Most homeowners think getting their own plans drawn up first gives them more control. More control over the design, more control over who builds it, more control over the price.
However, the opposite is actually more accurate.
Once the plans exist, the price is basically decided, and there isn't much left to control.
We get a version of the same phone call often enough that it's stopped being surprising. Someone's found a draftsperson or an architect, spent months on the design, and paid for a full set of plans and engineering, often $40,000 to $50,000 all up. They've got a number in their head, say $400,000, because that's roughly what everyone kept telling them it should cost. They ring us ready to build. We look at the plans, run the numbers, and the figure that comes back is closer to $800,000.
Some don't believe us. Others go quiet, because that's not the kind of gap you talk your way around. Either way, it happens more than you'd think, and it happens almost every time someone's gone through the whole design process without a builder anywhere near the table.
Why doesn't having your own plans give you more control?
Surely more information means more leverage, more room to shop around and negotiate. That's the assumption. It doesn't hold up.
A finished design shows exactly how big the slab is, how big the roof is, how much timber goes into the frame, how much tile goes on the floor, where every cabinet sits. That's the material list, more or less, and the labour needed to install it. Every competent builder is buying roughly the same materials and paying roughly the same trades to do roughly the same work, because the drawing tells them what to build, not how cheaply they feel like building it. Margins between established builders tend to sit in a similar band too. The plan fixes the cost long before anyone puts a price on it, it just doesn't feel that way at the time.
Underneath this is usually an assumption that builders are creaming a huge margin, so there must be tens of thousands of dollars sitting around to negotiate away. That's rarely where the money is. A building company carries admin staff, marketing, insurance, a fleet of vehicles and tools, and the risk that comes with running large projects. When something on site goes wrong, and eventually something does, it can cost tens of thousands of dollars that comes straight out of the same margin everyone assumes is padded. Shopping fifteen quotes and picking the cheapest one doesn't uncover hidden fat. It usually just finds whoever priced it wrong, or is planning to make it up later in variations.
It's tempting to listen to whoever says it shouldn't cost that much: a friend, a handy brother-in-law, a mate who does frames on the side. None of them are the ones signing a contract that legally commits them to building your home to Australian Standards for that price. If they're that confident it can be done for less, that's a business they're welcome to start.
So what actually gives you control over the price?
Choosing who you design with, before there's anything on paper yet. A builder who's involved from the start can tell you early whether $400,000 buys 200m² or 120m², rather than letting you fall in love with a design that was never going to land near your number. The budget and the design get worked out together, instead of the design getting finished first and the budget finding out about it later.
According to the 2026 State of Residential Construction Industry report, Australian builders lead every other country surveyed in fixed-price contract adoption, and 71.4 per cent of them skip cost-escalation clauses altogether, the fine print that lets a "fixed" price move once you're locked in. When the budget and the design get worked out together from the start, a fixed price actually means something.
None of this means you're stuck working only with a builder who does everything in-house. If you've already found a designer or architect you want to use, a builder can still come in early and work alongside them, sitting down together on the buildable details rather than approving the drawings over one phone call. What that team looks like depends on the project. A $3 million architectural build and a $350,000 internal renovation don't need the same level of documentation, but both need to be clear enough that trades can actually work from them without guessing.
How do you know if a set of plans is actually any good?
Two different things get judged under that one label, and they get confused constantly. One is whether the documentation is clear enough to build from: whether a builder and their trades can read it and follow it on site without stopping to guess. The other is whether the design itself works: whether the kitchen's in a sensible spot, the bedrooms are a usable size, the roofline makes sense.
A plan can be beautifully drawn and still put the laundry somewhere odd. A plan can nail the layout and still be nearly impossible to build from because half the details are missing. The second problem is worse. Messy documentation can be redrawn. A layout that doesn't work usually means starting again.
Why trust matters more than the contract type
None of this works without trust, and trust gets built the ordinary way: meeting the builder, talking to people they've built for before, asking how they handle it when something doesn't go to plan, because something usually shifts a little.
One thing worth asking for specifically is a look at the real numbers, the bill of quantities, what they expect to spend and what they expect to make. A builder willing to show you that isn't hiding anything, and it stops mattering much whether you end up signing a fixed price or a cost-plus contract. You've already seen there's nothing behind the curtain.
Why the number in your head is probably a few years out of date
It's also worth knowing why that $400,000-versus-$800,000 gap keeps turning up in the first place. The Housing Industry Association's own economic data, cited in its submission to South Australia's Building and Construction Industry Review, has construction costs in the state up 54.4 per cent since June 2020. Our own experience points to something closer to a doubling: a single storey extension or renovation that cost $250,000 to $300,000 in 2019 or 2020 is more often $500,000 or more now. The number a lot of people are carrying in their head is a few years out of date before they've even picked up the phone.
None of this is really an argument against getting plans done properly. It's an argument for the order you get them in. The building contract was never the moment the cost got decided, that happens earlier, in the drawing. Which means the real point of leverage isn't shopping your finished plans around, it's choosing who you draw them with in the first place, and doing that before a single line's on paper.
What this looks like at ZJ Building
At ZJ Building, this earlier stage is what our Preliminary Building Agreement is built around. It means getting a builder involved before the plans are finished, rather than waiting until they’re done.
If you’re not familiar with how our Preliminary Building Agreement works, we’ve explained the process in more detail here: [What Is a Preliminary Building Agreement?]
If you already have a designer or architect you’d like to work with, that’s fine too. We’re happy to work alongside them and help make sure the design, budget and build are considered together from the start.
If you’re thinking about an extension or renovation and want a clearer idea of what your budget needs to be, book a discovery call with our team. It’s worth having that conversation before you spend money on detailed plans, rather than finding out later that the numbers don’t work.
