Building A Granny Flat in WA: Everything You Need To Know

Are you one of many Aussies trying to maximise your backyard by building a granny flat? You’ve probably heard about all the great benefits a granny flat offers — somewhere private for your parents to stay, personal space for a teenage kid, or even a rental for private tenants. 

Deciding on building a granny flat probably isn’t what’s holding you back. It’s all the red tape behind legally building another home on your property. Before you go any further, you’ll need to understand the requirements for a granny flat build in Western Australia

You can build a granny flat or ancillary dwelling on your property in Western Australia if your property is large enough. WA residential density codes, or R-Codes, can affect the minimum land size needed for a granny flat. Some councils may have a different minimum land size, so it’s important to check with your local council. 

You’ll need to accommodate a disconnected structure with enough room remaining to adhere to the R-Codes in your area.  There are policies and limitations that have to be followed when considering a granny flat, and the minimum or maximum land size is just one of them. 

Knowing how confusing these codes can be, we took it upon ourselves to help you out by writing this guide to understand what you need to know when building a granny flat in Western Australia.

What Is a Granny Flat?

A granny flat is essentially a second home located on the same lot as a primary dwelling. They are also called ‘ancillary dwellings’ and can be separate from the main house, attached to the main house, or share some common space with the main house on the property. 

Most granny flats are self-contained and have everything you need to live independently, like a laundry, bathroom and kitchen. Recent changes to Western Australia’s State Planning Policy 3.1  mean that granny flats are no longer just for family members but are available to rent for anyone. This change makes building a granny flat an excellent option for homeowners who want to use their excess land to build another home they can rent out freely. 

What Is an Ancillary Dwelling?

An ancillary dwelling is a relatively small and self-contained dwelling standing on the same lot as a separate single house. It is also known as a secondary dwelling and can either be integrated into the main house, attached to the main house, or exist as a separate building altogether.

Secondary dwellings are often called granny flats. They typically have facilities a person would require to live in them independently, like a kitchen and a bathroom. Another type of secondary dwelling is a fonzie flat — a small self-contained dwelling that exists atop the main house’s garage. Because of their small size, second dwellings are usually occupied by an individual or a couple.

Is a Granny Flat a Secondary Dwelling?

Yes. ‘Granny Flat’ is a common term used to refer to secondary dwellings or ancillary dwellings, as they’re known in Western Australia. A granny flat is considered a secondary dwelling as long as it has a smaller footprint than the primary dwelling and is self-contained, requiring everything someone would need to live comfortably long term. 

Pros & Cons of Building a Granny Flat in WA

Building a granny flat has many advantages, which is why so many people consider it every year. However, it’s not a default great solution for every person and situation, as there are downsides you need to consider.

Pros of Building a Granny Flat in WA

Building a granny flat on your property has many merits. Here are some of the pros of building a granny flat:

  • Utilises extra space on your property – A granny flat will not only reduce the amount of space to maintain in your backyard, but it’s a great way to use extra land that may have been going to waste. Bigger properties in particular can gain a lot out of using the space that would ordinarily be left alone. 

  • Can generate extra income – Turning extra space on your land is a great way to generate some extra income. Renting out a granny flat you build will pay for the construction over time, and after that it becomes pure profit. 

  • Extra space for family or friends – A granny flat on your property is the perfect way to give your kids some extra room, have your parents close by or just have a place for visiting friends to sleep. 

  • Increases the value of your home – Although you will have to outlay a substantial amount of money to get the granny flat built, simply having it will add to the value of your home significantly. 

Cons of Building a Granny Flat in WA

Of course, there are very relevant drawbacks to building your own granny flat that need to be recognised. Some of the downsides of granny flats include:

  • Building a granny flat does require investment – Although over time you can recoup your investment, building a granny flat is still a significant investment. Just because you have the space to build one, doesn’t mean you can afford to. 

  • Unable to later subdivide your landSubdividing your Perth block and selling part of your land is a great idea for people with large properties looking to downsize and generate some extra money, especially later on in life. However, it’s important to note that if you build a granny flat on a plot of land, you won’t be able to subdivide it. 

  • Can be a lengthy process – A granny flat is still a small house, and like building a house in WA, it can be a long process from start to finish. Any number of factors can slow construction, like bad weather or finance availability.  

Can I Rent Out a Granny Flat in Western Australia?

Yes, Western Australia’s State Planning Policy 3.1 (SPP 3.1) changes under section 26 of the Planning and Development Act 2005 allowed non-family members of the primary dwelling to live in the ancillary dwelling/ granny flat. These changes don’t state a maximum number of people that can now live in a granny flat. 

Before the changes in Western Australia’s State Planning Policy 3.1 (SPP 3.1), planning regulations only permitted direct family members of the main house to occupy a second dwelling on the same lot. However, changes to the Residential Design Codes now allow for the occupation of non-family members in secondary dwellings. They can now provide housing for private tenants, students, carers, or unrelated seniors.

As part of the Tenancy Act, granny flats rented out must come with a suitable kitchen, window treatment, and flooring. In addition, as announced in the Federal Budget for 2021, the Government will soon treat granny flats as part of the primary dwelling for Capital Gains Tax (CGT). This means renting out a granny flat and generating an income will not be considered an investment property and will not attract CGT when selling the home.

However, these changes don’t automatically apply. You’ll need to make a local council application before you can legally rent out a granny flat in Western Australia. 

What Is the Minimum Land Size Required for a Granny Flat?

According to Western Australia’s Planning and Development Act 2005, the minimum lot size for a granny flat is 450m2 (unless otherwise stated by your local council). Granny flats are generally allowed to be a maximum of 70m2 but can be up to 100m2, depending on your local council.

Ultimately, the minimum land size required for a Granny Flat will depend on the property’s residential density under the R-Codes of Western Australia. Sizing thresholds differ across the different density codes — R20, R30, R40, R60, and R80.

Residential density is the maximum amount of units that can be built in a specific area. Western Australian R-Codes are a useful indicator of how dense an area can be and the minimum size needed to build a primary and secondary home. Councils and city planners use R-Codes to maintain a healthy amount of development in an area. 

R20: an average of 450m2 is required per dwelling

Existing Lot Size

450-899 m2

900-1,349 m2

1,350-1,799 m2

1,800-2,249 m2

2,250 m2 plus

Potential No. of Dwellings

Single and Grouped





5 or more

Multiple Dwellings





5 or more

R30: an average of 300m2 is required per dwelling

Existing Lot Size

300-599 m2

600-899 m2

900-1,199 m2

1,200-1,499 m2

1,500 m2 plus

Potential No. of Dwellings

Single and Grouped





5 or more

Multiple Dwellings





5 or more

R40: an average of 300m2 is required per dwelling

Existing Lot Size

220-439 m2

440-659 m2

660-879 m2

880-1,099 m2

1,100 m2 plus

Potential No. of Dwellings

Single and Grouped





5 or more

Multiple Dwellings





10 or more

R60: an average of 150m2 is required per dwelling

Existing Lot Size

150-299 m2

300-599 m2

600-749 m2

750-899 m2

900-1,049 m2 

Potential No. of Dwellings

Single and Grouped






Multiple Dwellings






R80: an average of 120m2 is required per dwelling

Existing Lot Size

180-359 m2

360-539 m2

540-719 m2

720-899 m2

900-1,299 m2 

Potential No. of Dwellings

Single and Grouped






Multiple Dwellings






There are still a few other factors to consider when building a granny flat that isn’t covered in this table, like;

  1. Whether an existing dwelling is retained
  2. Whether the proposed development will be used for a “battle-axe” type subdivision
  3. The R-Codes Clause 5.1.1 allows for modifying the minimum site area requirements.

In addition, the calculations for multiple dwellings in R40-R80 were made under the assumption that the average dwelling area is 65m2. This may not always be the case, as dwellings can be as small as 40m2.

To learn more, see our guide to R-codes in Western Australia.

Can I Build a Granny Flat on Vacant Land?

No, you can’t build a granny flat on vacant land. One condition for a granny flat being built on a property is that it has to be built on the same piece of land as an existing home and can’t be built if it would be the first building on that plot of land. 

How Many Granny Flats Can I Build on My Land?

Only one granny flat can be built on each plot of land, according to the Western Australia Planning and Development Act of 2005. In addition, you won’t be able to subdivide a block that has a granny flat built on it unless otherwise stated in the local planning scheme.

Do I Need Council Approval to Build a Granny Flat?

No, granny flats do not need council approval. However, they need formal building approval, either from the Development Approval (DA) or the Complying Development Certificate (CDC). Having building approval for your granny flat will ensure the building will be in line with the city’s Local Planning Scheme and Residential Design Codes.

Getting Approval for a Granny Flat in WA

For a granny flat or ancillary dwelling to be approved, it should comply with the requirements set forth by the Residential Design Codes of Western Australia (R-Codes), the Building Codes of Australia, and the local planning schemes of the city.

The planning requirements for a secondary dwelling are listed below. These dwellings must:

  • Be on the same lot as a single house, in which the lot is not less than 450m2.

  • Have a maximum floor area of 70m2.

  • Provide an additional car bay where the property is not within 800 metres of a train station or 250 metres of a high-frequency bus route.

  • Comply with all other R-Code provisions and requirements of the city’s Residential Development Local Planning Policy since these apply to single houses, except for the site area, street surveillance, and outdoor living areas.

  • Under the city’s Residential Development Local Planning Policy, it should also complement the existing house in terms of scale, roof design, detailing, window size, material, and colours.

If the second dwelling cannot satisfy these requirements, the applicant has the option to apply for an assessment against the relevant Design Principles of the R-Codes, which the city will then review to determine if it can be supported or not.

Planning approval is required to ensure that the development of a property within the city complies with the Local Planning Scheme, Residential Design Codes, and other relevant policies and legislations.

Building Permits to Build a Granny Flat (Ancillary Dwelling)

Building approval is required to ensure that the proposed building work complies with the Building Codes of Australia and other applicable standards and legislations.

Building permits should be secured from the relevant authority to build a secondary dwelling. However, a proposal for a second dwelling should first comply with the R-Codes and be confirmed by a decision-maker before issuing a building permit. 

In addition, building permit determinations should not seek to impose planning conditions and should not be refused on planning grounds, as this will create delays and result in invalid decisions.

Local Council Permits to Build a Granny Flat (Ancillary Dwelling)

Granny flats do not typically require local council approval, but if you’re uncertain, ask your builder or your local council. 

Larger ancillary dwellings or secondary dwellings may require council approval. Each city council will have relevant permits that allow the construction of a secondary dwelling. The Building Commission of Western Australia provides information on the Building Codes of Australia (BCA) and the steps required to certify plans for a secondary dwelling. Consult with the building approvals department at your local council to learn more about the process.

Ancillary Dwelling Building Codes in Australia

New secondary dwellings must have a building approval to ensure their compliance with the structural and fire safety, health, amenity and sustainability requirements provided for in the Building Code of Australia (BCA).

Do Mobile Granny Flats Require the Same Approval as Built Granny Flats?

No, mobile granny flats require council approval, while traditional granny flats only require building approval.

 Mobile granny flats require council approval, while traditional granny flats require either a Development Approval (DA) from your local council or a Complying Development Certificate (CDC).

How Close to Fence Can I Build a Granny Flat?

A granny flat must be at least three metres away from any fence. Although it seems like a great idea to use the space between the homes as effectively as possible, you can’t have a granny flat attached right next to a fence line.

The taller your granny flat is, the further away from the fence line it has to be. The exact allowable sizes will depend on your local council. You may also opt to build a higher back fence, subject to the permission of your local council.

How Much Does It Cost to Build a Granny Flat in WA?

The average cost to build a granny flat in Western Australia is between $100,000 and $120,000. However, this is just an average, and a small, simple single bedroom granny flat can cost less than $50,000 to build.

The cost to build your granny flat will ultimately depend on the size of the granny flat, the fees and rates associated with getting it built, and whether or not the granny flat is a premade or custom design.

Different councils and residential codes will affect the amount you will need to pay in fees and permits to have the granny flat built. Additionally, installing and connecting utilities will also add to your costs, and can be significant depending on your area. 

For a specific quote on your granny flat construction, consult a Perth custom builder

Can You Build Your Own Granny Flat in WA?

Although it is possible to build your own granny flat as an owner builder, it’s much harder, more time consuming and can even be more expensive. Because of the difficulty involved, many people find it safer and easier to work with a builder. 

It may seem like an appealing choice to build your own granny flat, work on it at your own pace, have complete control, and save money. But the reality is that the owner builder process in Western Australia is very challenging especially if you have no experience in construction or management, and you may not save the money you think you will.

To be able to build your own granny flat, you will need to be certified to do so safely. This involves getting an owner-builder certificate, a white card and you’ll still need to get licensed tradies in for things like electrical wiring and plumbing installation. Because you still have to manage labour, as well as any costs you incur from inexperienced building, it’s almost always best to work with a registered builder in Western Australia

If you’re looking to cut costs as much as possible, building to lock-up stage or ‘finish it yourself’ construction is a good compromise between the two options. Your structure will be built by the professionals, leaving you to organise the fixtures and finishings. 

Still not sure which option is right for you? Read our full guide to owner builder vs licensed builder construction

Who Builds Granny Flats in Western Australia?

Central Avenue Homes is one of the most experienced building companies in Western Australia. That’s why they’re a trusted name for building granny flats and other property development projects in Greater Perth. 

Working with a trusted home builder when adding a granny flat to your existing property is important. With over 500 custom design-and-build homes under our belt, you know you’ll get the best results Central Avenue Homes. 

Our property development specialties include:

Interested in adding a granny flat to your Perth property? Get in touch with Central Avenue Homes to find out more.

Greg Grainger

Greg has over 40 years of experience in the WA building industry starting as a carpenter joiner.

He is entrenched in the local industry and has served on the board for MBAWA (Master Builders Australia WA) for over 10 years and was a founding director of Wesbuilders Cooperative for over 11 years.

With this experience he is able to quote accurately on new projects without the huge increase to provisional sum allowances.

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