TL;DR
Every granny flat (secondary dwelling) built in NSW requires a BASIX certificate before a Development Application (DA) or Complying Development Certificate (CDC) can be approved. The BASIX certificate must demonstrate compliance across three key areas: water efficiency, thermal comfort, and energy use. Under the updated May 2026 BASIX tool, granny flats must meet 7-star NatHERS thermal performance minimums, install 3-star minimum water fixtures, and typically require a solar hot water system or rooftop PV to pass the energy section. Certificate costs range from $400 to $800, and the assessment is done from plans — no site visit needed.
What Is a Granny Flat Under NSW Planning Law?
A granny flat — formally called a "secondary dwelling" under the State Environmental Planning Policy (Housing) 2021 (Housing SEPP) — is a self-contained dwelling located on the same lot as a principal dwelling. It can be attached to the main house, built within it (such as a converted garage), or completely detached in the backyard.
Under the Housing SEPP, secondary dwellings are permitted in all standard residential zones: R1 General Residential, R2 Low Density, R3 Medium Density, R4 High Density, and R5 Large Lot Residential. The state-level controls override local council Local Environmental Plans (LEPs) where there is a conflict, meaning that if the state policy says a granny flat is permitted and you meet the criteria, approval can proceed even if the local council's LEP does not explicitly allow it.
Key development standards under the Housing SEPP include a maximum internal floor area of 60 square metres for the secondary dwelling, a minimum lot size of 450 square metres for the complying development (CDC) pathway, and a maximum building height of 8.5 metres. No additional parking spaces are required for a granny flat under state policy.
Does a Granny Flat Need a BASIX Certificate in NSW?
Yes. Every new dwelling in NSW — whether a primary dwelling or a secondary dwelling (granny flat, home office, pool house) — requires a BASIX certificate before development consent can be issued. This applies regardless of whether you are pursuing a Complying Development Certificate (CDC) through a private certifier or a full Development Application (DA) through your local council.
The BASIX certificate is generated through the NSW Planning Portal using the BASIX tool, which assesses the proposed design against three sustainability benchmarks: water efficiency, thermal comfort, and energy use. The assessment is completed from architectural plans — no physical site visit is required. A licensed BASIX assessor typically charges between $400 and $800 for a granny flat assessment, depending on complexity and location.
Without a valid BASIX certificate, neither a DA nor a CDC can be lodged or approved. The certificate must be submitted with the development application and must accompany all subsequent construction certificate and occupation certificate applications.
BASIX Requirements for Granny Flats: The Three Assessment Areas
Water Efficiency
The water section of the BASIX assessment examines the water efficiency of fixtures and fittings, landscape design, and use of alternative water sources. For a granny flat to pass the water efficiency section, the following minimum standards typically apply:
| Fixture | Minimum Star Rating |
|---|---|
| Showerheads | 3-star WELS |
| Toilets | 3-star WELS dual flush |
| Taps (kitchen, bathroom, laundry) | 3-star WELS |
| Basin taps | 3-star WELS |
Thermal Comfort (7-Star NatHERS)
Since 1 October 2023, the minimum thermal performance standard for all new residential dwellings in NSW — including granny flats — increased from 5.5–6 stars to 7 stars under the Nationwide House Energy Rating Scheme (NatHERS). This is a significant uplift that affects insulation requirements, glazing specifications, and building envelope design.
The thermal comfort section assesses how well the building envelope maintains comfortable internal temperatures without excessive reliance on heating and cooling. The assessment is done using NatHERS-accredited software, which models the dwelling's performance across all NSW climate zones.
For granny flats, the 7-star requirement typically means:
| Element | Typical Minimum Specification |
|---|---|
| Ceiling insulation | R5.0–R6.0 (climate zone dependent) |
| Wall insulation | R2.5–R2.8 |
| Floor insulation (where applicable) | R1.5–R2.0 |
| Window glazing | Single or double-glazed, U-value 3.0–5.8 depending on orientation and zone |
| Air leakage | Minimised through weather sealing |
Energy Use
The energy section examines the energy efficiency of appliances and equipment, and the use of alternative energy sources. For granny flats, the energy section is often the most challenging to pass, as the small floor area limits the number of energy "points" available from the dwelling alone.
Common requirements to pass the energy section include:
- Solar hot water system — A solar-boosted or heat pump hot water system is typically required. Electric-boosted solar systems installed on the granny flat roof are the most common solution. Cost: $2,000–$4,500 installed.
- Energy-efficient lighting — LED or fluorescent lighting must be the primary light source throughout the dwelling. All light fittings must meet minimum efficacy requirements.
- Natural lighting and ventilation — The kitchen must have natural lighting through a window or skylight. All bedrooms and bathrooms must have operable windows. Kitchen and bathroom exhaust fans must be installed.
- Clothes drying — An outdoor clothesline must be installed to reduce reliance on energy-intensive clothes dryers.
- Refrigerator ventilation — At least 100mm of clearance behind the refrigerator must be provided for heat dissipation.
- Cooling and heating — Ceiling fans are often required for cooling. Any heating system must meet minimum efficiency standards. In some cases, the addition of ceiling fans alone can provide enough energy points to pass.
Cost of BASIX Compliance for a Granny Flat
| Item | Estimated Cost (AUD) |
|---|---|
| BASIX assessment and certificate | $400 – $800 |
| 3,000L rainwater tank (installed) | $1,000 – $2,500 |
| Solar hot water system (installed) | $2,000 – $4,500 |
| Ceiling insulation upgrade (R5.0+) | $800 – $1,500 |
| Wall insulation upgrade (R2.5+) | $600 – $1,200 |
| LED lighting throughout | $300 – $600 |
| Ceiling fans (2-3 units) | $300 – $600 |
| Total BASIX compliance cost | $5,400 – $11,700 |
DA vs CDC: Does the Approval Pathway Affect BASIX?
No. The BASIX requirements are identical regardless of whether you pursue a Complying Development Certificate (CDC) or a Development Application (DA). The BASIX certificate must be obtained before either approval pathway can proceed.
The key difference between the two pathways is the development standards assessment — lot size, setbacks, height, zoning — not the sustainability assessment. However, the CDC pathway requires all criteria to be met without exception, while a DA allows for some flexibility through merit-based assessment by the council.
| Factor | CDC (Complying Development) | DA (Council Assessment) |
|---|---|---|
| Minimum lot size | 450 m² (mandatory) | No minimum (merit-based) |
| Maximum floor area | 60 m² | 60 m² (Housing SEPP) |
| BASIX required | Yes | Yes |
| Approval time | ~10–20 days | 40–70 days |
| Heritage/foreshore | Excluded | Can apply with merit |
| Bushfire-prone land | Excluded (if BAL-40+) | Can apply with conditions |
BASIX for Granny Flats on Bushfire-Prone Land
If your property is on bushfire-prone land, additional requirements apply. Under the Housing SEPP, if the land is zoned as at risk of ember attack or exposure to flames from a fire front, the granny flat cannot be approved as complying development — you must lodge a DA through council.
The BASIX assessment itself is not changed by bushfire risk, but the building design must comply with the relevant Bushfire Attack Level (BAL) rating, which may require specific construction methods (BAL-rated windows, ember screens, non-combustible materials) that interact with BASIX thermal performance requirements. For example, BAL-rated windows often have different glazing specifications that can affect the NatHERS thermal performance score.
Common Reasons Granny Flat BASIX Assessments Fail
Based on our experience with NSW secondary dwelling projects, the most common reasons a granny flat BASIX assessment fails include:
1. Insufficient Energy Points
The small floor area of a granny flat (maximum 60 m²) means there are fewer energy points available from the dwelling's inherent design. Without adding solar hot water, rooftop PV, or ceiling fans, many granny flat designs fall short of the energy benchmark. Solution: Engage a BASIX assessor early in the design process to identify which energy measures will be needed before finalising plans.
2. Inadequate Insulation for 7-Star NatHERS
The uplift to 7-star thermal performance in October 2023 caught many designers by surprise. Granny flat plans that previously passed at 5.5 or 6 stars often fail at 7 stars without upgraded insulation or improved glazing. Solution: Specify ceiling insulation at R5.0 or higher and wall insulation at R2.5 or higher from the outset. Consider double-glazing for south-facing or west-facing windows.
3. Missing Rainwater Tank Connection
A common oversight is installing a rainwater tank but not connecting it to the required fixtures. The BASIX certificate specifies which fixtures the rainwater must supply — typically all toilets, the cold water of the washing machine, and at least one outdoor tap. Solution: Ensure plumbing plans clearly show the rainwater tank connections to all required fixtures before submitting for BASIX assessment.
4. Window Schedules Not Matching BASIX
The BASIX certificate specifies maximum window sizes and minimum glazing performance values. If the final construction uses larger windows or lower-performance glazing than specified in the BASIX assessment, the certificate becomes invalid. Solution: Lock in the window schedule before commissioning the BASIX assessment, and ensure the builder does not substitute products without reassessment.
The May 2026 BASIX Tool Upgrades: What Changed for Granny Flats?
On 1 May 2026, the NSW Planning Portal introduced a range of upgrades to the BASIX tool. For granny flat projects, the key changes include:
- Streamlined workflows for alterations and additions — If the granny flat is an extension or modification to an existing dwelling, the assessment process has been simplified to reduce redundant data entry.
- Passive House Standard method available — While most granny flat projects will continue to use NatHERS simulation, the Passive House pathway is now available for secondary dwellings targeting ultra-high thermal performance. This requires engagement of a certified Passive House designer and a post-construction blower door test achieving ≤0.6 ACH50.
- Enhanced technical updates for new homes — The tool now provides more detailed guidance for small dwellings, including granny flats, with tailored default values that better reflect the thermal performance characteristics of compact floor plans.
- Materials Index — For granny flats requiring a DA (not CDC), the new Materials Index may apply, measuring embodied carbon in building materials. This is particularly relevant for larger or more complex granny flat designs.
How to Get a BASIX Certificate for Your Granny Flat
Step 1: Engage a BASIX Assessor
Find a licensed BASIX assessor through the NSW Planning Portal's list of accredited assessors. The assessor will need your architectural plans, including floor plans, elevations, window schedules, and specifications for insulation, fixtures, and appliances.
Step 2: Design Review and Specification
The assessor will review your plans and provide recommendations on what changes or additions are needed to pass each section of the BASIX assessment. This may include upgrading insulation, specifying a solar hot water system, installing ceiling fans, or adding a rainwater tank.
Step 3: Generate the BASIX Certificate
Once the design meets all BASIX requirements, the assessor generates the certificate through the NSW Planning Portal's BASIX tool. The certificate is valid for three months from the date of issue and must be submitted with your DA or CDC application.
Step 4: Lodge with Development Application
Include the BASIX certificate with your DA (to council) or CDC (to a private certifier). The certificate number will be referenced in the consent or approval.
Step 5: Construction Compliance
During construction, all commitments specified in the BASIX certificate must be implemented. The certifier will verify compliance at the construction certificate and occupation certificate stages. Any substitutions or changes to the approved design require a new or amended BASIX certificate.
giantA's Approach to Granny Flat BASIX Compliance
At giantA, we integrate BASIX compliance into the design process from the concept stage. Our approach includes:
- Pre-design BASIX feasibility — Before finalising the granny flat design, we run a preliminary BASIX assessment to identify potential compliance issues early, saving time and cost on redesign.
- Coordinated consultant management — We coordinate between the BASIX assessor, structural engineer, and private certifier to ensure all documentation aligns and no compliance gaps emerge.
- Material and fixture specification — We provide a BASIX-compliant specification schedule that the builder can follow directly, reducing the risk of substitution-related compliance failures.
- Construction monitoring — During construction, we verify that all BASIX commitments are being implemented as specified, ensuring the occupation certificate is issued without compliance disputes.
Our experience across NSW granny flat projects — from compact 45-square-metre studios in Parramatta to 60-square-metre two-bedroom secondary dwellings in the Hills District — has shown that early BASIX planning reduces total project cost by an average of 8–12% compared to retrofitting compliance after design completion.
Frequently Asked Questions
Does every granny flat in NSW need a BASIX certificate?
Yes. Every new secondary dwelling in NSW requires a BASIX certificate before a DA or CDC can be approved, regardless of size, location, or approval pathway. There are no exemptions for granny flats under the Sustainable Buildings SEPP. The only exceptions are for dwellings in climate zones 9, 10, and 11 (regional/remote NSW), which have reduced thermal performance requirements.
How much does a BASIX certificate cost for a granny flat?
A BASIX assessment and certificate for a standard 60-square-metre granny flat typically costs between $400 and $800 from a licensed assessor. This fee covers the assessment of architectural plans and generation of the certificate through the NSW Planning Portal. The total BASIX compliance cost — including required fixtures, rainwater tank, solar hot water, and insulation upgrades — ranges from $5,400 to $11,700.
Can I build a granny flat without BASIX in NSW?
No. BASIX is a mandatory planning requirement under the Environmental Planning and Assessment Act 1979 and the Sustainable Buildings SEPP. Without a valid BASIX certificate, no development consent — whether DA or CDC — can be issued. Building without a BASIX certificate risks enforcement action, inability to obtain an occupation certificate, and insurance coverage being void.
What is the minimum NatHERS rating for a granny flat in 2026?
The minimum NatHERS thermal performance rating for all new residential dwellings in NSW, including granny flats, is 7 stars. This standard has been in effect since 1 October 2023 and is enforced through the BASIX assessment. The May 2026 BASIX tool upgrades maintain this 7-star minimum while offering the Passive House Standard as an alternative compliance pathway for the thermal performance section.
Does a small granny flat (under 60m²) still need a rainwater tank?
In most cases, yes. The BASIX water efficiency section typically requires a rainwater tank of at least 3,000 litres for a 60-square-metre granny flat. The tank must be connected to all toilets, the cold water supply of the washing machine, and at least one outdoor tap. Some exemptions may apply in specific climate zones or where alternative water sources are available, but this is assessed on a case-by-case basis.
What happens if my granny flat fails the BASIX assessment?
If your granny flat design fails the BASIX assessment, the assessor will provide recommendations on what changes are needed to pass. Common fixes include upgrading insulation, installing a solar hot water system, adding ceiling fans, or specifying higher-performance glazing. The design is then reassessed. Engaging a BASIX assessor during the design phase — before plans are finalised — significantly reduces the risk of failure and the cost of redesign.
Does the May 2026 BASIX update affect existing granny flat approvals?
No. The May 2026 BASIX tool upgrades apply to new applications lodged after 1 May 2026. Existing approvals with valid BASIX certificates are not affected. If you are modifying an approved granny flat, you may need a new or amended BASIX certificate, which would be assessed under the updated tool.
Can I use the Passive House pathway for a granny flat?
Yes. The Passive House Standard method, introduced in the May 2026 BASIX tool upgrades, is available for secondary dwellings. However, it requires engagement of a certified Passive House designer, a PHPP software report, and a post-construction blower door test achieving ≤0.6 ACH50. For most granny flat projects, the standard NatHERS 7-star simulation pathway is more cost-effective, but Passive House certification may be worthwhile for projects targeting ultra-high energy performance.
Author: Franz Phan, Senior Planning Consultant, giantA Pty Ltd. With 15+ years of experience in NSW residential development, Franz has overseen more than 200 granny flat and secondary dwelling projects across Sydney and regional NSW. Sources: NSW Planning Portal BASIX (planningportal.nsw.gov.au/basix), State Environmental Planning Policy (Housing) 2021, Sustainable Buildings SEPP 2022, National Construction Code 2025, NSW Department of Planning, Housing and Infrastructure.