Like any development activity, to build a duplex you first need to have the land.

“If the family is ready to leave the nest, and you are sitting on a very large parcel of land, you may consider knocking down the house and building there,” says David Caruana, managing director of custom builders Arlington Homes in Victoria.

“Otherwise, you could buy a site, perhaps with the proceeds from the sale of the family home.”

Ideal land size for duplexes

Size is an issue. An ideal site is 700 to 750 square metres (although builders do squeeze duplexes on something smaller) and you’re more likely to find this in a regional area if you’re not using your older, larger, family home site, he says.

Rectangular blocks can generally fit more units than irregular blocks, width is important for vehicle access, and sloping land can add restraint on the design, says Caruana.

Of course, if land size or availability is a problem, or you don’t want to go through the building process yourself, you may want to buy a newly-built duplex from one of a number of companies springing up to service pre-retirees’ and retirees’ housing needs.

Navigating the planning system

Now get ready to wade through some serious paperwork.

Robert Gullo, operations manager of duplex specialists Rawson Homes, says: “The town planning rules surrounding duplexes are complex.”

“While a builder may be able to point you in the right direction, you’ll need experts, such as a town planner to assist you, unless they have these specialists at their company.”

Planning and building is a tricky business so you are likely to need a range of experts to prepare and submit the application, before the first concrete pour on the site.

Key planning issues to consider

For starters, it's always a good idea to look at building a duplex which complements the character of the local neighbourhood, while at the same time ensuring you meet all applicable planning regulations.

For this reason, it is well worth consulting not only a builder but also a town planner and potentially also an architect or building designer to prepare your building plans. (Architects are formally registered under State laws, while building designers are not - for this reason building designers tend to be less expensive than hiring architects).

Alternatively, you could select a builder's off-the-shelf design which looks to be in character with the local area and integrates well with your proposed building site.

You are certainly likely to require a new site survey, including understanding not only the site's boundaries but also the current sewerage, slope, tree and soil conditions, along with any title or easement limitations on your proposed block. If you're looking at subdividing the block, a proposed subdivision plan will also need to be prepared.

Remember as well that the council will want to know how you are proposing to drain stormwater from your property, so a block capable of draining towards the street is always preferable. 

Otherwise, you may need to look at an on-site stormwater retention tank or seek to create a drainage easement through a neighbouring property.

In fact, if you're submitting an application to a council it will want to make sure all possible problems with the development are solved before construction starts - this includes for instance resolving any flooding or overland water issues and managing sensitive vegetation.

Fast-tracked planning changes ahead

Fortunately, some jurisdictions are introducing streamlined planning for duplexes.

If you live in NSW, you may benefit from the State Government’s new Low Rise Housing Diversity Code, introduced in July 2020. 

This code allows well-designed semi-detached dwelling projects (in other words duplexes) to be carried out under a complying development approval pathway, in which you should receive your approval in as little as 20 days.

The complying development approval pathways allows you to seek approval for your project for a private or council certifier that you pay for, rather than submitting an application to the local council for a full merit assessment.

This process should deliver a fast and more certain outcome, if you follow the fairly prescriptive rules set out in the code.

In addition, the complying development process means your application will be notified to the neighbours only for their information, but not for their feedback on the proposal.

This approval pathway is best suited for typical and standard residential blocks of land which have few development constraints.

For instance, you can't use this approval pathway for land in a heritage conservation area, and it is more difficult to use it in bushfire or flood prone land.

The Western Australian Government is also delivering a program of reforms related to medium density residential development including duplexes.

The Medium Density Housing Code will form part of Western Australia’s Residential Design Codes Volume 1 – known as the R-Codes – and is open for public comment until 16 April 2021.

Comment from our CEO

Downsizing.com.au CEO Amanda Graham said many State and local governments were happy to support the development of duplexes as they provided an attractive additional housing option for extended families, including those looking to support elderly parents, downsize themselves, or help their adult children start out on the housing ladder.

“This means local planning schemes are likely to increasingly provide opportunities to construct duplexes, particularly on land near retail centres or close to transport,” Ms Graham said.

“So it is definitely worth investigating further if this may suit your family situation or your current block of land.”

Find out more:

See all the stories in our duplex series:

Downsizing.com.au has prepared a series of stories about duplexes, to help guide downsizers who are considering embarking on a duplex build. 

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Helen Hawkes is a business and lifestyle journalist who has written for clients including CPA Australia, Westpac, Colonial First State, The Australian Financial Review, QSuper and the Australian Institute of Superannuation Trustees. She is also the owner of interior styling and homewares business The Little Green Room. Helen has bought and sold five houses and is currently planning a comfortable retirement.