A duplex is one of the few homes that can help pay for part of itself, and it's a big reason people ask us to build a duplex across Wollongong, the Illawarra and the Southern Highlands. You get two dwellings on one block, so you can rent both, live in one and rent the other, or sell one down the track to bring the mortgage down. Before any of that adds up though, you want a rough number to work with.
What a duplex costs to build
The honest answer is a range, because a duplex is really two homes sharing a wall and a slab. As a rough guide, a single dwelling in a duplex tends to land somewhere around $400,000 to $650,000 to build, so a full pair often sits in the $800,000 to $1.3 million band before land, demolition and council costs. Bigger footprints, two storeys and higher-end finishes push past that.
Treat those figures as a starting point, not a quote. The only way to get a number you can actually plan around is a proper design and a fixed-price proposal on your specific block.
What that duplex build price does and doesn't cover
The $400,000 to $650,000 per dwelling is the build: the slab, the structure, the fit-out and the finishes. It doesn't include the land, demolition of whatever is on it now, council contributions and certification, or the surveying and legal work if you plan to split the titles later.
Those extras bite harder on a duplex than on a single home. You're often paying two sets of service connections, and some councils charge contributions per dwelling rather than per block. Budget them as their own line rather than hoping they squeeze inside the build number.
Does a dual occupancy cost the same as a duplex?
Close to it, when the two homes are attached. Dual occupancy is the planning term for two dwellings on one lot, and a duplex is the attached version, so an attached dual occ prices much like the per-dwelling range above.
A detached dual occupancy, two separate buildings on the same block, generally costs more for the same total floor area. You're paying for two roofs, two slabs and two full sets of external walls instead of one shared structure, plus a longer run of driveway, drainage and services to reach the rear dwelling. What you get back is privacy between the two homes and, on the right block, a cleaner subdivision.
What dual occupancy means in NSW covers attached versus detached properly, and dual occupancy designs for the Illawarra shows how the two shapes sit on local blocks.
What drives the final number
Two duplex builds on paper can cost very differently once you look at the site. The main things that move the price:
- Your block. Slope, soil type and access all matter. A flat, easy block off a quiet Illawarra street is cheaper to build on than a steep or tight one that needs retaining and craned-in materials.
- Single or double storey. Going up costs more per square metre than going out, but a two-storey duplex is sometimes the only way to fit two homes on a narrower block.
- Size and layout. Three bedrooms versus four, single versus double garage, how many bathrooms. Every extra room and wet area adds up.
- Finishes. Standard kitchens and bathrooms versus stone benchtops, better tiling and joinery. This is where budgets stretch the fastest.
- Demolition and site works. If there's an existing house to remove, or a lot of excavation, that's a real line item before you pour a slab.
- Council and certification. Contributions and approval costs vary by council, so check these with your local council or a certifier rather than assuming.
Why owners build duplexes
The appeal is usually money, but it shows up in a few ways.
Some owners rent out both halves and treat the duplex as an income property. Some live in one side and rent the other, which can cover a decent chunk of the loan. Others build to keep family close, one side for the parents and one for the kids, without everyone sharing a kitchen.
The bigger play is subdividing. If your block and your council allow it, you can split the duplex into two separately owned homes and sell one, or hold both as separate assets. That's often where the real value sits, and it's the reason a duplex can make sense on a block where a single knockdown rebuild wouldn't.
Subdivision and titles
This is the part worth getting right early. In NSW, dual occupancy and subdivision are controlled by your council's local rules, and they usually hinge on things like minimum lot size and zoning. Two common paths are Torrens title, where each home ends up on its own separate lot, and strata title, where the two homes share the land under a strata scheme.
Which one is possible, and whether you can subdivide at all, depends entirely on your block. Confirm it with your local council or a private certifier before you get too far into design, because it shapes both what you can build and what each dwelling is worth at the end.
Getting a real duplex build cost for your block
A duplex has more moving parts than a standard home, so the planning stage carries more weight. The design, the council approval and the build all need to line up, and small early decisions, like single versus double storey or which title you're aiming for, move the budget more than the paint colours ever will.
We run the design, the council approval and the build as one job with a single point of contact, which keeps a two-home project from turning into two separate headaches. The price lands in a fixed-price proposal built around your block, so a real duplex cost comes from walking your actual site, not a per-square-metre rule of thumb. That's the point where the range on this page turns into a number you can plan around.
Want a hand with this at your place? Get a free quote or call 0434 037 851.