Duplex and dual occupancy

Are duplexes allowed in R2 zones?

From 1 July 2024, duplexes and semi-detached homes are permitted in every R2 zone (low density residential) in NSW, even where the council plan previously prohibited them. Approval is still required. Certain land is excluded, for example bush fire prone land and heritage items.

There is a catch. If a duplex is allowed in R2 only because of the new state rule, not your council's plan, you cannot use the fast-track CDC route (a quick approval from a private certifier). You need a development application (DA) to council instead.

How large must my lot be?

Under the state fast-track duplex rules (the Codes SEPP's Low Rise Housing Diversity rules), your council's own duplex minimum sets the lot size. Where council has no minimum, 400 square metres applies. The rules only operate in zones RU5 and R1 to R3, and only where council already permits duplexes.

From 28 February 2025, low and mid rise housing areas follow different rules. These areas are residential zones an 800 metre walk or less from a nominated station or town centre. In them, council cannot refuse a duplex on these grounds when it passes four tests. The lot must be 450 square metres or larger, and 12 metres wide where the front building line sits. The floor space ratio (floor area compared to lot area) is 0.65:1. Height tops out at 9.5 metres.

Can I subdivide later?

Within those station and town centre areas, zones R1 to R3 also have rules for dividing a duplex into two lots. Every new lot must be 225 square metres or larger, with 6 metres or more of width at its front building line. No battle-axe lots are allowed.

They only apply where the duplex approval was granted, or the combined application was lodged, from 28 February 2025 onwards. Strata subdivision is not covered. Any subdivision needs a separate approval plus a registered surveyor. Bring it up before your design is locked in.

What does the shared wall need?

An attached duplex is two houses joined by a separating wall. Under the building code, the wall must have a fire rating of at least 60/60/60, or be masonry at least 90 mm thick. It must start at the footings or ground slab.

The wall must reach right up to the roof covering. Any gap must be packed with fire-resisting material such as mineral fibre. Timber and other materials that burn cannot cross it, apart from small roof battens (75 x 50 mm or smaller) and roof sarking. If eaves or a verandah run across both homes, a non-combustible vertical lining must divide them.

The same wall must also block sound between the homes. Sort out fire and sound details together on the drawings.

Which land rules out a duplex CDC?

Fast-track duplex approvals come from Part 3B of the Codes SEPP (State Environmental Planning Policy (Exempt and Complying Development Codes) 2008). Before detailed design starts, compare your site with that policy's list of excluded land. A single hit means no complying development certificate (CDC), and the duplex then needs a development application (DA) lodged with council.

A site containing a heritage item, or a draft heritage item, is shut out of Part 3B entirely. Broader exclusions also cover heritage conservation areas, sites set aside for public purposes, certain acid sulfate soil classes, significantly contaminated sites, foreshore areas, coastal hazard areas and sites inside the 25 ANEF contour (a measure of aircraft noise) or a noisier one.

Failing an exclusion does not always end the project. Where the zone permits a dual occupancy, council can still assess it on its merits. Knowing this before the architect starts saves paying for CDC drawings that cannot be lodged.

Bush fire risk and the CDC route

A bush fire prone map overlay alone will not block a CDC duplex. What counts is the bush fire attack level (BAL), which rates how exposed the actual building site would be.

The Codes SEPP bars complying development on any site rated BAL-FZ (flame zone), and on any site rated BAL-40. Council can determine the rating, or a consultant the Rural Fire Service recognises can, applying the Rural Fire Service's published assessment method.

The restriction applies only to the affected part of the lot. If both homes sit on a part rated below BAL-40, the CDC can proceed there even though another corner of the lot is worse. Landscaping, non-combustible fences, swimming pools and non-habitable detached structures more than 6 metres from a dwelling are also outside this rule.

Order the BAL report early. It shows whether the spot chosen for the two homes qualifies for complying development, before money goes into drawings.

Does Sydney Water need to see a duplex?

In Sydney Water's area, dual occupancy development is on the list of projects referred for a Section 73 compliance certificate. Subdividing or keeping one lot makes no difference to this. Sydney Water notes that the consent authority makes the final referral decision, so read the conditions on your consent or CDC.

Sydney Water separately assesses building plans against its pipes and structures through its Tap in service. Its own procedure says a building plan cannot be approved while the development still needs a Section 73 application, so the two steps have to be done in the right order.

That referral list also names subdivision by Torrens, strata and community title. Begin the Section 73 process early in the build, so water and sewer paperwork does not become the final item holding up the project.

Can I owner-build a duplex?

Only in limited cases. A permit is required in NSW when an owner, rather than a licensed builder, carries out or supervises work worth over $10,000 including GST. For a dual occupancy, the permit is issued only where special circumstances exist, because the land could be subdivided or sold as separate properties.

The owner must plan to live in one of the two homes after construction. For work costing $20,000 or more, the required owner-builder training comes first. Generally only one permit is issued in any five year period, unless the earlier permit was for the same land or special circumstances apply.

The permit never covers specialist trades. Unless you personally hold the licence, specialist work stays with licensed trades: electrical, plumbing and gasfitting, plus air conditioning and refrigeration. Plan for this in the budget and program before deciding to owner-build.