GMP vs. Cost-Plus vs. Lump Sum: The Three Main Construction Contract Types

Construction contracts come in three basic pricing forms. Lump sum — the contractor commits to a fixed price for the defined scope. Cost-plus — the owner pays actual costs plus a negotiated fee or percentage. Guaranteed maximum price (GMP) — the contractor commits to a ceiling on costs while the actual price is based on actuals up to that ceiling. Every commercial construction contract in the US is a variant of one of these three structures, or a hybrid combining elements.

The contract pricing type matters more than almost any other contract feature because it determines who bears cost-overrun risk, who benefits from under-runs, what transparency the owner has into the contractor's operations, and how the finance and AP workflows operate on both sides. Understanding the tradeoffs is the foundation for negotiating the right structure for a given project.

Lump Sum (Fixed Price)

Lump sum contracts commit the contractor to build the defined scope for a single fixed price. If the contractor's actual costs come in lower than expected, they keep the savings. If actual costs exceed expectations, they absorb the loss. The owner's exposure is limited to the contract amount plus approved change orders.

Lump sum is used when the scope is well-defined upfront — complete drawings and specifications, clear site conditions, predictable durations. It works well for repetitive or standardized project types (warehouses, multi-family, standardized commercial) where experienced contractors can predict costs accurately. It works poorly for projects with design uncertainty, unusual site conditions, or fast-track schedules where scope evolves during construction.

Cost-Plus

Cost-plus contracts reimburse the contractor for actual costs incurred, plus a fee that compensates for overhead and profit. The fee can be structured in several ways — a percentage of costs (cost-plus-percentage), a fixed fee (cost-plus-fixed-fee), or a fee based on milestones or performance (cost-plus-incentive-fee). The contractor's 'price' is essentially open: the final cost is whatever the actual costs plus the fee turn out to be.

Cost-plus is used when scope is unclear at signing — fast-track projects where construction starts before design is complete, highly complex or first-of-kind projects, or situations where the owner and contractor have an established trust relationship. The owner trades price certainty for flexibility and speed; the contractor trades cost-overrun risk for a guaranteed fee.

Open-book requirements are central to cost-plus. Because the owner is paying actual costs, they have the right to see what those costs are — labor detail, material invoices, subcontractor agreements, equipment usage. The contractor's financial records become audit-relevant in a way they wouldn't be on a lump sum job. Billing on cost-plus is typically based on actuals accumulated through the billing period, with supporting invoices attached.

Guaranteed Maximum Price (GMP)

GMP contracts combine elements of lump sum and cost-plus. The contractor commits to a maximum price (the guaranteed maximum) that the owner will pay. Actual billings are based on cost-plus mechanics up to that ceiling. If actual costs come in below the GMP, a savings-sharing provision typically splits the savings between owner and contractor (e.g. 50/50, 70/30). If actual costs exceed the GMP, the contractor absorbs the overrun.

GMP is the contract type of choice for many commercial projects because it gives the owner a price ceiling (like lump sum) while preserving the open-book transparency of cost-plus. The contractor's upside is capped by the GMP but can be enhanced by the savings-sharing, which aligns incentives to manage costs well even though the structure is nominally cost-reimbursable.

The Risk Allocation Spectrum

The three contract types sit along a spectrum of risk allocation from owner to contractor:

The right contract type for a given project depends on scope definition quality, schedule pressure, owner sophistication, and market conditions. On a well-designed project with stable scope, lump sum maximizes owner value. On a fast-track project with evolving scope, cost-plus or GMP preserves flexibility. No structure is inherently better — the structure has to match the project's characteristics.

Operational Implications for Finance

The contract type dramatically affects how finance and AP workflows operate on both sides of the table.

Lump Sum Operations

On lump sum contracts, AP operations are relatively straightforward. The contractor's costs are an internal concern — the owner only sees the pay applications against the approved schedule of values. Revenue is recognized on the contractor side using percentage-of-completion based on cost-to-cost (actual costs vs. estimated total costs). Margin is fully captured by the contractor if costs come in below estimate.

Cost-Plus Operations

Cost-plus contracts dramatically expand AP's workload for the contractor. Every invoice, every timesheet, every piece of equipment cost needs to be documented with sufficient detail that the owner's representative can audit it. Monthly billings pull from the actual job cost detail and are supported by underlying invoices and records. The owner's AP team mirrors the contractor's — they review the detailed backup as part of approving payment, not just the summary.

GMP Operations

GMP projects have cost-plus operational characteristics below the GMP ceiling and lump-sum characteristics above it. Monthly billings require the same open-book detail as pure cost-plus. Savings-sharing reconciliation happens at project close — final actual cost is compared to the GMP, and any savings are split per the contract. Contractor finance teams need to track the GMP remaining throughout the project to avoid unauthorized overruns.

Hybrid and Specialized Structures

Real construction contracts often blend elements. A GMP contract might have lump-sum line items for specific well-defined scopes (standard cabinetry, commodity materials). A lump-sum contract might have time-and-materials allowances for specific uncertain scopes (hazmat abatement, foundation conditions). Unit-price contracts — where the contractor bids unit rates and the owner pays for actual quantities — are essentially a cost-plus variant with predefined rates.

The contract form (AIA, ConsensusDocs, DBIA, or custom) generally accommodates the pricing structure chosen. AIA A101 is the standard lump sum form. A102 is the standard cost-plus with GMP form. A103 is cost-plus without GMP. ConsensusDocs 200 series covers similar structures with somewhat different allocations. The choice of form matters less than the pricing structure it implements.

Change Orders Across Contract Types

Change orders work differently depending on contract type. On lump sum, change orders are essentially mini-contracts for the changed scope — priced separately, approved separately, and added to the base contract amount. On cost-plus, changes to scope are handled through scope amendments that update the open-book cost tracking, often without formal change orders. On GMP, scope additions typically require a GMP amendment that increases the ceiling.

Bottom Line

Lump sum, cost-plus, and GMP are the three pricing structures that organize essentially all commercial construction work. Each allocates cost-overrun risk differently, requires different levels of transparency from the contractor, and drives different operational patterns for finance and AP on both sides. Sophisticated owners and contractors select the structure that matches the project's characteristics rather than defaulting to a favorite. The contract pricing type is not an accounting detail — it's the architecture of the entire commercial relationship for the project.