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Fixed Price vs Cost Plus: Which Contract Should a Builder Use?
It All Comes Down to One Question: Who Carries the Risk?
Every builder eventually gets asked it across the kitchen table. "So, is this fixed price, or are we doing cost plus?" How you answer shapes your profit, your risk, and whether the job ends with a handshake or a headache.
The two contract types sound like a technicality, but they are not. They are two completely different ways of sharing the risk on a job, and picking the wrong one for the wrong project is how builders lose a job's worth of margin. The whole thing comes down to a single question: if the job costs more than expected, who pays for it?
Under a fixed price contract, you do. Under a cost plus contract, the client does. Everything else about these two contracts flows from that one difference. Understand it properly and you will know exactly which one to reach for on any given job.
TL;DR
Fixed price sets one total price upfront. If costs blow out, the builder absorbs it
Cost plus bills the actual costs plus an agreed fee or percentage. The client carries the overrun risk
Fixed price suits clearly defined jobs with a locked scope. Cost plus suits uncertain or evolving ones
Fixed price rewards you for efficiency. Cost plus protects you when the scope is unknown
The right choice depends on how well the scope is defined before you start
Why This Is About Risk, Not Just Price
It helps to see these two contracts for what they really are: two ways of deciding who takes the gamble on a job costing more than planned. A fixed price contract is you promising a number and betting you can deliver within it. A cost plus contract is the client agreeing to cover whatever it actually costs, plus your fee, so the gamble is theirs. Neither is better in the abstract. The right one depends entirely on how much is unknown when you sign, which is why a clear scope changes everything.
Licensed Builder and Renovator
I quoted an old cottage renovation fixed price to win the job, when half of it was hidden behind hundred-year-old walls. The surprises ate my entire margin and then some. That exact job should have been cost plus. Now I match the contract to how much I can actually see, not to what feels easier to sell.
What a Fixed Price Contract Is
A fixed price contract, sometimes called a lump sum contract, sets one agreed total for the whole job before work starts. The client knows exactly what they will pay, and you commit to delivering the scope for that number.
The trade-off is that you carry the risk. If the job runs over, because materials rose, because you hit problems, or because you underestimated, that overrun comes out of your margin, not the client's pocket. But it works the other way too: if you deliver efficiently and come in under your estimate, you keep the difference. Fixed price rewards a builder who knows their costs and runs a tight job.
Best for:
Jobs where the scope is clear and fully defined before you start
Clients who want certainty and a single number they can budget to
Straightforward work where you can price the risk accurately
Most standard residential jobs where you can see what you are dealing with
What a Cost Plus Contract Is
A cost plus contract works completely differently. Instead of one fixed number, the client agrees to pay the actual costs of the job, materials, labour, subcontractors, plus an agreed fee on top for your profit and overhead. That fee is usually either a fixed amount or a percentage of the costs.
Here the client carries the risk. If the job costs more than expected, they pay the extra, and you are still covered because your fee sits on top of whatever the real cost turns out to be. This protects you on jobs where you genuinely cannot predict the final cost. The trade-off is that the client does not know the final price upfront, which requires trust, and it requires you to track and document every cost transparently.
Best for:
Jobs where the scope is unclear or likely to change
Renovations and old buildings full of unknowns behind the walls
Projects where the client wants to make decisions and changes as they go
Work where pricing a fixed number would mean padding it heavily to cover risk
The Honest Comparison
Fixed price | Cost plus | |
|---|---|---|
The price | One set total, agreed upfront | Actual costs plus an agreed fee |
Who carries overrun risk | The builder | The client |
Client knows final cost upfront | Yes | No |
Best when scope is | Clear and defined | Uncertain or changing |
Your upside | You keep savings if efficient | Fee is protected whatever it costs |
Main demand on you | Accurate pricing upfront | Transparent cost tracking throughout |
Requires | A tight scope | Client trust and good records |
As a neutral guide, business and accounting sources describe the same split: smaller, straightforward projects with a defined scope suit a fixed price contract, while larger, longer projects where the scope is hard to determine upfront suit cost plus. NetSuite
How to Choose the Right One
The deciding factor is almost always the same: how well can you define the scope before you start?
Choose fixed price when you can see the whole job. If the scope is clear, the drawings are done, the selections are made, and you can price the work accurately, fixed price is usually the right call. The client gets certainty, and you get rewarded for running an efficient job. This is the bread and butter of most residential work.
Choose cost plus when the job is full of unknowns. If you are renovating an old building, the scope is likely to change, or the client wants to make decisions as they go, cost plus protects you from carrying risk you cannot price. Rather than padding a fixed number heavily to cover the unknowns, you bill what it actually costs and keep your fee either way.
The mistake to avoid is forcing a fixed price onto a job full of unknowns just because it is easier to sell. That is exactly how builders end up funding other people's surprises. If you genuinely cannot see what you are dealing with, either price the risk properly with a real contingency and clear exclusions, or use cost plus.
A Note on Trust and Records
Whichever you use, the paperwork underneath matters. A fixed price contract is only safe if the scope of work is tight and the exclusions are clear, because that scope is what defines exactly what your fixed number covers and what becomes a variation. A cost plus contract lives or dies on transparent records, because the client is trusting you to bill honestly, so every cost needs to be documented and easy to show. Get the documents right and either contract works. Get them wrong and either can turn into a dispute.
The Bottom Line
Fixed price and cost plus are two ways of sharing risk, not just two ways of writing a number. Fixed price puts the risk on you and rewards efficiency, and suits jobs you can see and price accurately. Cost plus puts the risk on the client and protects you on jobs full of unknowns, but demands trust and transparent records. The right choice comes down to how well you can define the scope before you start. See the whole job clearly, go fixed price. Cannot see what is behind the walls, seriously consider cost plus. Match the contract to the risk, and you protect your margin either way.
This is general information, not legal advice, and contract law varies by region, so get proper advice before settling on a contract for a specific job.
Where Jobdocs Fits
Whichever contract you use, it is only as strong as the documents behind it. Jobdocs helps you build clear quotes, scopes and contracts with your inclusions, exclusions and terms laid out properly, so a fixed price job has the tight scope it needs and a cost plus job has the clear paper trail it depends on. Get the documents right, and both contract types work in your favour.
Try it free for 14 days at jobdocs.ai
