The guide on this site covers what a dual occupancy is and how it is approved, and the land size page covers whether your lot qualifies. This page starts where those end: you have a lot that can carry two homes, and now they have to be built.
1. The layout decides the build
A dual occupancy is attached or detached, and the construction consequences follow directly from that choice.
| Attached | Detached | |
|---|---|---|
| Shared elements | A separating wall between the two homes, usually under one continuous roof | No shared wall; two separate buildings |
| Fire rules for the shared wall | NCC Housing Provisions Part 9.3 applies | Not a separating wall; the external wall rules for buildings close together apply instead |
| Sound rules | NCC Housing Provisions Part 10.7 applies to the separating wall | Not applicable to a separating wall |
| Where the effort goes | Detailing one wall very well | Two of every external envelope element |
Neither is simply cheaper. An attached pair shares a wall and often a roof, but that wall is the most heavily regulated element in the build. A detached pair avoids it, but builds two complete envelopes. Which one wins depends on the lot, and that is decided at design, not on site.
2. The separating wall: fire
Where two attached homes share a wall, the National Construction Code treats it as a separating wall between Class 1 buildings. Under Part 9.3 of the NCC 2022 Housing Provisions, it must:
- have a fire resistance level (FRL) of not less than 60/60/60, or be masonry not less than 90 mm thick;
- extend to the underside of the roof covering where the roof covering is non-combustible, or to not less than 450 mm above it where the roof covering is combustible;
- not be crossed by timber or other combustible building elements, except roof battens of 75 × 50 mm or less, or roof sarking;
- have any gap between the top of the wall and the underside of the roof covering packed with mineral fibre or other suitable fire-resisting material.
Two more clauses catch people. Services that pass through the wall must be treated to the requirements for service openings. And combustible roof lights and skylights in a roof that needs a non-combustible covering must sit at least 900 mm from the vertical line of the separating wall.
Where this goes wrong
Not in the wall itself, which is easy to draw, but where the wall meets the roof and where plumbing and cabling cross it. Those details are covered up early, so they need checking before the roof and linings go on.
3. The separating wall: sound
The same wall carries sound rules. Under Part 10.7 of the Housing Provisions, a separating wall between Class 1 buildings must have an airborne sound rating of Rw + Ctr not less than 50.
There is a second requirement that shapes the floor plan. The wall must be of discontinuous construction if it separates a bathroom, sanitary compartment, laundry or kitchen in one home from a habitable room (other than a kitchen) in the other. Discontinuous construction adds cost and thickness. The simplest way to limit it is to plan the two homes so wet areas and kitchens back onto each other, rather than onto a neighbour's bedroom or living room. That is a drawing decision, and it is free if it is made before the plans are lodged.
4. Water and wastewater
In Sydney Water's area of operations, which includes the Illawarra, a new water or wastewater connection for any type of dwelling, including dual occupancy units, needs building plan approval, a Section 73 Compliance Certificate if one is required for your development, and a Sydney Water Tap in application.
The Section 73 certificate is the one to plan around. It confirms the development has adequate water, wastewater and stormwater services, and Sydney Water says councils generally will not provide occupancy certificates for new properties without it. Your council or certifier will tell you whether you need one. If you do, start early: any works Sydney Water requires become part of your build.
5. One BASIX certificate for both homes
For new dwellings, the NSW Planning Portal says the application must include one BASIX certificate covering all dwellings. For a dual occupancy with strata title, separate certificates can be submitted, but only where common areas and shared services can be apportioned between them and the consent authority approves it first.
Since 1 October 2023 the thermal standard for new homes in the Illawarra has been 7 stars. On an attached pair, the separating wall helps both homes thermally, one of the few places the shared wall saves money rather than costs it.
6. Build order: together or one at a time
This is judgement rather than regulation. My default is to build both homes together under one contract: one site set-up, one set of footings and slab pours, one frame stage, one roof, and trades moving straight from one home to the next. The separating wall in an attached pair also has to be built as one element, so staging an attached pair rarely makes sense.
Staging a detached pair can work, for example building the rear home first and living in it while the front home is built. The cost is doing the site set-up, the approvals admin and parts of the servicing twice, and it needs a clear plan for access past the finished home. Decide it before the contract is priced, because it changes the price.
Checklist before you sign a dual occupancy contract
- Attached or detached decided, and why.
- Separating wall system named, with its FRL and Rw + Ctr, and the roof junction detailed.
- Wet areas and kitchens planned against each other where they meet the separating wall.
- Whether a Section 73 Compliance Certificate is needed, and who is applying.
- One BASIX certificate covering both homes, unless the consent authority has agreed otherwise.
- Build order agreed and reflected in the price and the progress payments.
Sources
Checked on 28 September 2026.
- Australian Building Codes Board, NCC 2022 Housing Provisions Part 9.3, Fire protection of separating walls and floors: FRL, extent to the roof, crossing elements, gap packing, service openings, roof lights.
- Australian Building Codes Board, NCC 2022 Housing Provisions Part 10.7, Sound insulation: Rw + Ctr 50 and discontinuous construction.
- Sydney Water, Connections: new connections for dual occupancy units.
- Sydney Water, Section 73 Compliance Certificates: purpose, and occupancy certificates.
- NSW Planning Portal, When should more than one BASIX certificate be submitted?: one certificate covering all dwellings, and the strata exception.
General information from a builder, not engineering, certification or planning advice. The NCC edition that applies depends on when your application is lodged; your certifier confirms the requirements for your approval.