The Difference Between a Fixed Price Contract and a Cost Plus Contract

When you're planning a custom home or renovation in the Blue Mountains, one of the most important decisions you'll make before construction starts isn't about design or materials. It's about how your contract is structured.

Most clients don't fully understand the difference between a fixed price contract and a cost plus contract until they're already in one. By then, the implications are locked in. Here's what each model actually means, and how to think about which one is right for your project.

What a Fixed Price Contract Is

A fixed price contract sets the total cost of the build upfront. The builder agrees to complete the scope of work for a specified sum. If the builder's costs come in higher than anticipated, that's the builder's problem to manage. If they come in lower, the saving stays with the builder.

For the client, a fixed price contract provides certainty. You know what you're committing to before construction begins. Your finance is structured around a known number. Your budget planning is straightforward.

The catch is that a fixed price contract is only as reliable as the documentation behind it. A well-specified fixed price contract, with selections resolved, engineering complete, and provisional sums clearly flagged, gives you genuine certainty. A fixed price contract issued on incomplete documentation, with large provisional sums and unresolved scope, is a fixed price in name only. The variations that arrive during construction close that gap at your expense.

This is why the pre-construction process matters so much. A fixed price contract is only meaningful when the work done before contract is thorough.

What a Cost Plus Contract Is

A cost plus contract works differently. Instead of a fixed total, the client pays the actual cost of all labour and materials, plus a margin charged by the builder, either a fixed fee or a percentage of costs.

The theoretical appeal of cost plus is transparency. You see every invoice. You pay what things actually cost. There's no markup hidden inside a fixed price.

The practical reality is more complicated. Cost plus transfers all cost risk to the client. If materials prices rise, you pay more. If the project takes longer than expected, you pay more. If unforeseen conditions are encountered, you pay more. The builder's margin is protected regardless of what happens to the overall cost. Cost plus also makes budget planning difficult. The final cost of the project is genuinely unknown until the last invoice arrives.

When Cost Plus Makes Sense

Cost plus isn't inherently bad. There are situations where it's the more appropriate model.

For highly complex or experimental projects where the scope genuinely can't be fully defined upfront, cost plus allows the work to proceed without forcing a fixed price on something that's inherently variable.

For renovation work where hidden conditions are likely and the extent of works won't be clear until demolition is complete, cost plus can be a reasonable way to handle genuine unknowns.

The key distinction is whether cost plus is being used because it genuinely suits the project, or because the builder hasn't done the pre-construction work needed to issue a reliable fixed price. Those are very different situations with very different implications for the client.

Fixed Price Contracts and Value Management

One of the core reasons we work under a Design and Construct model is that it allows us to issue genuinely fixed price contracts. Because we're involved from the design stage, we can manage cost implications as the design develops, resolve selections before contract, and complete engineering and reports before we price the job.

By the time we issue a contract, the scope is fully defined. The selections are priced at actual cost. The provisional sums that remain are for genuinely uncertain items, clearly flagged and explained. The number we give the client is the number we're accountable to.

This is what Value Management actually means in practice. Not finding cheap substitutes for what the client wants. Managing the design and procurement process so that the contract price is honest, and staying accountable to it through construction.

Provisional Sums — The Grey Area in Fixed Price Contracts

Even in a well-run fixed price contract, some items can't be priced with certainty before construction begins. These are handled as provisional sums, budget figures included in the contract for work where the final cost can't yet be determined.

Excavation is the most common example in the Blue Mountains. Rocky ground, variable soil, and steep grades mean that what the earthworks contractor will actually encounter on a specific site can't always be fully known in advance. We set provisional sums for excavation based on the best available information and are upfront with clients about the realistic range of outcomes.

Ask your builder to walk you through every provisional sum in the contract before you sign. Where did each number come from? What's the realistic range? How is a variation triggered and processed if the actual cost differs? A builder who can answer clearly has done the work. One who can't is one whose contract deserves more scrutiny.

What to Look For Before You Sign

Whether you're being offered a fixed price or cost plus contract, the questions worth asking are similar.

For a fixed price contract: What's included and what isn't? What are the provisional sums and what's the basis for each figure? What triggers a variation and how are they priced? What documentation is in place before the contract is signed?

For a cost plus contract: What is the builder's margin and how is it calculated? Is it a fixed fee or a percentage of costs? What oversight do you have over costs being incurred? What happens if costs run significantly over expectations?

A builder who can answer these questions clearly and specifically has thought about them properly. One who deflects or gives vague answers is one whose contract deserves more scrutiny before you commit.

The Bottom Line

A fixed price contract with thorough pre-construction documentation is the strongest position a client can be in when they start a build. It gives you certainty, protects your budget, and keeps the builder accountable to the number they gave you.

A cost plus contract can be appropriate in specific circumstances, but it requires careful scrutiny of the margin structure, cost controls, and reporting obligations before you sign.

In either case, understanding what you're signing before you sign it is the most important due diligence you can do.

Get in touch with the team at Eberones if you'd like to understand how we structure our contracts and what's included in our fixed price.