Construction Contract Types: Lump-Sum, GMP, and Cost-Plus Explained

The contract you sign with your general contractor determines how risk, cost, and incentives distribute across your project. Get it right and the project stays aligned with your underwriting. Get it wrong and you spend the next twelve months arguing about change orders.

Three contract types dominate commercial construction: lump-sum (also called stipulated-sum or fixed-price), guaranteed maximum price (GMP), and cost-plus with fee. Each one works well in specific conditions and poorly in others.

This post covers what each contract actually does, when each makes sense, what each optimizes for, and what to watch for before you sign. If you're a developer, asset manager, or owner's representative deciding between contract types on an upcoming project, this is the explainer we wish someone had given us when we were learning this the first time.

Lump-Sum Contracts (Fixed-Price)

What it is: You agree to a single total price for a defined scope of work. The contractor delivers that scope for that price. Any cost overruns are the contractor's problem. Any savings are the contractor's profit.

When it works best:

• The project scope is fully defined at contract signing
• Construction documents are complete, not conceptual
• The owner has little appetite for scope changes mid-project
• The owner wants maximum cost certainty

When it fails:

• Scope is still evolving at contract signing (contractor prices aggressive contingency, owner pays a premium for risk the contractor absorbs)
• Design documents have significant gaps (contractor either under-bids and files change orders, or over-bids and owner pays more than necessary)
• The project requires fast-track delivery with design-construction overlap

What to watch for:

• Hidden markups on allowances and unit pricing
• Aggressive contingencies hidden in line-item pricing
• Change order pricing that's never defined — the owner discovers real costs only when something changes
• Unclear scope inclusions

Lump-sum is the right contract when you know exactly what you want and you need the price locked in. It's the wrong contract when scope is fluid or documents aren't complete.

GMP Contracts (Guaranteed Maximum Price)

What it is: The contractor guarantees a maximum total price. Actual costs are tracked on an open-book basis — the owner sees every invoice, every labor hour, every material receipt. If actual costs come in under the GMP, savings flow according to a pre-agreed formula (often 50/50 owner/contractor, sometimes 100% to owner). If actual costs exceed the GMP, the contractor absorbs the overage.

When it works best:

• Scope is known but some details are still evolving
• The owner wants cost certainty with an incentive for the contractor to drive savings
• The project is complex enough that open-book transparency is worth the overhead
• The owner values collaborative delivery over adversarial contract dynamics

When it fails:

• Scope changes are so aggressive that GMP has to be renegotiated mid-project
• The owner doesn't have the bandwidth for open-book invoice review
• The contractor inflates the GMP aggressively to ensure they never go over

What to watch for:

• How the GMP is developed (conceptual, schematic, or construction documents — each produces different GMP quality)
• Contingencies in the GMP vs. owner's separate contingency
• Savings-share formula and what qualifies as savings
• Allowances for undefined scope items and how they trend in practice

GMP is the right contract when scope is mostly known and the owner wants the best of both worlds — price certainty with incentive alignment for cost discipline.

Cost-Plus Contracts

What it is: The owner pays the contractor's actual cost plus an agreed fee — either a percentage of cost or a fixed fee. The contractor has no incentive to drive costs down; the owner bears all cost risk.

When it works best:

• Scope is genuinely undefined at contract signing (emergency work, complex renovations with unknown conditions, fast-track projects)
• The owner wants maximum flexibility to change direction
• The owner has strong project management and isn't relying on the contractor for cost discipline
• Speed is more important than cost certainty

When it fails:

• The owner doesn't have the operational bandwidth to manage a cost-plus engagement
• The contractor's cost accounting isn't transparent or auditable
• Scope is more defined than the contract structure implies

What to watch for:

• Fee structure (percentage vs. fixed fee — each has different incentive implications)
• Cost audit rights and cost accounting transparency
• What counts as "cost" (labor burden, equipment rental, small tools, office overhead)
• Whether there's a not-to-exceed cap that converts the contract to an effective GMP

Cost-plus is the right contract when truly evolving scope and speed matter more than cost certainty. It's the wrong contract for most defined-scope commercial work.

How to Choose

For most commercial construction projects:

1. Defined scope, complete documents, no appetite for change → lump-sum
2. Mostly defined scope, some evolving details, want cost certainty with incentive alignment → GMP
3. Genuinely undefined scope, speed-critical, strong owner-side project management → cost-plus

The worst outcome is using the wrong contract type for your project reality. A lump-sum contract for a scope still being designed creates adversarial change-order fights. A cost-plus contract for fully defined scope overpays for flexibility you don't need.

What We Do at Pillars of Seven

We deliver under all three contract types depending on what makes sense for the project: lump-sum for ground-up and renovation work where scope is fully defined; GMP for complex projects where owners want open-book accounting with cost incentive alignment; cost-plus for emergency work, fast-track projects, and scope that's genuinely still evolving.

Our preconstruction services develop the budget and schedule discipline that makes any contract type work. Our construction management practice handles open-book GMP accounting and change order management with the transparency the contract requires.

If you're structuring a commercial project and thinking through contract type, let's talk. We'll walk through the trade-offs specific to your scope, timeline, and risk tolerance.