What our analysis shows
Analysis by Granny Flat Numbers, data as of 23 October 2026
- On an easy Sydney block, a builder price that covers construction and the CDC ($184,500 for our 60 m² reference flat) becomes $220,000 once site work, connections, design and the council contribution are added: 19% more.
- On a steep block with the sewer 40 m away the same "from" price becomes $263,000, 43% more. The building price tells you least about the cost on difficult sites.
- Across the states the uplift on an easy block runs from 14% to 19%. NSW is at the top because of the council contribution; the ACT is lowest because its high build rates make the fixed site and connection costs a smaller share.
- Owners report the gap at contract stage, not at advertisement stage: a $9,000 variation after approval for slab, edge beams and piers is the typical example. Fixed site costs after a soil test are the single best protection.
What to add to a builder's 'from' price, easy block
- New South Wales+19%
- South Australia+19%
- Tasmania+18%
- Western Australia+18%
- Victoria+18%
- Queensland+18%
- Australian Capital Territory+14%
Our model: 60 m² standard flat, flat block, services within 12 m. 'From' price = construction plus fast-track approval.
From a "from" price to the real one
| State | Building + approval | Turnkey, easy block | Add |
|---|---|---|---|
| New South Wales | $184,500 | $220,000 | +19% |
| South Australia | $157,000 | $187,000 | +19% |
| Tasmania | $162,900 | $193,000 | +18% |
| Western Australia | $168,300 | $198,500 | +18% |
| Victoria | $168,800 | $199,000 | +18% |
| Queensland | $168,500 | $198,500 | +18% |
| Australian Capital Territory | $207,600 | $237,500 | +14% |
Use the percentage as a first check on an advertised price, then replace it with real figures for your block. The cost calculator shows each line.
The lines every quote should show
Ask each builder to price these separately, or to state in writing that a line is excluded:
| Line | Our Sydney figure | Often missing because |
|---|---|---|
| Construction | $180,000 | Check what finishes and appliances are included |
| Site preparation (flat, easy access) | $12,000 | Priced as an allowance until the soil test |
| Sewer, water, power and stormwater connections | $10,000 | Depends on distance and depth; quoted as a provisional sum |
| Design, engineering and BASIX/energy report | $8,000 | Sometimes in the build price, sometimes charged before the contract |
| Approval: Complying development certificate (CDC) | $4,500 | Certifier fees may be separate |
| Council s7.11 contribution (typical) | $5,500 | Paid to council, so builders leave it out |
| Driveway, paths, fencing, landscaping | $10,000 | Almost always excluded |
| Separate electricity meter | $3,000 | Needed only if the tenant pays power separately |
Provisional sums and variations
A provisional sum is the builder's estimate for work that cannot be priced yet. A prime cost item is an allowance for something you choose later, such as tiles or appliances. Both can rise, and the rise is yours to pay. A quote with large provisional sums for excavation, rock, sewer or retaining walls is not comparable with one where they are fixed.
Before signing, get a soil test and contour survey (together a few thousand dollars) and the sewer service diagram, and ask builders to convert provisional sums into fixed prices. If a builder will not fix site costs after seeing those reports, treat the quote as a starting point.
Deposits and insurance
| State | Maximum deposit |
|---|---|
| New South Wales | 10% of the contract price |
| Victoria | 5% if the contract is $20,000 or more (10% below) |
| Queensland | 5% if the contract is $20,000 or more (10% below) |
| Western Australia | 6.5% of the contract price |
Statutory home warranty insurance must be in place before the builder takes a deposit: in NSW for work over $20,000, in Victoria over $16,000, in Queensland over $3,300 and in Western Australia over $20,000. Ask for the certificate, not just a statement that it is included.
Questions to ask
- Is the price fixed, and which items are provisional sums?
- Does it include the certifier, BASIX or energy report, engineering and soil test?
- Does it include sewer, water, power and stormwater connections to the street or main house, and over what distance?
- Who pays the council contribution or infrastructure charge?
- What is the build time from slab to handover, and what happens if it runs over?
- Which appliances, floor coverings and window coverings are included?
Our site costs guide explains the lines that move most, and what owners report collects the variations people have been hit with.
This week's worked example
Week of 19 October 2026. A new example, rule and owner report every Monday.
A 55 m² two-bedroom flat with a standard finish on a steep block, services 8 m from the house, in Ballarat: our model puts it at $191,000 turnkey ($168,000–$214,000), or $3,470 per m². At the area's median 2-bedroom bond rent of $370 a week it would return 7.6% net and pay back in 16.7 years after tax.
Rule check: Water authorities control building within a sewer main's zone of influence; building directly over a main can mean concrete encasement at roughly $1,000 a metre.
From an owner (QLD, 2025): A family near Caboolture was quoted $300,000 for a small two-bedroom brick granny flat, which they called the price of an entry-level house.
Questions people ask
Why are granny flat quotes so different?
Usually because they include different things. One builder's price may include site works, connections and the certifier; another's may list them as provisional sums or leave them out. Compare quotes only after putting them on the same list of lines.
What is a provisional sum in a granny flat quote?
An allowance for work the builder cannot price yet, such as excavation or sewer connection. If the work costs more, you pay the difference. Ask for site costs to be fixed once the soil test and survey are done.
How much deposit can a granny flat builder ask for?
It is capped by state law: 10% in NSW, 5% in Victoria and Queensland for contracts of $20,000 or more, and 6.5% in Western Australia.