Last reviewed: August 2026
The advertised price of a granny flat covers the building. It usually does not cover what your block does to that building. Published 2026 guides put the additional site, approval and connection costs at roughly $14,000 to $41,000 on top, and on a difficult block it goes higher.
None of these are hidden in the sense of being deliberately concealed. They are hidden in the sense that nobody mentions them until the contract is in front of you.
The costs that sit outside the build price
| Cost | What triggers it |
| Site excavation and levelling | Any slope at all |
| Rock removal | Discovered by the soil test, priced by volume |
| Retaining walls | Significant slope, or cut and fill |
| Engineered slab upgrade | Reactive clay soil classification |
| Sewer connection or diversion | Distance to the main, or a main under the build area |
| Stormwater and drainage | Distance, levels, and council requirements |
| Water and power connection | Authority fees, and any main upgrade |
| NBN connection | Separate from power and water |
| Tree removal and arborist reports | Any tree in or near the build zone |
| Driveway crossing | If council requires a new or widened crossing |
| Survey, soil test, engineering | Required for any approval |
| BASIX and energy assessment | Required in NSW |
| Approval fees | Certifier or council, plus any consultant reports |
| Long service levy | Payable on work above the statutory threshold |
| Section 7.11 or 7.12 contributions | Set by your council, varies a lot |
The four that cause the most damage
1. Rock
Rock is the most common budget blowout in Sydney, and it is almost impossible to price accurately before a soil test.
Large parts of Sydney sit on sandstone. If your excavation hits it, removal is charged by volume, and the machine and method both change. A block that looked flat and simple can add a five figure sum at this stage.
The protection is a soil test before the contract, not after. If a builder is willing to quote a fixed price without one, ask what happens if rock is found. The answer is usually that you pay for it.
2. Slope
A block that looks gently sloping to the eye can need substantial cut and fill.
Slope brings three costs at once: the excavation, the retaining walls to hold the cut, and often a stepped or dropped-edge slab. It also affects the height of the finished building, which can push you into larger required setbacks, which can force a redesign.
3. Sewer
Two separate problems live here.
The first is distance. The further your granny flat sits from the connection point, the more pipe, trenching and reinstatement you pay for.
The second is worse. If a sewer main runs under where you want to build, you need approval to build over it, or the main needs diverting. Both are expensive and both add time. A sewer diversion can reshape the whole project.
A sewer service diagram will tell you where the lines run. It is worth pulling before you fall in love with a position for the building.
4. Council contributions
Section 7.11 and 7.12 contributions are levied by councils to fund local infrastructure. They vary enormously between councils, and two neighbouring council areas can charge very different amounts for the same granny flat.
This one is entirely predictable, which is what makes it frustrating when it is left out. It is published in your council’s contributions plan.
How to get these priced before you commit
The whole problem here is sequencing. Most of these costs cannot be known until someone has inspected the block, done a soil test and pulled the service diagrams. Yet most people get a price before any of that happens.
The way through it:
- Get a site inspection before you get a price. A number produced without one is an estimate wearing a price tag.
- Get the soil test done early. It sets the slab design and reveals rock.
- Pull the sewer service diagram. Know where the lines are before you position the building.
- Ask which items are provisional sums. A provisional sum is an allowance, not a price. If the real cost is higher, you pay the difference.
- Ask for the exclusions in writing. Every builder has them. The question is whether they will tell you.
What a fixed price should actually mean
A fixed price should mean that after the site inspection, soil test and design are complete, the number does not move unless you change something or something genuinely unforeseeable turns up.
It should not mean a headline number with a page of provisional sums attached.
Ask the direct question: what specifically can change this price after I sign? A builder who can answer that clearly is one worth dealing with. BlueGum quotes fixed price after a site assessment, and manages approvals in-house so the approval costs sit inside the same scope.
Frequently asked questions
How much should I budget on top of the build price?
Published 2026 guides suggest $14,000 to $41,000 for approvals, excavation and connections on a typical block. The range is wide because the block is what decides it. On a flat, clear site close to services you will be at the bottom. Rock, slope or a sewer problem takes you past the top.
Can a builder tell me the site costs before inspecting?
Not accurately. They can give an allowance based on similar blocks, but rock, soil classification and service locations are specific to your land. Anyone quoting site costs without a soil test is estimating.
What happens if rock is found after I have signed?
It depends entirely on your contract. If site costs were a provisional sum, you pay the difference. If they were fixed after a soil test, they should not move. This is the single most important thing to clarify before signing.
Are council contributions always charged?
It depends on your council and on the scale of the work. Some councils levy contributions on secondary dwellings, others do not, and the amounts differ. Your council’s contributions plan is public, and a builder who works in your area will know.
Does a granny flat increase my council rates or land tax?
It can affect your rates, and it may have land tax implications depending on how the property is used and your overall holdings. That is a question for your accountant rather than your builder, and it is worth asking before you build rather than after.
Get the full number, not the headline number
The only way to know what your granny flat costs is to have the block looked at. Everything else is a range.
Book a site assessment, or call BlueGum on 1300 060 651.
BlueGum Granny Flats is a licensed NSW builder, licence number 247388C. Cost ranges in this article are drawn from published 2026 Sydney market guides and are indicative only. They are not a quote. Tax and rates questions should go to a qualified accountant.
