Cost-plus vs fixed-price construction contracts

The contractor agrees to deliver a defined scope for a defined price. If actual costs come in lower, the contractor keeps the difference as profit. If actual costs come in higher, the contractor absorbs the loss. The owner's exposure is capped at the contract price plus any approved change orders.

Pros: Owner has cost certainty. Contractor has incentive to manage costs tightly. Simpler accounting — the owner pays per the schedule of values.

Cons: Requires complete scope at contract signing — any ambiguity becomes a change order battle. Contractor builds contingency into the bid to cover risk, so the owner often pays a premium for cost certainty. Less collaborative — disputes over what's "in scope" can become adversarial.

Verification focus: Match every invoice line to the schedule of values. Check for cumulative overbilling. Catch missing change orders. Validate retainage. The contractor's actual costs are not the owner's concern — only what was contracted for.

The owner reimburses the contractor for actual costs (labor, materials, subs) plus a fee — either a percentage of cost (10–20% is common) or a flat management fee. The owner pays for what's actually spent, plus the contractor's compensation for managing it.

Cost-plus contracts often include a Guaranteed Maximum Price (GMP) — a ceiling above which the owner doesn't pay regardless of actual costs. With a GMP, cost-plus behaves more like fixed-price on the upside while preserving the cost-transparency benefits on the downside.

Pros: No premium for cost-certainty padding. Transparent — owner sees actual costs. More collaborative — fewer "in scope" disputes since changes are simply billed at actual cost. Better for projects where scope is evolving (high-end custom, design-build).

Cons: Owner carries cost risk. Requires significant document review on every invoice — actual costs need to be verified. Risk of mark-ups on subcontractor invoices being inflated. Less incentive for the contractor to control costs (mitigated by GMP).

Verification focus: Every actual cost needs documentation — sub invoice with markup, supplier receipts, time sheets, equipment rentals. Validate that markups are at the contracted rate. Check for personal expenses creeping in. Verify subs aren't being marked up on top of their own internal markup. Track running total against GMP.

Fixed-price failure modes: Scope ambiguity at signing leads to change order disputes mid-project. Contractor cuts corners to preserve margin when actual costs run high. Quality suffers. Allowance overruns get hidden in the fixed price.

Cost-plus failure modes: Markups on top of markups (sub marks up their materials, GC marks up the sub's invoice). Personal expenses billed to the project. Time sheets inflated. Equipment rentals continue past actual usage. GMP exceeded due to undisciplined change-order handling.

Both models require document verification — but the verification is different. Fixed-price verification is mostly scope-of-work matching. Cost-plus verification is cost-substantiation: every dollar billed has to tie back to a third-party document (sub invoice, supplier receipt, time card, rental agreement).

Ella reads the contract on day one and knows which model applies. On fixed-price contracts, she focuses verification on schedule-of-values matching, change order alignment, cumulative overbilling, and retainage. On cost-plus contracts, she additionally verifies every actual cost — sub invoice markups, supplier receipts, time sheet rationality, rental durations — and tracks running totals against GMP.

For GMP contracts approaching the ceiling, she alerts the team early so the conversation about the overrun happens before the work is done, not after.