Planning guide
Can I build a granny flat on my block?
The size limit is the question everyone asks first and the last one that actually matters. Whether a second dwelling is allowed at all is decided by your zone, your block's area, the setbacks and any overlay on the land, in that order.
Written and verified by Struqt’s registered architects · Last verified June 2026
Whether you can build a granny flat is settled before size or cost comes into it, and it is settled by your address rather than by a national rule. Four things have to line up: your zone has to allow a second dwelling at all, your block has to meet the minimum site area that goes with that pathway, the design has to fit the setbacks, open space and parking your council's scheme requires, and any overlay on the land has to be satisfied. Miss one of the four and the maximum floor area never becomes the question.
The four things that decide it
There is no single national answer to this, and the state-level pages that rank for it cannot give you one either, because the rule that governs your block is attached to your address. What does generalise is the order of the questions.
- Is a second dwelling allowed in your zone at all?This one is binary, it is the cheapest to check, and it is the one that ends the conversation when the answer is no. A secondary dwelling is a use, and a zone either contemplates that use or it does not. Plenty of land that looks like ordinary suburbia sits in a zone that permits one house and nothing more, and no amount of spare backyard changes that.Decided by: Your zone, through your council's planning scheme, or through the state instrument in the jurisdictions that set the pathway centrally.
- Does the block itself meet the minimum site area?Most fast-track pathways attach a minimum area for the existing block, not for the leftover space behind the house. A block that falls short does not usually get refused outright, it drops out of the streamlined pathway and into a full merit assessment, which is slower, dearer and genuinely uncertain. This is the gate that catches subdivided inner-suburban blocks most often.Decided by: The same instrument that grants the pathway. The figure is attached to your zone and, in some states, to your specific address.
- Does the design fit inside the envelope?Size is only one of the controls and rarely the binding one. Setbacks from each boundary, site coverage, private open space for both dwellings, car parking, and a separate access path to the street all constrain where a second dwelling can physically sit. On a narrow block the setbacks and the driveway usually decide the outcome long before the floor-area cap does.Decided by: Your council's scheme, read together with any state-wide residential design code that applies.
- Does an overlay change the answer?An overlay sits on top of the zone and can add a requirement, add a referral to another authority, or remove the fast-track pathway altogether. Heritage, flood, bushfire and significant-vegetation overlays are the four that most often reshape a granny flat proposal, and a mature tree near the only buildable corner does it more often than any of them.Decided by: The overlay maps in your council's scheme, plus any referral body an overlay triggers.
Two of the four have guides of their own: flood overlays and heritage overlays. If the plan is to split the block instead of adding to it, minimum lot size and frontage is the equivalent question for subdivision.
Key takeaway: Work them in that order. Gate 1 costs nothing to check and settles it outright when the answer is no, while gate 3 is the one that needs a drawing. Paying for a design before checking the zone is the most expensive way to find out.
What counts as a granny flat?
A granny flat — also called ancillary accommodation, a secondary dwelling or a small second dwelling — is a self-contained dwelling on the same block as an existing main dwelling.
Granny flats often range from about 40m² up to 80m²+, usually with one or two bedrooms, a living space, a bathroom and a kitchenette. Most states set a maximum size for the streamlined approval pathway, and many require a minimum existing site area before you can add a secondary dwelling.
How big can a granny flat be?
| State / territory | Typical maximum size |
|---|---|
| Victoria, Tasmania, New South Wales | 60 m² |
| South Australia, Western Australia | 70 m² |
| Queensland | 80 m² |
| Northern Territory | 50–80 m²Varies by scenario. |
Maximum floor area generally allowed under each state’s streamlined approval pathway. Most states also require a minimum existing site area before you can add a secondary dwelling. These are ceilings for the fast-track pathway, not a guarantee that a second dwelling is permitted on your block.
What catches people out
- A maximum size is a ceiling, not a permission. Finding out that your state allows up to a given floor area tells you what you could not exceed if you were allowed to build at all. It says nothing about whether your zone permits a second dwelling, or whether your block is big enough to qualify. Most pages that answer this question answer it with the ceiling, which is the last of the four gates rather than the first.
- The minimum site area is measured on the whole block. It is the existing lot that has to meet the figure, not the piece of yard the granny flat would sit on. People routinely measure the wrong thing here, conclude they qualify, and discover otherwise at lodgement.
- A streamlined pathway is not an approval. Every state runs some form of fast track for a second dwelling that stays inside the rules, and none of them make it automatic. It is a faster and more certain assessment, with the certainty bought by staying inside every criterion at once. Miss one and you are in the ordinary pathway, which is where the cost and the timeline change.
- Whether you can rent it out is a separate question from whether you can build it. Some schemes attach conditions to who may occupy a secondary dwelling, or to whether it can be sold or leased separately from the main house, and those conditions are set independently of the building rules. If your plan depends on rental income, settle that question before you model the return, not after.
What a granny flat costs to build
A granny flat in Australia typically costs from about $80,000 for a small studio up to $300,000+ for a two-bedroom, custom-designed build. A prefabricated granny flat is the cheaper entry point; a custom design on a tricky block sits at the top of the range. Site preparation, approvals, construction and utility connections each move the figure.
A studio (often prefabricated) sits at the low end; a two-bedroom custom build at the top.
Depends on access, soil type and land topography.
New water, electrical and other services extended from the existing home.
What impacts the price of your build
Sloping or difficult land
$20,000–$100,000+Retaining walls, earthworks (cut & fill) and split-level designs all add cost on a sloping block.
Soil type
$10,000–$80,000+Your soil classification drives footing cost. Reactive clay needs more extensive footings and reinforcement; rocky ground costs more to excavate.
Planning layers
$15,000–$60,000+Heritage, flooding and bushfire planning layers can add significant cost to a build.
What is overlay?Service connections
$10,000–$30,000Sites without access to sewer, water or other services can need on-site solutions such as septic tanks or large rainwater tanks.
Internal fitout
Tens of thousandsFlooring, joinery, tiling heights, air conditioning and plumbing fixtures vary greatly in cost. Higher specifications can add tens of thousands of dollars.
External works
Often excluded from contractFootpaths, driveways, fencing and landscaping are commonly left out of contract pricing but are important to budget for.
Frequently asked questions
Can I build a granny flat on my block?
It depends on four things, and only one of them is size. Your zone has to allow a second dwelling, your block has to meet the minimum site area attached to that pathway, the design has to fit the setbacks, private open space and parking your council requires, and any overlay on the land has to be satisfied. There is no national rule and, in most of Australia, no single state figure either, because the controls that decide it are attached to your address.
Do I need council approval for a granny flat?
Almost always. A secondary dwelling is a separate dwelling, so it needs approval in every state and territory. What differs is the pathway: each state runs a faster, more certain assessment for a proposal that stays inside every one of its criteria, and an ordinary merit assessment for anything that does not. Falling outside the fast track is not a refusal, but it does change the timeline, the cost and the certainty.
What stops most granny flats from being approved?
Site area and setbacks, far more often than floor area. A block that falls under the minimum area for the streamlined pathway drops into a full assessment, and on a narrow block the boundary setbacks and the requirement for separate access to the street usually decide where a second dwelling can physically go before the size cap is ever reached.
Does a granny flat need its own driveway or access?
Usually it needs its own defined access to the street, though not always its own crossover. This is where corner blocks and narrow frontages come unstuck: a street tree, a power pole or an existing crossover can leave only one workable driveway position on the whole site, which caps the site at one dwelling regardless of how much land is left over.
How big can a granny flat be?
It depends on your state. Victoria, Tasmania and NSW generally allow up to 60m² for the streamlined pathway; South Australia and WA up to 70m²; Queensland up to 80m²; and the Northern Territory 50–80m² depending on the scenario. Most states also require a minimum existing site area.
Is a prefab granny flat cheaper than a custom build?
Usually, yes. A prefabricated granny flat is a cheaper entry point — often near the bottom of the $80,000+ range — while a custom-designed, two-bedroom build sits at the top, $300,000 or more, especially on a block with access, slope or soil challenges.
Do I need council approval for a granny flat?
Almost always. A secondary dwelling normally needs approval, and most states run a streamlined pathway if you stay within the size and site-area rules. Statutory approvals typically cost $4,000–$18,000. Run your address through Struqt to see the pathway and rules for your block.
Disclaimer. The figures in this guide are estimates intended as a planning guide only. Actual costs vary significantly depending on your site, design, specification and current market conditions. Always seek detailed quotes from licensed builders, and factor in a contingency of 10–20% before committing to a budget.
Check what your block actually allows
The four gates all resolve to your address, not to your state. A Struqt report reads the zone, the site area, the setbacks and every overlay on your land, tells you which approval pathway a second dwelling would take, and says plainly where the rule is your council\u2019s rather than your state\u2019s.