General informationThis is not planning or legal advice. Check the definitions that apply to your lot on the NSW Planning Portal and with your council; in Queensland, check your council’s planning scheme.
The NSW definitions, word for word
The NSW Planning Portal’s page on the Low Rise Housing Diversity Code sets out the two kinds of dual occupancy in a sentence each:
“Dual occupancy (attached) means 2 dwellings on one lot of land that are attached to each other but does not include a secondary dwelling.”
“Dual occupancy (detached) means 2 detached dwellings on one lot of land but does not include a secondary dwelling.”
NSW Planning Portal, Low Rise Housing Diversity Code
Three things in those two sentences decide most of what follows. The two homes stand on one lot, so the land has not been divided. There are exactly two of them. And a secondary dwelling sits outside the definition: the planning department’s example of one is a granny flat, which it says belongs on the same lot as a principal dwelling.
Attached or detached: how the two homes sit
Drawn for this guide from the arrangements the NSW planning department describes, with the street along the bottom of each lot. Not to scale and not designs.
Attached, side by side. The two homes share a common wall in what the department calls a semi-detached configuration. Both usually face the street, keep a front setback and a larger back yard, and are set back from the side neighbours. The department says this arrangement is best suited to wider lots, and that its scale is generally consistent with a single freestanding house.
Detached, side by side. Two separate homes next to each other, both facing the street. The department says this is usually seen on a corner lot, with one home facing the main road and the other facing the side road, and suits narrow, long lots or corner lots.
Detached, front to back. One home at the front facing the street and the other behind it. This arrangement typically has a right of carriageway down one side of the lot to reach the rear home, and the department says it suits lots that are narrow and deep.
The department’s own way of putting the choice is that the arrangement depends on the configuration of the lot of land.
The words that sit next door
Several planning terms sit close to a duplex, and each one leads to different rules. Here they are in plain words, each from its official source.
| Term | What the official source says | Where it is defined |
|---|---|---|
| Dual occupancy (attached) | Two dwellings on one lot, attached to each other; does not include a secondary dwelling. | NSW Planning Portal |
| Dual occupancy (detached) | Two detached dwellings on one lot; does not include a secondary dwelling. | NSW Planning Portal |
| Semi-detached homes | Dual occupancies built side by side and then subdivided down the middle, giving each house a lot of its own; often called semis. | NSW planning department |
| Secondary dwelling | Not part of a dual occupancy; the department gives a granny flat as the example. | NSW planning department |
| Manor house | A residential flat building of 3 or 4 dwellings, each attached to another by a common wall or floor, with at least one partly or wholly above another, and no more than 2 storeys excluding any basement. | NSW Planning Portal |
| Multi-dwelling housing (terraces) | 3 or more attached dwellings on one lot, each with access at ground level, none above another, facing and generally aligned along one or more public roads. | NSW Planning Portal |
What the building code calls each half
The National Construction Code classes buildings by use. Its Class 1a is “one or more buildings, which together form a single dwelling”, and the examples include “One of a group of two or more attached dwellings, each being a building, separated by a fire-resisting wall”. So in an attached duplex each home is a building of its own, and the wall between them is a working part of the design, set out on the party wall page.
The code adds a limit: “a Class 1 building cannot be located above or below another dwelling or another class of building, other than a private garage.” Its explanatory note says a residential unit over another residential unit will always be a Class 2 or Class 3 building. Two homes stacked one above the other are therefore not a Class 1 pair in the code’s terms.
Queensland draws the line wider
NSW
Two dwellings on one lot. Once a side-by-side pair is subdivided so each house sits on its own lot, the department calls them semi-detached homes.
Queensland
The Planning Regulation 2017 defines dual occupancy as a residential use involving “2 dwellings (whether attached or detached) on a single lot or 2 dwellings (whether attached or detached) on separate lots that share a common property”, with any associated domestic outbuilding, and not a secondary dwelling.
Brisbane City Council uses dual occupancy as the City Plan term for duplex developments. Its page explains that a proposal is either code assessable or impact assessable: an impact assessable one needs a development application with public notification before council decides, while a code assessable one does not need public notification. Which category applies depends on the zone and on conditions the page sets for each zone, such as building height. That is one council’s scheme; check your own council’s planning scheme for the rules where you are.
Why the word matters
The planning word decides which rules a design is measured against. In NSW, the Low Rise Housing Diversity Code lets dual occupancies, manor houses and terraces of up to two storeys be approved through the fast-track complying development pathway. The building class decides what the shared wall must do. Even the BASIX tool asks the question: its multi-dwelling section lists dual occupancy dwellings on one parcel of land with common property or shared services among its typical projects, which is covered in BASIX for a duplex.
Dividing a finished dual occupancy into separate lots, by strata or Torrens title, is a later step this guide does not cover.