What our analysis shows
Our calculations, data as of 23 October 2026
- On a 60 m² Sydney flat, prefab came out −$2,400 against site-built in our model. We assume the factory builds 8% cheaper than a site crew and allow $12,000 for transport, crane and set-down.
- Below about 51 m² in Sydney, prefab costs more than building on site, because the transport cost stays the same however small the module. A 30 m² studio comes out +$3,900.
- Where build rates are lower, the module has to be larger before it saves money: 44 m² in the ACT, 59 m² in South Australia.
- The block matters far more than the method. A steep site adds about $33,000 in our model, many times the prefab saving, and it applies to both.
Smallest flat at which prefab saves money
- Australian Capital Territory44 m²
- New South Wales51 m²
- Victoria55 m²
- Western Australia55 m²
- Queensland56 m²
- Tasmania57 m²
- South Australia59 m²
Our model, standard finish, metro build rates. Prefab construction at 92% of the site-built rate plus $12,000 transport and crane; site, connections and approval the same for both.
Size by size
| Floor area, Sydney | Site-built | Prefab | Difference |
|---|---|---|---|
| 30 m² | $140,800 | $144,700 | +$3,900 |
| 40 m² | $167,200 | $169,000 | +$1,800 |
| 50 m² | $190,000 | $190,000 | $0 |
| 60 m² | $220,000 | $217,600 | −$2,400 |
NSW caps a granny flat at 60 m², so the table stops there. In Victoria, where 80 m² is now allowed in the planning rules, an 80 m² prefab comes out −$5,500 against building on site. The bigger the module, the more of the factory saving survives the trip.
Time on site
Prefab's real advantage is time and disruption. Manufacturers quote 8–16 weeks of factory time, and the module arrives with its kitchen and bathroom finished. On site, the footings and connections take a few weeks and the set-down a day. A site-built flat has trades in your yard for three to four months. If you live in the house, or a parent needs to move in soon, that can be worth more than the price difference. Approval takes the same time either way: a module needs the same planning route and building approval as any other flat.
Transport and cranes
Loads wider than 2.5 m are oversize on Australian roads and need permits, signs and set routes; wider still, and pilot vehicles are required. Most granny flat modules are 3–3.5 m wide, so transport is planned like any oversize load. Getting the module into the backyard usually needs a mobile crane, and the crane size depends on how far it must reach over the house. Expect a few thousand dollars for a standard lift and more for a long reach or a road closure. If no crane can reach the backyard, a module is not an option, and a panel or kit system carried in by hand is the alternative.
What the factory cannot fix
Slope, reactive clay, rock and a long run to the sewer cost the same whichever way the flat is built, and they can add $15,000–40,000 to a difficult site. A module's fixed window positions can also make the energy rating harder on a badly oriented block, which may mean better glazing or extra insulation to reach 7 stars. Price the site first; our site costs guide shows how far it moves the total.
Reading a prefab quote
Check what "installed" covers: some prices stop at set-down on piers, with connections, steps, decks and footings extra. Ask for the module's certification against the National Construction Code for your climate zone, and confirm who holds the home warranty insurance if the manufacturer is not the licensed builder on site. Get transport and crane priced for your address rather than as "from" figures. If you are borrowing, ask the lender early: some will not release funds for a module until it is fixed to its footings. For the cheapest route of all, see kit granny flats; a 60 m² kit finished by an owner-builder comes to about $151,600 in our model.
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 $259,700 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.