Lump Sum, GMP, or Cost-Plus: Choosing the Right Contract

7 min read

Every construction contract answers one question: who absorbs the difference between what we thought this would cost and what it actually costs. The rest is detail.

Choose the structure that matches how much you know at the time you sign, and how much visibility you want while you build.

Lump sum (stipulated sum)

The contractor commits to a fixed price for a defined scope.

Best when: drawings are complete, the scope is stable, and the market is competitive.
Owner carries: scope definition risk. Anything the drawings do not show becomes a change order.
Contractor carries: performance and pricing risk, and prices it into the number.
What you give up: visibility. You do not see buyout savings, and you cannot see how the number was built.

Guaranteed maximum price (GMP)

Cost of the work plus fee, with a cap. Savings below the cap are shared or returned per the contract.

Best when: you want an early price with incomplete documents, and you want transparency.
Owner carries: risk of an inflated contingency and of scope gaps inside the assumptions.
Contractor carries: overrun above the cap, subject to the qualifications in the GMP exhibit.
Watch: the qualifications and assumptions exhibit is the real contract. Read it more carefully than the price.

Cost-plus with no cap

Actual cost plus a fee, fully open book.

Best when: scope genuinely cannot be defined — complex renovation, disaster repair, fast-tracked work.
Owner carries: nearly all cost risk.
Requires: strong owner-side controls, defined reimbursable costs, and audit rights that are actually exercised.

Design-build and CM at risk

Both change who holds coordination risk rather than only how price is set. Design-build gives you a single point of responsibility for design and construction, which reduces gap disputes and usually speeds delivery, at the cost of some design control. CM at risk brings the builder in during design as an advisor, then converts to a GMP — useful when constructability and early pricing matter.

The clauses that decide outcomes

Regardless of type, these terms determine what actually happens when things go wrong:

Contingency: whose money is it, what can it be spent on, and who approves each draw.
Savings: shared, returned, or retained, and measured against what baseline.
Allowances: defined scope, reconciliation method, and what happens to overages.
Escalation: whether material escalation is the contractor risk, and any shared thresholds.
Change orders: pricing method, markup caps, notice period, and required documentation.
Schedule: substantial completion definition, excusable delay, liquidated damages.
Retainage: percentage, reduction milestones, and release conditions.
Payment: application timing, lien waiver requirements, and audit rights.

Matching structure to your situation

Complete documents plus competitive market: lump sum.
Need an early number and want open books: GMP with a strong qualifications exhibit.
Undefined scope and capable owner team: cost-plus with tight controls.
Speed and single-point responsibility: design-build.
Complex build with heavy preconstruction value: CM at risk converting to GMP.

The takeaway

There is no universally cheaper contract type. There is only the type whose risk allocation matches your document completeness, your market, and your ability to manage. Decide the structure deliberately, then spend your attention on contingency, changes, and escalation — the three clauses that write most of the checks.