What our analysis shows
Analysis by Granny Flat Numbers, data as of 23 October 2026
- On a 60 m² Sydney flat, prefab came out $2,500 cheaper than site-built in our model. We assume the factory builds 8% cheaper than a site crew, and charge $12,000 for transport, crane and set-down.
- Below about 52 m² in Sydney, prefab costs more than a site-built flat, because the fixed transport cost outweighs the factory saving. On a 30 m² studio the gap is $3,500 in favour of building on site.
- The cheaper the state's build rates, the bigger a prefab needs to be to save money: from 46 m² in the ACT to 61 m² in South Australia.
- Owners on forums report the same thing from the other side: in 2025 a Brisbane-area family found quality prefab units priced close to a small new house. A kit built by an owner-builder ($151,500 for 60 m² in our model) is the option that is clearly cheaper.
Prefab minus site-built, 60 m² standard flat
- New South Wales−$2,500
- Victoria−$1,500
- Queensland−$1,000
- Western Australia−$1,500
- South Australia$0
- Tasmania−$1,000
- Australian Capital Territory−$4,000
Prefab: construction at 92% of the site-built rate plus $12,000 transport and crane. Same site, connection and approval costs.
Size by size
| Floor area, Sydney | Site-built | Prefab | Difference |
|---|---|---|---|
| 30 m² | $141,000 | $144,500 | +$3,500 |
| 40 m² | $167,000 | $169,000 | +$2,000 |
| 50 m² | $190,000 | $190,000 | −$0 |
| 60 m² | $220,000 | $217,500 | −$2,500 |
| 70 m² | $250,000 | $245,000 | −$5,000 |
| 80 m² | $280,000 | $273,000 | −$7,000 |
The site, connections, design and approval cost the same whichever way the flat is built. Only the construction line and the transport line differ, and the transport line does not shrink with the flat. That is why small prefabs rarely save money.
Where prefab starts to pay
| State | Prefab cheaper from | Difference at 60 m² |
|---|---|---|
| New South Wales | 52 m² | −$2,500 |
| Victoria | 56 m² | −$1,500 |
| Queensland | 57 m² | −$1,000 |
| Western Australia | 56 m² | −$1,500 |
| South Australia | 61 m² | $0 |
| Tasmania | 59 m² | −$1,000 |
| Australian Capital Territory | 46 m² | −$4,000 |
Metro build rates, standard finish. Regional transport is often dearer than our $12,000 allowance, which pushes these sizes up.
Time on site
The real advantage of prefab is time and disruption, not price. Manufacturers quote 8–16 weeks of factory time, and the module arrives finished inside. On site, the slab or piers and connections take a few weeks; the set-down takes a day. A site-built flat has trades in your yard for three to four months. If you live in the main house, or the flat is for a parent who needs to move soon, that can be worth more than the price difference.
Access and cranes
A module up to about 3.4 m wide travels on a standard truck; wider ones need escorts and permits. Getting it into a backyard usually needs a crane, and the crane size depends on the lift distance over the house. Published crane hire prices run from about $500 for a short, easy lift to $7,000 for a long reach. If a mobile crane cannot reach your backyard, a prefab may not be possible at all, and panelised or kit systems carried in by hand are the alternative.
What to check in a prefab quote
- What "installed" means. Some prices stop at set-down on piers; connections, steps, decks and the slab or footings are extra.
- Certification. The module must meet the National Construction Code, including the energy rules in force when the flat is approved. Ask for the certification documents for your climate zone.
- Home warranty insurance. If the manufacturer is not the licensed builder on site, check who carries the statutory insurance.
- Transport and crane. Get both priced for your address, not "from" figures.
For the kit and owner-builder route, see kit granny flats.
This week's worked example
Week of 19 October 2026. A new example, rule and owner report every Monday.
A 80 m² two-bedroom flat with a standard finish on a flat block, services 30 m from the house, in Whittlesea: our model puts it at $259,500 turnkey ($228,500–$291,000), or $3,250 per m². At the area's median 2-bedroom bond rent of $480 a week it would return 7.4% net and pay back in 17.2 years after tax.
Rule check: Builder deposits are capped by law: 10% in NSW, 5% in Victoria and Queensland for contracts of $20,000 or more, and 6.5% in Western Australia.
From an owner (NSW, 2021): A 7 x 4 m cedar studio cost $30,000 delivered and assembled to lock-up; with the family's own fit-out and bathroom it came to about $45,000.
Questions people ask
Is a prefab granny flat cheaper?
Only slightly, and only for larger flats. In our Sydney model a 60 m² prefab saves about $2,500; a 40 m² prefab costs $2,000 more than building on site, once transport and crane are included.
How long does a prefab granny flat take?
Manufacturers quote 8–16 weeks for factory production, and the module is usually set down and connected in days. Approval, site work and connections take the same time as for a site-built flat.
Do prefab granny flats need council approval?
Yes, exactly like a site-built flat: the same size limits, setbacks and approval route apply. Many manufacturers handle the application as part of their price.