Passive and thermal design for South Coast homes: what it means and what it costs

The thermal performance of your home determines how comfortable it is in winter and summer, and what you spend on energy over the next 30 years. For a custom home on the NSW South Coast, good thermal design means working with a specific climate: temperate and coastal, with mild winters and warm summers. Getting it right at the design stage costs far less than correcting it later, and the benefit compounds across the life of the building. For context on overall build costs on the South Coast, see our cost to build guide.

What the 7-star BASIX requirement means for your build

From 1 October 2023, all new residential buildings in NSW must meet BASIX thermal performance targets consistent with a 7-star NatHERS rating, up from an average of 5.5 to 6 stars under the previous standard. The increase aligns NSW with the National Construction Code 2022, confirmed by the NSW Planning Portal. Homes in remote climate zones 9, 10, and 11 in north-eastern NSW are exempt; the South Coast is not.

A BASIX certificate is required before lodging a development application or complying development certificate. It is produced by an accredited NatHERS assessor who models the home's heating and cooling load against the caps set for your climate zone. The certificate carries through to the construction certificate stage. This means thermal design decisions, including orientation, glazing, insulation, and shading, must be resolved during the design phase rather than after DA approval. See our DA and CC approvals guide for more on the approval process, and our design-and-build process guide for how thermal performance is integrated from the first brief.

How the South Coast climate shapes good design

The Eurobodalla coast sits in a temperate coastal climate. Batemans Bay records a mean maximum of around 21.8 degrees Celsius and a mean minimum of around 10 degrees (Bureau of Meteorology). Winters are mild by Australian standards; summers are warm but rarely extreme. Consistent afternoon sea breezes are a reliable natural cooling resource. See our Batemans Bay builder guide for what local site conditions mean for your build.

A home designed for this climate should:

• harvest low winter sun through north-facing glazing
• use thermal mass to moderate temperature swings overnight
• block the high summer sun with correctly sized eaves
• create cross-ventilation paths to capture the prevailing coastal afternoon breezes

The five passive design moves that make the most difference

Orientation. Orienting living areas to face true north, within 15 degrees west or 20 degrees east, is the single most cost-effective passive design decision. It captures the low winter sun while the high summer sun passes overhead. Orientation is set at the planning stage and cannot be retrofitted.

Eaves and shading. Correctly sized eaves block the high summer sun from north-facing glass while allowing the lower winter sun to warm the floor and thermal mass. For typical window-to-eave heights on the South Coast, overhangs of 600 to 900 mm are standard. External shutters or adjustable louvres address east and west faces where horizontal eaves give less protection.

Thermal mass. Exposed concrete slabs, masonry walls, and tiled floors absorb heat from winter sun during the day and release it slowly overnight. In summer, when shaded from direct sun, the same surfaces moderate indoor temperatures. Thermal mass works best when paired with correct orientation and shading; mass in the wrong position traps unwanted heat.

Glazing performance. Double glazing with low-E coatings reduces heat loss in winter and limits solar heat gain on east, west, and south faces. For most South Coast custom homes, double glazing delivers a practical balance between cost and performance. On BAL-rated sites, glazing must also meet bushfire construction requirements; see our BAL ratings guide for how bushfire ratings interact with your building envelope.

Cross-ventilation. Openings on the prevailing breeze side, typically south and east on the Eurobodalla coast, draw air through the home without mechanical cooling. Cross-ventilation is a layout question: the floor plan needs to create clear airflow paths, not block them. On a well-oriented South Coast home, natural ventilation can handle summer temperatures on most days without air conditioning.

What Passive House certification means (and who it is for)

Passive House is a certification standard that sits well above the 7-star BASIX minimum. It requires annual space heating and cooling demand of no more than 15 kWh per square metre per year and a peak load of no more than 10 W per square metre (yourhome.gov.au). Certified Passive Houses use 80 to 90 per cent less energy for space conditioning than conventionally built homes, based on Australian build comparisons and data from yourhome.gov.au. The standard requires continuous insulation, elimination of thermal bridges, an airtight building membrane, and a mechanical ventilation unit with heat recovery.

On the mild South Coast, Passive House principles are adapted to balance winter warmth and summer cooling: the tight envelope remains, but external shading and natural ventilation paths complement the mechanical ventilation unit. Perrem's off-grid Solstice home at Depot Beach, a finalist in the 2026 HIA Australian GreenSmart Awards (Custom Built Home category), demonstrates what high thermal performance looks like in this climate: 19 kWh solar with battery storage, 60,000 litres of onsite water, and a building envelope designed for an exposed coastal site. Learn more about Perrem's passive house and thermal performance work.

How thermal design affects your build cost

There is no single cost figure for passive design, because the most effective lever, orientation, has no cost at all. It is a planning decision. The items that do carry a premium, and indicative ranges for a South Coast custom home:

• Double glazing (all openings): $8,000 to $20,000+ depending on window area and product specification
• Insulation upgrade above base specification (wall, roof, underfloor): $3,000 to $8,000
• Extended eaves and external shading above standard roof projection: $3,000 to $10,000
• Full Passive House certification (airtight envelope, heat recovery ventilation, triple glazing on exposed facades): $30,000 to $80,000+ above standard construction for a typical South Coast home

These figures are indicative. For most custom home clients, the practical approach is to design for 7-star BASIX compliance from the outset, covering orientation, eaves, thermal mass, double glazing, and good insulation, without pursuing formal Passive House certification. That combination delivers a very comfortable home for a modest premium above standard construction.

Does my new home need a BASIX certificate on the South Coast?

Yes. All new residential buildings in NSW, including in the Eurobodalla and Shoalhaven shires, require a BASIX certificate before lodging a development application or complying development certificate. The certificate confirms the design meets the thermal performance, water, and energy targets set by NSW Planning. There is no exemption for coastal or regional areas; only homes in remote climate zones 9, 10, and 11 in north-eastern NSW are exempt.

What is a NatHERS star rating and how does it relate to BASIX?

NatHERS (Nationwide House Energy Rating Scheme) rates a home's thermal performance from 0 to 10 stars using accredited modelling software. BASIX is the NSW-specific sustainability index that uses NatHERS as its thermal assessment method and adds water and energy targets. The BASIX certificate is the NSW compliance document; the NatHERS star rating is the thermal metric within it. Since 1 October 2023, NSW requires results consistent with 7 stars, a significant step up from the previous 5.5 to 6 star range.

Is Passive House design suited to the South Coast's coastal climate?

Yes, with adaptations. The Passive House standard originated in cold European climates where heating dominates. On the mild South Coast, cooling and ventilation matter as much as winter warmth. Designers apply Passive House principles, including an airtight envelope, eliminated thermal bridges, and mechanical ventilation with heat recovery, alongside local passive cooling strategies: cross-ventilation paths, external shading, and well-placed thermal mass. The result is a home that stays cool in summer and warm in winter with minimal mechanical assistance.

Does good thermal design add time to the build?

No. Thermal design decisions are made during the design phase, before DA lodgement. The BASIX certificate and any NatHERS modelling are completed at the design stage as part of the certification process. Construction timelines are unchanged; the difference is in what the home is designed to achieve, not in how long it takes to build.

Thermal performance is a design question, not a product to buy at the end. Perrem works through it from the first site visit: orientation, envelope, glazing, and ventilation are resolved before construction begins, not added as upgrades. If you are planning a custom home on the South Coast and want to understand what good thermal design looks like for your block and budget, start with our passive house and thermal performance service page or get in touch directly.