Will you get hit with extra costs after signing your build contract?
It's the number one question we get asked. And honestly, it's the right question to be asking.
You've found your block of land, chosen a builder, gone through the quoting process, and now you're staring at a build contract with a number on it. The question running through your head is: is this actually what I'm going to pay?
The short answer is, it depends. But more importantly, it depends on how well you understand what you're looking at right now, before you sign.
What "fixed price" actually means
Most volume builders in Australia will tell you their contract is a fixed price contract. And technically, they're right. The contract price is fixed based on the information available at the time of signing.
But here's what a lot of people miss. There's usually a section in the contract, sometimes buried on page 4 of 40, that says something along the lines of: "Warning: this price is not a fixed price contract" followed by a list of clauses that allow the price to change.
That's not the builder being dodgy. It's the reality of building a home. There are things that genuinely can't be locked in until certain information comes back, like soil test results, engineering reports, or developer requirements. The problem is, most people don't know that section exists, or if they do, they don't understand what it means for them.
Where the extra costs actually come from
In our experience working with hundreds of homebuyers across Australia, the most common areas where costs increase after contract signing are:
Site costs. This is the big one. Your builder will include an allowance for site works based on whatever information they have at the time. If the soil test or engineering comes back worse than expected, that allowance might not be enough. The difference comes out of your pocket.
Provisional sums. These are line items in your contract where the builder is essentially saying, "we think it's going to cost this much, but we're not sure yet." They're estimates, not fixed prices. Once the actual cost is known, you'll receive a variation for the difference, up or down.
Energy rating compliance. Your home needs to meet a certain energy rating to get a building permit. If the design doesn't hit that rating, upgrades are needed, things like better windows, additional insulation, or changes to the orientation. These are often not included in the original quote.
Developer requirements. If you're building in a new estate, the developer will have specific requirements around things like fencing, landscaping, facade materials, and driveway finishes. Not every builder includes these in the quote. Some do, some don't.
Plan changes and variations. Any change you make after signing, whether it's moving a window, adding a power point, or upgrading a benchtop, will come through as a variation with a cost attached.
The key question most people don't ask
The question isn't really "will the price change?" because in most cases, there's a chance it will. The real question is: what's the likelihood of each of these items affecting me, and by how much?
That's the conversation most people aren't having with their builder, and it's the conversation that matters most.
For example, if your builder has included a provisional sum for site works based on a flat block with normal soil conditions, but your block is on a slope or in an area known for rock, the likelihood of that cost increasing is high. You need to know that before you sign, not after.
On the other hand, if a clause says the builder can increase the price due to material shortages, that might be standard wording that's been in every contract since COVID. It might never apply to your build. But you need someone to tell you that, rather than reading it at 11pm and losing sleep over it.
What you can do about it
Before you sign your build contract, here are four things you should be doing:
Ask the builder to explain every provisional sum. What is it for, why can't it be fixed, and what's the realistic range? Don't accept "it's standard" as an answer.
Cross-check the inclusions against the developer's requirements. If the developer says you need a certain type of fencing or landscaping, make sure it's actually in the contract. If it's not, you're paying for it later.
Understand what information the builder is still waiting on. If they haven't done the soil test or ordered the engineering yet, there are costs that simply can't be fixed right now. Know what they are and what the potential impact is.
Ask the builder directly: based on what you know today, what's the likelihood of the price increasing, and by how much? A good builder will give you an honest answer. If they won't, that tells you something too.
The bottom line
A build contract is not a finish line. It's a starting point. There are more documents, more decisions, and more costs that come after signing. That's normal. What's not normal is being caught off guard by them.
The people who have the smoothest builds aren't the ones who got lucky with their builder. They're the ones who understood what they were signing, asked the right questions up front, and knew what to expect before it happened.