In an attached duplex, the shared wall does more work than any other wall in the building. It has to hold back a fire in one home long enough for the neighbours to get out, and it has to keep everyday noise from carrying between living spaces.
Why the wall matters
The National Construction Code treats each home in a side by side duplex as its own Class 1a building. That status depends on the homes being divided by a fire-resisting wall. The detailed rules sit in the ABCB Housing Provisions, which NSW applies through the Building Code of Australia.
Fire rules
Under Part 9.3 of the Housing Provisions, a separating wall must either have a fire resistance level (FRL) of at least 60/60/60, or be masonry at least 90 mm thick. The three numbers refer to minutes of structural adequacy, integrity and insulation.
The wall must also:
- start at the footings or ground slab
- rise all the way to the roof covering, with any gap at the top packed with mineral fibre or a similar fire-resisting material
- not be crossed by timber or other combustible parts, except roof battens no larger than 75 x 50 mm and roof sarking
- where it meets a masonry veneer outer skin, leave a gap of no more than 50 mm, packed with fire-resisting material
Eaves, verandahs and similar spaces that open into a roof space shared by both homes need a non-combustible vertical lining to divide them.
Services through the wall
Every hole in the wall is a weak point. The code limits the size of openings for cables and other services, requires gaps around them to be packed with fire-resisting material, and controls where power points and switches can sit back to back. Where possible, run plumbing and wiring so it does not need to cross the wall at all.
Sound rules
Part 10.7 of the Housing Provisions sets the acoustic standard. A separating wall between Class 1 buildings must achieve an airborne sound rating (Rw + Ctr) of at least 50. If a bathroom, toilet, laundry or kitchen in one home sits against a habitable room other than a kitchen in the other, that part of the wall must be of discontinuous construction. In practice, that usually means two separate stud frames with a gap between them.
Other acoustic details include:
- the wall rising right up to the roof, or to a ceiling that gives the required insulation
- water pipes for one home kept off the wall leaf on the other side, with a clearance of at least 10 mm
- power outlets on opposite sides offset by at least 100 mm in masonry or 300 mm in framed walls
- a second layer of plasterboard laid so its joints do not line up with the first
Proprietary wall systems
Many duplexes use lightweight fire and acoustic wall systems instead of masonry. These only achieve their rating when built exactly as tested, including the right studs, fixings, sealants and board layers. Keep the manufacturer's installation details on site and ask the installer for certification, because the principal certifier will want it before the occupation certificate.
What the certifier looks at
The frame inspection is the key time to see the wall before linings hide it. Expect the certifier to check the wall type against the approved plans, packing at the roof and eaves, service penetrations and the separation of frames. Book at least 48 hours ahead through our inspection booking page.
NSW plans to adopt NCC 2025 from 1 May 2027, and your certifier will confirm which edition applies to your application. Learn more about how we certify duplex and dual occupancy projects, or send drawings for plan assessment before construction.
Local help across NSW
CertifiBuild certifies Class 1 and Class 10 work in 39 council areas. Local rules on these council pages are a good place to start: Wollondilly Shire Council, Willoughby City Council, Burwood Council, City of Ryde, City of Sydney, Campbelltown City Council.
Suburb pages with local planning notes: Buxton, Castle Cove, Castlecrag, Chatswood, Chatswood West, Croydon Park, East Ryde, Forest Lodge, Glen Alpine, Hermitage Flat. You can also browse every service area.
Sources
- Separating wall fire rules (Housing Provisions Part 9.3), ABCB
- Sound insulation of separating walls (Housing Provisions Part 10.7), ABCB
- Guide to NCC building classes, ABCB