considerably more complex once energy, water and thermal comfort targets are considered. This guide to BASIX requirements explains where BASIX fits into a NSW project, what the assessment considers, and why those decisions should be made well before plans are lodged for approval.

BASIX stands for the Building Sustainability Index. It is a NSW planning requirement intended to reduce water use, improve energy performance and deliver homes that are more comfortable to live in. It is not a separate building style or a box-ticking exercise at the end of design. BASIX affects the design, specification and approval documentation for many residential projects.

When BASIX requirements apply

BASIX generally applies to new residential dwellings in NSW, including houses, duplexes, townhouses, attached dwellings and secondary dwellings such as granny flats. It also applies to many alterations and additions with a project value of $50,000 or more. Larger swimming pools and spas may have BASIX requirements as well.

The exact pathway depends on the scope of work and the approval route. A Development Application through council and a Complying Development Certificate can both require BASIX documentation. For a project involving an existing home, the assessment may focus on the proposed works rather than the performance of every part of the original dwelling. That distinction matters when planning an extension, upper-floor addition or substantial renovation.

A BASIX certificate is prepared using the NSW Government's online assessment tool. The certificate sets out the commitments that must be reflected in the plans and specifications submitted for approval, then carried through to construction. A certifier or council will look for consistency between the certificate and the documentation. If the plans show one thing and the BASIX commitments show another, delays can follow.

The three parts of a BASIX assessment

A BASIX assessment addresses water, energy and thermal comfort. The targets are calculated according to the project location, dwelling type, size and design. There is no single set of inclusions that works for every site.

Water efficiency

Water commitments commonly relate to fixtures, fittings and, depending on the project, rainwater harvesting or other water-saving measures. A rainwater tank may be nominated to supply toilets, washing machines or external taps. The required tank size, connected roof area and end uses must be coordinated with the hydraulic and architectural plans.

For homeowners, this is where practical design matters. A tank that technically satisfies the assessment but cannot be sensibly located, maintained or connected can create avoidable cost during construction. Tank placement, access, overflow and pump requirements should be considered early, particularly on constrained Sydney sites.

Energy use

The energy component considers fixed services and appliances such as hot water systems, heating and cooling, lighting, cooking and solar power. Efficient electric systems, solar photovoltaic panels, ceiling fans and appropriately selected hot water systems can all influence the outcome.

The most suitable solution depends on the home and the owner’s priorities. An all-electric design may suit a new build with a well-designed solar system, while an extension may need a more targeted upgrade that works with existing services. The goal is not to add products at random. It is to select measures that meet the required target while making sense for the budget, building form and future running costs.

Thermal comfort

Thermal comfort is often the part of BASIX that has the greatest influence on the building design. It assesses how the dwelling performs through the year, including how it responds to summer heat and winter cold. Orientation, glazing, shading, insulation, roof colour, building materials, ventilation and room layout all play a role.

A home with extensive west-facing glass may look appealing in a concept image but be difficult to make comfortable without careful shading and glazing choices. Conversely, a well-oriented living area, eaves that suit the sun path and a balanced window layout can improve the assessment while creating a better home to occupy.

Thermal performance is especially relevant for sites with limited solar access, steep slopes, bushland settings or tightly spaced neighbouring homes. These conditions do not prevent a compliant design, but they require a more considered response than a standard plan copied onto a new block.

Guide to BASIX requirements: start with the site

The most cost-effective BASIX decisions are made during concept design, not after a complete set of plans has been drawn. Before finalising the floor plan, assess the site orientation, overshadowing from neighbours, views, setbacks, slope and available roof area for solar panels.

This early work helps identify opportunities and constraints. A north-facing living space may support good winter solar access, but privacy requirements could limit window locations. A generous roof area may be ideal for solar panels, while a heritage streetscape or difficult roof geometry may require another approach. Every project involves trade-offs.

For an addition, it is also worth looking closely at how the new rooms connect with the existing house. Adding a large glazed rear living area without shading can affect more than the extension itself. It may change how the adjoining original rooms perform and increase reliance on mechanical cooling. Thoughtful window placement, insulation continuity and external shading can often resolve issues more effectively than simply increasing the capacity of an air-conditioning system.

What to prepare before the assessment

A BASIX assessment is only as reliable as the information supplied. It should be based on a reasonably developed design, rather than broad assumptions that later need to be revised. For a new home or secondary dwelling, the assessor will typically need:

• site address and local climate information

• floor plans, elevations and roof plans

• window and door sizes, locations and proposed glazing

• construction details for walls, floors, roofs and insulation

• proposed hot water, heating, cooling, lighting and solar systems

• rainwater tank details, where required.

Selections do not need to be limited to a particular brand at the early stage, but the performance assumptions need to be realistic. If the certificate relies on high-performance glazing, a specific insulation level or a solar system of a certain capacity, those commitments need to remain achievable once quotes are obtained.

Changes after the certificate is issued are possible, but they should not be treated casually. Altering the window schedule, changing from one hot water type to another, reducing solar capacity or substituting insulation can require the BASIX assessment to be updated. The revised certificate and plans may then need to be coordinated before approval or construction continues.

Avoid the common causes of BASIX delays

The most common issue is leaving BASIX until the plans are otherwise finished. At that point, the design may have little flexibility, and compliance can require expensive changes to glazing, shading, insulation or services.

Another issue is treating the certificate as separate from the drawings. The rainwater tank, solar panels, window specifications and other commitments need to appear clearly in the approval documentation. Builders also need enough detail to price and deliver the work correctly.

Cost cutting can create a further problem. Some substitutions are reasonable, but replacing specified items without checking their BASIX impact can place the final build out of line with the approved commitments. A practical design team will identify measures that are compliant, available and appropriate for the project from the outset.

BASIX, approvals and construction documentation

BASIX is one component of the broader approval process. A compliant BASIX certificate does not remove the need to satisfy planning controls, building regulations, bushfire requirements, flood constraints, stormwater requirements or council conditions. It needs to work alongside them.

For example, a proposed eave or external screen that improves thermal comfort still needs to comply with setbacks, building height and relevant planning controls. Solar panels need to be accommodated on the roof design. Rainwater tank locations need to consider access, drainage and available space. This is why coordinated design and approval documentation is valuable.

At GAP Designers, BASIX considerations are addressed as part of the wider design and approval process, so the concept, plans and compliance documentation are working towards the same outcome. For homeowners and small developers, that approach can reduce redesign and provide clearer direction before the project reaches the builder.

A well-designed NSW home should not merely meet a target on paper. It should be comfortable in summer, warmer in winter, economical to run and practical to build on the site you own. Bringing BASIX into the conversation at the first design meeting gives the project the best chance of achieving all four.