Cost-plus vs fixed price: which contract fits your build?

Short answer: A fixed-price contract locks in one number before you start. A cost-plus contract charges the actual cost of the build plus an agreed margin. Fixed price suits a repeat design you won't change. Cost-plus suits a custom home you want to shape as you go, because you can adjust without a variation invoice every time. In the Australian building industry, cost-plus margins are usually quoted between 10% and 25%, most often landing around 15% to 20%. We mostly build custom, one job at a time, so we mostly work cost-plus. We can do fixed price too, but only once there's a detailed enough quotation to price it properly. This is general information, not a quote. Your contract should always be set up around your specific build.

What each contract actually means

A fixed-price contract, sometimes called a lump-sum contract, gives you one agreed number for the whole job. The builder carries the risk of the build coming in over that number. To price it, they need the design and finishes locked down first.

A cost-plus contract works differently. You pay the actual cost of materials and labour, plus an agreed margin on top. The margin is set upfront and doesn't change. What changes is the build cost underneath it, because that follows what the job actually needs.

Neither one is a trick. They just suit different builds.

Where fixed price bites: variations

Most project builders and volume builders quote fixed prices. They can do that because they build the same design again and again. It works well right up until you want to change something.

That's when the variation invoices show up. Move a wall, upgrade a benchtop, change the tiles, and each change comes back as a separate charge on top of the fixed price. The headline number looked certain. The final number often isn't.

If your design is genuinely finished and you're not going to touch it, fixed price can work fine. If you're still making decisions, it can get expensive in a way you didn't plan for.

Why we mostly build cost-plus

We build custom homes, duplexes and co-living, one project at a time. No two are the same. So cost-plus fits how we actually work.

You pay the real cost of your build plus our agreed margin. Because the margin is fixed upfront, you can tailor the house as you go without a fresh markup every time you change your mind. Change a finish and you pay the real cost of that finish, not a penalty for deciding late.

We'll talk you through our margin figure on the feasibility call rather than publish it here, because it depends on the scope of your project. What we won't do is bury a change behind a variation charge you didn't see coming.

When fixed price makes sense

Cost-plus isn't always the answer. If you already have a detailed design and a full specification, and you know you won't change any of it, a fixed price gives you a locked number to plan around. Some clients want that certainty, and that's a fair call.

If you'd like fixed price, we need a detailed enough quotation to price it properly first. If you can't provide that detail, we charge for producing the detailed quotation. That fee is absorbed back into the project if you go ahead with us, so it isn't money lost.

Does the contract change your timeline?

No. A straightforward duplex, once everything's in place, typically takes 8 to 12 months from signing to handover. A basement, a more complex design, or bad weather can push that out a bit.

The timeline is the same whether you go cost-plus or fixed price. Cost-plus lowers your risk on changes. It doesn't slow the build down.

So which should you choose?

It comes down to one question: how settled is your design?

• Design fully locked, no changes coming: fixed price gives you a clean number to plan against.
• Still shaping the home, or building custom: cost-plus lets you make decisions as you go without paying a variation penalty each time.
• Not sure yet: that's what the feasibility call is for. We'll look at your plans, your finishes and your block, and tell you honestly which contract fits.

You don't need every decision made before you call. Whether you already own the block or you're still looking, the earlier we're involved, the better the outcome. If you want the cost side first, our duplex cost guide walks through what a build actually runs to in Western Sydney.

These figures are indicative 2026 market ranges for general planning purposes only, not a quote. Auzzy Projects is a builder — book a feasibility check to set up the right contract for your build.

Common questions

What is the difference between cost-plus and fixed-price building contracts?
Fixed price is one locked number agreed before the build, with variation charges if you change anything. Cost-plus covers the actual build cost plus an agreed margin, so you can adjust the project as you go without a fresh markup each time.

Is cost-plus more expensive than fixed price?
Not by design. With cost-plus you pay the real cost plus an agreed margin. With fixed price you pay a number that already has the builder's risk built into it, plus variation charges for any changes. Which works out cheaper depends on how much your design changes during the build.

What margin do builders charge on a cost-plus contract?
In the Australian building industry, cost-plus margins are usually quoted between 10% and 25%, most often around 15% to 20%. The exact figure depends on the scope of the project.

Can Auzzy do a fixed-price contract?
Yes. We need a detailed enough quotation to price it properly first. If you can't provide that detail, we charge to produce it, and that fee is absorbed back into the project if you build with us.