Fixed price or cost-plus: a risk decision, not a preference

Cost-plus: transparency and its own risks

Cost-plus removes pricing risk and introduces different problems, most of which are about trust and administration rather than money.

The fee structure matters. A percentage fee — commonly 10% to 20% of cost — is simple but creates a structural conflict: your fee grows when costs grow. Sophisticated clients notice this immediately. A fixed fee, agreed in advance for a defined scope, removes the conflict and is the more defensible structure on larger projects.

A guaranteed maximum price caps the client's exposure at a stated number, converting cost-plus into something closer to fixed price above the cap. It is a common compromise, and it means you have accepted the overrun risk beyond the ceiling — so it should be priced accordingly, with contingency inside the maximum.

Administration is the hidden cost. Cost-plus requires receipts, allocated invoices, documented labor hours by job and clean monthly billing packages. Builders who take cost-plus work without the accounting discipline to support it end up in disputes over invoices they cannot substantiate.

Define "cost" explicitly. Is supervision a cost or covered by the fee? Small tools? Truck time? Rework caused by a subcontractor? The disputes in cost-plus are almost never about the big items. They are about what is included in the base.

In fixed price the argument is about scope. In cost-plus the argument is about the definition of cost. Both arguments are avoidable, and both are avoided in the contract rather than in the conversation.

Choosing correctly, project by project

Fixed price fits new construction from complete drawings; defined additions with known conditions; repetitive work you have priced many times; projects where the client's primary need is budget certainty; and any situation where you have strong historical cost data for the exact scope.

Cost-plus fits renovation of older buildings with unknown conditions; historic and structural work; projects starting before design is complete; owners who want to select finishes as they go; insurance restoration where the scope emerges during demolition; and any job where an honest fixed price would require so much contingency that the number becomes uncompetitive anyway.

The hybrid that often works best: fixed price for the defined portions and cost-plus for the genuinely unknown ones — demolition and structural repair at cost-plus, everything after the framing is exposed at fixed price. It gives the client most of the certainty they want without asking you to price what nobody can see.

The same project, both ways

A whole-house renovation of a 1930s home. Estimated cost $412,000. Real conditions unknown until demolition.

Fixed price at 20% margin: contract $515,000.

What was found: failing knob-and-tube wiring in two walls, rotted sill plate along one elevation, a chimney requiring rebuild, and plumbing that could not be reused. Additional cost: $68,000.

• Approved change orders for genuinely out-of-scope items: $41,000 (at 20% margin, $8,200 profit)
• Disputed items the owner argued should have been anticipated: $27,000, absorbed after negotiation
• Final profit: $103,000 + $8,200 − $27,000 = $84,200 (15.1%)
• Plus three weeks of delay while change orders were negotiated, and a relationship that ended coolly

Cost-plus with a 15% fee:

• Actual cost including all discoveries: $480,000
• Fee: $72,000
• Total to owner: $552,000
• No disputes — each discovery was documented, shown to the owner and priced as it appeared
• No delay from change order negotiation
• Profit: $72,000 (13% of contract)

The fixed price produced slightly more profit and a worse experience for everyone. Change one variable — make the discoveries $110,000 instead of $68,000, which is entirely plausible in a 1930s house — and the fixed price job produces a loss while the cost-plus job produces the same fee.

Contingency: the line most builders hide

Every honest fixed price on a renovation contains contingency. The only question is whether the owner is told about it, and the instinct to bury it inside the line items is usually a mistake.

Hidden contingency creates two problems. If it is not spent, you have quietly taken money for a risk that never occurred, which is defensible but invisible — and the owner who later learns the job came in easily feels overcharged. If it is spent, you cannot point to it, so the expenditure looks like an overrun rather than the plan working as designed.

A disclosed contingency does the opposite. Listed as its own line with a stated purpose — "unforeseen conditions in existing structure, 6% of construction cost" — it educates the owner before the work starts, makes the first discovery a normal event rather than a crisis, and gives you a defined pool to draw from with a defined approval process.

How to size it honestly: base it on your own history by project type rather than a habit. New construction from complete drawings might carry 2% to 4%; a straightforward addition 5% to 8%; a gut renovation of a pre-1950 house 10% to 15% or more. If your own records show that renovations of a certain vintage routinely run 12% over, that is the number, and pricing 5% is not competitiveness — it is a decision to lose money on a predictable schedule.

Two rules make disclosed contingency work in practice. State what happens to unused contingency — returned to the owner, retained, or split — before the contract is signed, because discovering that at the end is a bad conversation. And document each draw against it with photographs and a short written note, so that at completion there is a record of exactly what the money handled.

Explaining the choice to an owner

Most owners want fixed price because it feels safe, and many have been told cost-plus is a blank check. Both beliefs deserve an honest answer rather than agreement.

What works is naming the trade directly: "I can give you a fixed price on this. To do it responsibly on a house of this age I have to include contingency for what we cannot see, which means you will probably pay for risk that never materializes. Or we can do it at cost with a fixed fee, where you see every invoice and pay for what the house actually needs. Either is fine — but a fixed price without contingency is not a real price, it is a number that becomes a change order argument in week three."

Owners respond well to that framing because it is true, and because it explains the competitor's suspiciously low fixed bid without disparaging anyone.

For cost-plus, remove the blank-check fear concretely: a not-to-exceed or guaranteed maximum, a fixed fee rather than a percentage, monthly billing packages with receipts, and approval required above a stated threshold.

Five mistakes that cost the most

Taking fixed price on an undefined scope. The contingency required to do it honestly usually loses the bid anyway, so the job is won by underpricing risk.

Percentage fees on large cost-plus projects. The conflict is obvious to sophisticated clients and damages trust.

Not defining cost in a cost-plus contract. Supervision, tools, truck time and rework are where the disputes live.

Cost-plus without the accounting to support it. Invoices you cannot substantiate become invoices you do not collect.

Guaranteed maximum priced like ordinary cost-plus. The cap transfers overrun risk back to you and has to be paid for.

The five numbers to decide on

• Estimate accuracy by project type — actual cost against estimated, across completed jobs. This tells you where fixed price is safe.
• Change order value as a percentage of contract, which reveals how well scopes are being defined.
• Contingency used versus carried, so the allowance can be calibrated rather than guessed.
• Margin variance — the spread between estimated and realized margin, which is the real measure of risk exposure.
• Days lost to change order negotiation, a cost that never appears on any invoice.

Neither contract is better. The mistake is using one for everything, which in practice means using fixed price for projects that cannot be defined — and that is where most construction losses are born, long before anyone picks up a tool.

Frequently asked questions

When should a contractor use cost-plus instead of fixed price?

When the scope cannot be defined precisely before starting. Cost-plus fits renovation of older buildings with unknown conditions, historic and structural work, projects that begin before design is complete, owners who intend to select finishes as the job progresses, insurance restoration where scope emerges during demolition, and any job where an honest fixed price would need so much contingency that the number stops being competitive. Fixed price fits the opposite: new construction from complete drawings, defined additions with known conditions, repetitive work you have priced many times, and clients whose primary need is budget certainty. A hybrid often works best on renovations — cost-plus for demolition and structural repair, fixed price for everything after the framing is exposed — which gives the owner most of the certainty they want without asking the builder to price what nobody can see.

How much contingency should a construction contract carry?

It should come from your own history by project type rather than from habit. As general orientation, new construction from complete drawings might carry 2% to 4%, a straightforward addition 5% to 8%, and a gut renovation of a pre-1950 house 10% to 15% or more. The more important decision is whether the contingency is disclosed. Hidden contingency creates two problems: unspent, it becomes money taken for a risk that never occurred and the owner feels overcharged if the job goes easily; spent, it looks like an overrun because you cannot point to it. Disclosed as its own line with a stated purpose, it educates the owner before work begins, makes the first discovery a normal event instead of a crisis, and creates a defined pool with a defined approval process. State what happens to unused contingency before signing, and document each draw with photographs and a short note.

Is a percentage fee or a fixed fee better in cost-plus?

A fixed fee is more defensible on larger projects. A percentage fee, commonly 10% to 20% of cost, is simple to administer but creates a structural conflict: the builder's compensation grows as costs grow, and sophisticated clients notice immediately. A fixed fee agreed in advance for a defined scope removes that conflict entirely. A guaranteed maximum price is a common middle ground, capping the owner's exposure at a stated number, but it transfers overrun risk above the ceiling back to the builder and therefore needs to be priced with contingency inside the maximum rather than treated like ordinary cost-plus. Whichever structure is used, cost must be defined explicitly in the contract — whether supervision, small tools, truck time and rework caused by a subcontractor are billable costs or covered by the fee is where nearly all cost-plus disputes actually occur.