The phrase “fixed price contract” does a lot of heavy lifting in the new home building industry. Builders use it in advertising, sales consultants repeat it reassuringly during initial meetings, and prospective buyers hear it as a promise that the number on the contract is the number they will pay. That is not always how it works out, and the gap between what “fixed price” implies and what it actually delivers is worth understanding before you sign anything.
This is not an argument against fixed price contracts. They are the standard arrangement for most residential builds in Australia, and when they work as intended they do provide genuine cost certainty. But they come with conditions, exclusions and qualifications that are worth reading carefully, because a fixed price contract is only as fixed as what it includes.
What a fixed price contract actually is
A fixed price building contract is an agreement in which the builder commits to completing your home for a set price, regardless of changes in their labour or material costs during the build. If timber prices rise between signing and frame stage, that is the builder’s problem, not yours. If a subcontractor charges more than anticipated, the builder wears it. This is the core value proposition, and it is real.
The alternative is a cost-plus contract, where you pay the actual costs incurred plus a margin for the builder. Cost-plus arrangements are more common in custom and high-end builds, and while they offer transparency about what things actually cost, they transfer the financial risk of blowouts to you. Fixed price shifts that risk to the builder, which is why most volume builders and project home companies work this way.
What “fixed” does not mean
Here is where the nuance lives. A fixed price contract fixes the price for the scope of work described in the contract documents. Anything outside that scope is a variation, and variations are priced separately. Some of these are initiated by you. Others are not.
Provisional sums are a common source of confusion. A provisional sum is an estimated allowance for an item where the actual cost cannot be confirmed at the time of signing, typically things like site costs, retaining walls, or connections to services. The builder puts a figure in the contract, but that figure is an estimate. If the actual cost is higher, you pay the difference. If it is lower, you get a credit. Provisional sums are not fixed, and in some contracts they can be significant.
Prime cost items work similarly. These are allowances for selections you have not yet made, such as tiling, tapware or appliances. The contract nominates a dollar amount per item, and if your chosen product exceeds that allowance, the overage is added to your contract sum. In project home contracts the prime cost allowances are sometimes set at figures that make the base price look attractive but leave little room to choose anything you would actually want.
Site costs are the other reliable source of variation. Most builders offer a fixed price subject to site costs being confirmed after the soil test and survey. Until those results are in, the site cost component is often a provisional sum or an assumption that the site meets certain standard conditions. If your block has a significant slope, reactive soil, poor access, or any number of other complications, site costs can add tens of thousands of dollars to the contract price before construction has started.
The contract documents matter
A fixed price contract is only as good as what the contract documents describe. The drawings, specifications and inclusions schedule define the scope, and therefore define what is fixed. Vague specifications are an opportunity for disputes later. If your contract says “carpet to bedrooms” without specifying grade or brand, there is room for interpretation. If it says “reconstituted stone benchtops” without specifying thickness or brand, same problem.
Before signing, it is worth going through the inclusions schedule carefully and asking your builder to clarify anything that is ambiguous. This is not paranoia. It is basic due diligence, and most builders will accommodate reasonable questions from a serious buyer.
The drawings also matter. If you have agreed to a modification that is reflected in a hand-drawn markup or a verbal conversation but not in the formally stamped plans, it may not be covered by the fixed price. Get changes documented in writing, attached to the contract, and confirmed by your building consultant before you assume they are included.
Variations during construction
Even with the most comprehensive contract documents, variations happen during construction. Some are initiated by you because you change your mind about something. Others arise from site conditions that could not be fully anticipated. A few are the result of the builder discovering something during construction that requires a different approach than what was specified.
In a well-structured fixed price contract, variations require written approval from you before the additional work proceeds. You should receive a variation quote, sign it, and receive a copy. Be cautious about verbal agreements to proceed with variations without paperwork, as these can become contentious later when the final account is settled.
It is also worth knowing that builders in most Australian states have obligations under domestic building legislation about how and when variations can be charged. These differ by state, so if you are concerned about a specific variation or feel you are being charged for something that should have been included, your state’s domestic building authority is a good first port of call.
Reading the progress payment schedule
Fixed price contracts are typically structured around a progress payment schedule tied to construction milestones. Base, frame, lock-up, fixing and practical completion are the most common stages, though the exact terminology and split varies between builders and states. Each payment is a percentage of the contract sum, and payment becomes due when the relevant stage is reached.
The reason this matters in the context of fixed price is that variations and site cost adjustments are often settled at specific points in the payment schedule rather than as they arise. Understanding when additional costs will be charged, and having those amounts confirmed in writing before the relevant stage, avoids surprises on your progress claim invoice.
Cooling-off periods
Most Australian states provide a cooling-off period after you sign a domestic building contract, during which you can withdraw without forfeiting your full deposit. The length of the period and the conditions attached to it vary by state, so the specifics depend on where you are building.
In Victoria, for example, the Domestic Building Contracts Act provides a five-business-day cooling-off period for contracts valued above a certain threshold. During this period you can rescind the contract in writing, though the builder is entitled to retain a small portion of any deposit paid to cover their preliminary costs. Similar provisions exist in other states, though the cooling-off period in some jurisdictions only applies to certain contract types or above certain contract values.
The cooling-off period exists for good reason. Building contracts are long, detailed documents, and the sales process does not always allow adequate time to read them properly before signing. If you sign at a display centre on a Saturday afternoon with a promotion expiring at the end of the month, the cooling-off period is the mechanism that gives you time to actually read what you agreed to, have a solicitor or building consultant review the contract, and make a considered decision rather than one driven by sales pressure.
Using the cooling-off period is not a sign of bad faith. It is a legal right, and exercising it is sensible if you have any unresolved concerns about the contract terms, the inclusions, the site cost assumptions, or anything else that was glossed over during the sales process. A builder who reacts poorly to a buyer taking the cooling-off period seriously is also telling you something useful about how they handle the rest of the relationship.
What you should not do is sign a building contract assuming you will sort out the details later. The cooling-off period is a window, not a guarantee that everything you raise will be accommodated. If the builder is unwilling to make changes you consider important, your options are to proceed on their terms, negotiate within the window, or rescind and look elsewhere.
Statutory warranties
Regardless of what your building contract says, Australian domestic building legislation provides statutory warranties that apply to all residential building work. These are implied by law and cannot be contracted out of, meaning a builder cannot include a clause in the contract that removes them.
The warranties vary slightly by state but generally cover the same core obligations. The builder warrants that the work will be carried out in a proper and workmanlike manner, that materials used will be good and suitable for their purpose, and that the work will comply with all applicable laws and regulations. There is also typically a warranty that the home will be fit for occupation upon completion, and that the work will be completed within a reasonable time.
The more practically important warranties, from a homeowner’s perspective, are the defect warranties that apply after practical completion. Again, the specifics differ by state, but the general structure in most jurisdictions provides for a warranty period during which the builder is obliged to rectify defects. In Victoria, the builder is liable for structural defects for ten years after completion. For non-structural defects the period is shorter, typically two years, though this too varies.
Knowing your statutory warranty entitlements matters because builders do not always volunteer the information when defects arise. If you discover a problem after practical completion and the builder is unresponsive or disputes liability, understanding what the legislation actually requires of them is the starting point for any formal action. Your state’s domestic building authority can tell you the specific warranty periods and the process for making a claim.
One thing the statutory warranties do not do is replace the need for a thorough pre-handover inspection. Identifying defects before you take possession of the home is almost always easier than pursuing them after the fact. An independent building inspector at pre-handover is money well spent, and their report gives you a documented basis for requiring rectification before you make the final progress payment.
What to ask before you sign
There are a few things worth clarifying with any builder before committing to a fixed price contract. Ask them to identify every provisional sum and prime cost item in the contract and explain the basis for each estimate. Ask what the contract assumes about site conditions and what happens if the soil test or survey produces a different result. Ask how variations are handled and documented. Ask whether the inclusions schedule covers everything you discussed during the sales process.
None of these questions are unusual or unreasonable. A builder who is uncomfortable answering them is telling you something useful.
A fixed price contract is a sensible way to build a home. It gives you a framework for budgeting, allocates commercial risk appropriately, and provides a basis for holding your builder accountable. It just requires you to understand that “fixed” describes the pricing mechanism, not a guarantee that the final figure will match the number on the cover page. The more you understand about what is and is not inside that fixed price, the better equipped you are to manage the ones that are not.
