Fixed Price vs Cost-Plus: Which Contract Is Right for Your Build?

The contract type you choose is one of the most important decisions you make on a job, and most builders just default to whichever one they have always used. Fixed price and cost-plus are not good or bad, they suit different jobs. Pick the wrong one for the situation and you either carry risk you should not, or you put a strain on the client relationship that never quite recovers. Here is how I think about it.

The two, plainly

Fixed price is a lump sum. You quote a number, the client signs it, and you deliver the build for that price. The client gets certainty. You carry the risk of having estimated it right. If you missed something, that is your problem, not theirs.

Cost-plus means the client pays the actual cost of the build plus an agreed margin or fee. Your risk is lower because you are not betting on a fixed number. But the client carries the uncertainty, so it only works when there is real trust and genuine transparency in how you track and present the costs.

Side by side

	Fixed price	Cost-plus
Who carries the cost risk	The builder	The client
What the client gets	Certainty on price	Certainty they pay true cost, not a padded one
What it demands of you	An accurate estimate and complete documentation	Transparent, open-book cost tracking and trust
Best when	Scope is well defined and plans are complete	Scope is still evolving or the build is complex
The failure mode	Variations and disputes when scope was incomplete	Client feels ripped off when tracking is not transparent

When fixed price is the right call

Fixed price suits a job with complete documentation, a defined scope, and a client who wants the comfort of a single number. If the plans are finished, the selections are locked, and you trust your estimate, fixed price rewards you for being good at pricing. The client relaxes because they know their number, and you get paid a premium for carrying the risk.

The trap is doing fixed price on an incomplete scope. If you quote a lump sum off half-finished plans, every unknown becomes a variation, and variations are where margin and goodwill both leak away.

When cost-plus is the right call

Cost-plus comes into its own when the scope is still moving. High-end and complex builds, jobs where the client will drive a lot of change, renovations where you genuinely cannot see inside the walls yet. In those situations a fixed price is really just a guess with a number on it, and someone loses.

But cost-plus lives or dies on transparency. If you cannot show the client, clearly and regularly, exactly what has been spent and what the margin is, they will feel like they are signing a blank cheque. This is exactly where running your costs live matters, and it is a big reason I keep going on about knowing your numbers as the job runs, not after.

My take

Match the contract to the job, not to habit. A well-documented, price-sensitive job wants fixed price. A complex, evolving, high-trust job wants cost-plus with open-book tracking. And whichever you choose, it only works if you know your true cost of building in the first place. That is the foundation under both, and I broke it down in how to actually price a job. Getting a sharp scope down before you commit helps just as much, which is where a good scope of work earns its keep.

Pick on purpose, and the contract works for you instead of against you.

Cheers Luke