NBN for a duplex or small subdivision: what to sort before design
An eight-step checklist for duplexes, triplexes and subdivisions of up to about 20 lots, so the NBN pit and pipe design is quick and right first time.
Published 14 October 2026. Reviewed against NBN-TE-CTO-194 Revision 13.0
In short: NBN applies the same pit and pipe rules to two premises as to two hundred. Before design starts, confirm whether the site is SDU or MDU, fix the lot count, mark driveways, transformers, poles and retaining walls, and send a clean DWG. Each premises then gets its own P20 service drop from a pit no more than 25 m away.
Small developments are where NBN pit and pipe causes the most surprises. A two-lot subdivision or a duplex feels too small to need a telecommunications design, but NBN applies the same rules whatever the size. The good news is that a small job is quick to design well, as long as a few things are sorted before the designer starts.
1. Confirm how NBN will see your development
NBN's rules depend on what each premises is. If every dwelling sits on its own title and fronts a road, it is treated as a single dwelling unit (SDU). Each premises then needs its own P20 service drop conduit from an NBN pit. A duplex is two premises, so it needs two conduits, not one shared conduit.
If several dwellings share a lot and common property, the site is a multi dwelling unit (MDU) or a super lot instead, and the pit and pipe runs across common property. Super lots follow thresholds: three or fewer premises get individual P20 lead-ins, 4 to 24 get an internal P50 network, and 25 or more get a P100 local network with P50 laterals. Our SDU or MDU guide covers each type.
2. Fix the lot layout and premises count
The premises count in the design must match your stage application with NBN. If the lot count changes after design, the design and the application both need updating. Wait until the lot layout is stable, then start.
3. Put driveways and crossovers on the plan
NBN's standard keeps pits clear of driveways and proposed driveways. Where there is genuinely no alternative, a high strength pit is required. On a small site there may be only one sensible place for a pit, so driveway positions need to be known before design, not chosen afterwards.
4. Find the transformers and poles
Pits must sit at least 15 m from pad-mount transformers and poles with transformers, and at least 3 m from other poles. On a narrow frontage this can rule out most of the verge. Mark existing poles, streetlights and any proposed kiosk on the plan.
5. Check the 25 m rule
The furthest service drop run from a pit to the property boundary entry cannot exceed 25 m. On battle-axe lots and deep blocks this is the rule most likely to force an extra pit.
6. Note retaining walls and easements
Where a service drop passes under a retaining wall, the conduit must be installed before the wall is built. Easements and existing services also limit where conduit can run. Put them on the plan.
7. Send clean CAD
A DWG of the current plan, at true scale, with lot boundaries and numbers, kerbs, driveways and existing services, is the single biggest factor in a fast, accurate design. PDF works for small sites, and NBN accepts designs for developments of up to 20 premises as PDF files, but a traced PDF takes longer and carries more risk. See how to prepare CAD plans.
8. Plan the build and as-builts
Once NBN accepts the design, your contractor builds to it: white PVC conduit to AS/NZS 1477, sealed ends, a draw cord in every conduit, and each service drop starting 1 m inside the lot with an end cap and a label in the pit showing the lot, left or right, and distance from the boundary. After construction the as-built design goes to NBN with the Notice of Practical Completion. Builds that match the design make this step quick, and as-builts are included in our price.
The short version
- Confirm SDU, MDU or super lot
- Fix the lot layout and premises count
- Mark driveways and crossovers
- Mark transformers, poles and streetlights
- Check every lot is within 25 m of a pit
- Mark retaining walls and easements
- Send a clean, true-scale DWG
- Brief your contractor on build and as-built requirements
Duplex and small subdivision questions
Does a two-lot subdivision need an NBN pit and pipe design?
Yes, where NBN services the development through pit and pipe. NBN applies the same design rules regardless of size, and accepts designs for developments of up to 20 premises as PDF files.
Can a duplex share one NBN conduit?
No. NBN requires one P20 service drop conduit per premises, so each dwelling in a duplex gets its own conduit from the pit.
How much does NBN design cost for a duplex?
NBN Design charges $120 per unit for the first 10 units with no minimum fee, so a duplex is $240.
Also published on NBN Design's LinkedIn page. Requirements summarised from NBN-TE-CTO-194 Revision 13.0 for general guidance; always check the current revision.