Why Sydney Property Owners Are Building Duplexes: A Builder's View on the Investment Case

A few years ago, most of the people who came to us wanted one house on one block. These days, a growing share of the conversations I have start with a different question: "Could I fit two homes here?" Usually it's someone who has owned an older house for years and wonders whether the land could work harder.

I'm a builder, not an accountant or a financial adviser, so I won't tell you whether a duplex is the right investment for you. What I can share is what I see on site and in early planning meetings: why owners consider it, what makes a block suitable, and where projects tend to go right or wrong.

What a duplex actually is in Sydney

For international readers, a duplex in Australia usually means two attached homes on one block, sharing a common wall. They are often built side by side, sometimes front and back. In New South Wales (NSW) the two homes can often be strata or Torrens titled separately, which means each can potentially be sold, financed or rented on its own. Whether that is possible depends on the site and the local council, so it is one of the first things to check.

A common way in is a "knockdown rebuild". That's where an owner demolishes an older, tired house and builds new on the same land. Plenty of Sydney suburbs have post-war houses on generous blocks, and that combination is often what starts the duplex conversation.

The three strategies I see most often

Live in one, rent the other. This is probably the most common. The owner gets a new home to live in, and the second dwelling brings in rental income.

Build two, sell one. Some owners plan from the start to sell one half when it is finished and keep the other. The idea is that the sale helps offset the cost of the build. Whether that works depends entirely on the numbers in your market at the time, which is exactly why you should run it past an accountant and a local agent before committing.

Multigenerational living. This is one I find really rewarding to build. Parents in one home, adult children and grandkids in the other. Everyone has their own front door, kitchen and privacy, but help is next door. Families tell me it eases childcare and ageing-parent care without anyone feeling crowded.

What makes a block suitable

Not every block can take a duplex, and finding out early saves a lot of money and disappointment. The things I look at first:

- Zoning. The planning controls on the land need to allow dual occupancy (the NSW planning term for two dwellings on one lot). Your council's planning controls will tell you.

- Size and frontage. Councils usually set minimum lot sizes and street frontage widths for duplexes. These vary between councils, so check the current rules for your address rather than relying on what a neighbour did.

- Shape and slope. A regular, fairly level block is simpler and usually cheaper to build on. A steep or oddly shaped one can still work, but expect more engineering and site costs.

- Services and easements. Where the sewer, stormwater and power run matters. An easement or a sewer main through the middle of the block can change the whole layout.

- Constraints on the land. Things like flood or bushfire overlays, heritage listings and large protected trees can all affect what is approved.

Approvals in general terms

In NSW there are generally two paths to approval. A Development Application (DA) goes to the local council for assessment. It is the more flexible route but usually takes longer. A Complying Development Certificate (CDC) is a faster, more standardised approval that a private certifier can issue, but only if the site and design meet a fixed set of criteria.

Which path is available depends on your block and your design, and the rules change from time to time. My advice is always the same: talk to your council or an accredited certifier about your specific address before you fall in love with a design.

Design choices that affect rentability and resale

This is where a good investment duplex separates itself from one that just ticks boxes. In my experience, buyers and tenants respond to homes that feel like proper standalone houses, so I push for a few things on every project:

- Two genuinely separate homes. Separate entries, driveways where possible, and layouts that don't look into each other's living areas.

- Separate private yards. Even a modest courtyard matters to families and pet owners.

- An acoustic party wall. The shared wall needs to be built properly to limit noise between the two homes. Nothing undermines a duplex faster than hearing the neighbour's TV.

- Separate services and metering. Individual water, power and gas meters where possible make renting, billing and selling much simpler later on.

- Sensible, durable finishes. A rental needs to stand up to wear. Good-quality flooring, hard-wearing benchtops and plenty of storage go a long way.

If you'd like to see how these principles play out on real projects, you can look at how Buildana's duplex team approaches the planning and layout stage.

Budgeting and managing risk

I won't put a figure on what a duplex costs, because it genuinely depends on the block, the design, the finishes and the approval pathway. Two sites on the same street can come in quite differently once you factor in demolition, soil conditions and service connections.

A few things I'd suggest:

- Get site-specific quotes. Make sure the quote covers site costs, demolition, connections and any council requirements, not just the build itself.

- Hold a contingency. Older blocks in particular can hide surprises underground. Set money aside for the unexpected.

- Allow for holding costs. If you're moving out during the build, account for rent or temporary accommodation, plus loan repayments while there's no income.

- Choose a licensed builder with duplex experience. In NSW, check the builder's licence on the NSW Fair Trading website, and ask to see duplexes they have actually completed. Duplexes involve things a standard single home doesn't, like fire separation and separate services.

- Get the right advice on the money side. Tax, depreciation, finance and titling decisions should go through an accountant and a financial adviser. Those choices can shape the design, so bring them in early.

A final word

A duplex can be a smart way to make more of land you already own, whether that means a new home with rental income, a staged sale, or a place where the whole family can live close together. In my experience, the projects that go well are the ones where the owner does the homework first: confirms the block is suitable, understands the approval path, designs two homes people actually want to live in, and budgets honestly. Do that groundwork with the right builder and advisers beside you, and you'll be making the decision with your eyes open.

About the author

Oliver Alameri is the founder of Buildana, a licensed builder based in Fairfield, Western Sydney (NSW licence 487805C). Buildana builds custom homes, duplexes, knockdown rebuilds, granny flats and extensions, and is a member of the Housing Industry Association (HIA) and Master Builders NSW. 

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