Duplex projects often involve a more complex thermal relationship than standard single dwellings.

Because two homes are being designed together on the same site, decisions around orientation, glazing, ventilation and solar access become increasingly interconnected.

In many NSW residential developments, BASIX assessments for duplexes are less about evaluating one isolated building and more about understanding how two dwellings interact thermally across a constrained site.

This creates a different layer of architectural and environmental complexity.

Quick Answer

Does BASIX apply to duplex projects in NSW?

Yes.

Most duplex developments in NSW require BASIX assessments as part of the development approval process.

Duplex BASIX assessments commonly consider:

• thermal comfort
• energy performance
• water efficiency
• orientation
• glazing exposure
• ventilation
• overshadowing between dwellings

Because duplexes involve multiple attached homes on one site, thermal performance often becomes more complex than standard detached housing.

Why duplex thermal performance is more complex

Single dwellings are generally designed around one thermal strategy.

Duplexes often involve balancing two different thermal conditions simultaneously.

This may include:

• different orientations
• unequal solar exposure
• varying glazing conditions
• shared walls
• overshadowing between dwellings
• constrained ventilation pathways

Even mirrored floorplans may perform differently thermally depending on how each side of the site receives sunlight throughout the year.

Good BASIX outcomes therefore depend heavily on careful architectural coordination rather than repeating identical layouts mechanically.

Orientation imbalance between dwellings

One of the most common challenges in duplex projects is orientation imbalance.

On many suburban sites, one dwelling may receive stronger northern solar access while the other experiences:

• increased western exposure
• reduced daylight
• overshadowing
• limited passive heating opportunities

This can create uneven thermal behaviour between the two homes.

A floorplan that performs effectively on one side of the site may behave very differently when mirrored onto the opposite side.

This is why duplex design often requires thermal thinking beyond simple layout duplication.

Shared walls and thermal behaviour

Attached dwellings also introduce different thermal conditions through shared construction systems.

Shared walls may influence:

• heat transfer
• thermal stability
• ventilation opportunities
• internal temperature behaviour

In some situations, shared walls can help reduce exposed external surface area.

However, duplex projects may also create internal thermal imbalance if one dwelling experiences significantly different solar conditions than the other.

Understanding how attached building forms behave thermally becomes increasingly important during BASIX modelling.

Glazing pressure in duplex developments

Glazing design is often one of the defining thermal factors in duplex projects.

Because suburban duplex sites are frequently narrow, glazing may become concentrated toward:

• side setbacks
• rear courtyards
• upper levels
• western exposures

Without careful shading and orientation control, this may increase:

• overheating risk
• cooling demand
• thermal instability
• compliance pressure

Large glazing areas introduced primarily for daylight or visual openness may create more complex thermal conditions if passive solar behaviour is not considered simultaneously.

Overshadowing and solar access

Overshadowing plays a significant role in many duplex developments.

This may occur through:

• neighbouring structures
• upper floor forms
• attached building mass
• fencing
• surrounding urban density

Solar access becomes especially important when balancing:

• passive heating
• daylight quality
• private open space
• internal comfort

Good duplex design often requires careful adjustment of massing, setbacks and glazing placement to maintain balanced solar performance across both dwellings.

Ventilation and airflow constraints

Natural ventilation can become more difficult in attached residential developments.

Narrow lot conditions may limit opportunities for cross ventilation or airflow separation between spaces.

This may increase reliance on:

• front-to-back airflow
• courtyard ventilation
• upper-level openings
• carefully positioned operable windows

Ventilation strategies often need to be integrated early in the planning process rather than added later as isolated solutions.

Balancing privacy and thermal performance

Duplex design frequently involves balancing competing priorities.

For example:

• privacy screening may reduce daylight
• solar access may conflict with overlooking controls
• glazing placement may affect both ventilation and privacy simultaneously

This is why thermal performance in duplex projects is rarely solved through one isolated adjustment.

The building envelope behaves as an interconnected system where multiple design decisions influence one another.

Why mirrored layouts are not always ideal

Mirrored floorplans are common in duplex developments because they simplify planning and construction efficiency.

However, mirrored layouts do not always create equal thermal outcomes.

Different site conditions may produce:

• unequal solar exposure
• inconsistent daylight quality
• varying overheating conditions
• different ventilation opportunities

In some projects, slight architectural variation between dwellings may improve overall thermal balance significantly.

Good BASIX outcomes often emerge through refinement rather than strict duplication.

Early thermal integration matters

One of the most common duplex problems occurs when thermal performance is considered too late.

By the time BASIX modelling begins, many critical decisions may already be fixed, including:

• floorplans
• glazing areas
• orientation response
• building form
• shading opportunities

Late-stage adjustments may then become difficult because changes to one dwelling can affect the other simultaneously.

Projects that integrate thermal thinking early often create smoother assessment pathways and more balanced long-term performance.

Designing duplexes as complete thermal systems

Strong duplex performance generally emerges when the entire development is considered as one interconnected environmental system.

This includes balancing:

• orientation
• shading
• glazing
• ventilation
• solar access
• building form
• private open space

When these elements work together, duplexes may achieve not only smoother BASIX outcomes, but also stronger long-term liveability and indoor comfort across both dwellings.

In many NSW residential developments, successful duplex performance is ultimately less about compliance alone and more about how carefully the architecture responds to climate, density and shared living conditions over time.

Related Reading

To understand how homes maintain stable indoor temperatures, explore understanding thermal comfort in BASIX.

For a broader overview of climate-responsive architecture, read passive design and BASIX.

For the full overview, return to the BASIX Knowledge Hub.