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

The structure of a construction contract determines how project risk is allocated between owner and contractor, how costs are tracked, and under what conditions scope changes trigger financial adjustments. Four contract types dominate US commercial and public construction: lump sum (fixed price), guaranteed maximum price (GMP), cost-plus, and unit price. Each carries distinct mechanisms for payment, cost control, and dispute exposure that shape project outcomes from preconstruction through final completion.

Definition and scope

Construction contracts establish the legal and financial framework within which a project is delivered. The American Institute of Architects (AIA) and the Engineers Joint Contract Documents Committee (EJCDC) publish standard contract forms used across the industry, and the Federal Acquisition Regulation (FAR), codified at 48 CFR Part 16, governs contract type selection on federal construction projects. State procurement codes impose parallel requirements on publicly funded work.

The four primary contract types divide along two axes: whether the total price is fixed at contract execution, and whether the contractor or owner bears the risk of cost overruns.

1. Lump Sum (Fixed Price): A single agreed price covers all work within a defined scope. The contractor absorbs cost overruns; savings become contractor profit.
2. Guaranteed Maximum Price (GMP): The owner pays actual costs plus a fee, capped at a ceiling price. Costs above the GMP fall to the contractor unless a change order is issued.
3. Cost-Plus (Cost-Reimbursable): The owner reimburses all allowable project costs plus a fee, either fixed or percentage-based. No ceiling price exists unless the parties negotiate one.
4. Unit Price: Payment is calculated by multiplying a fixed per-unit rate against quantities of work actually installed or completed. Total contract value fluctuates with final measured quantities.

How it works

Lump sum contracts require a fully developed set of construction documents before bid. Bidders price all labor, materials, equipment, subcontracts, overhead, and contingency into a single number. The International Building Code (IBC), published by the International Code Council (ICC), defines the technical scope of work that must be covered; permit-driven inspections confirm compliance independent of contract type. Because the scope is fixed, owners gain budget certainty, but any scope change requires a formal change order that reopens price negotiation.

GMP contracts are common in design-build and construction management at-risk (CMAR) delivery. The contractor establishes a cost ceiling during design development, often when documents are 60–90% complete. The AIA A133 and A134 contract forms are the standard instruments for CMAR-GMP arrangements (AIA Contract Documents). Savings below the GMP are typically shared between owner and contractor under a split defined in the contract—commonly 50/50 or 75/25 in favor of the owner.

Cost-plus contracts shift nearly all financial risk to the owner. Allowable costs are defined contractually and typically follow categories established under FAR Part 31 for federal work, which distinguishes direct costs, indirect costs, and unallowable costs. Fee structures are either a fixed dollar amount (cost-plus fixed fee) or a percentage of costs (cost-plus percentage fee); federal procurement discourages percentage-fee arrangements because they create incentives to inflate costs.

Unit price contracts are structured around a schedule of quantities. The engineer or owner estimates quantities; the contractor bids per-unit rates. Final payment reflects actual field-measured quantities, which may deviate significantly from estimates. EJCDC C-700 and AIA A101 both accommodate unit price schedules. Projects with building listings that involve earthwork, paving, or utility installation routinely use unit price structures because subsurface conditions make precise quantity prediction unreliable.

Common scenarios

Lump sum is appropriate for projects with complete, well-defined construction documents, stable scope, and low design uncertainty. Tenant improvement buildouts, pre-engineered metal buildings, and repetitive residential construction are typical applications. The contractor's contingency is priced into the bid and is not transparent to the owner.

GMP is the dominant structure for large commercial projects delivered under CMAR. Hospital construction, institutional facilities, and complex mixed-use developments frequently use GMP because it allows construction to begin before design is complete while providing an owner cost ceiling. The CMAR contractor's involvement during preconstruction — estimating, constructability review, and subcontractor procurement — is addressed under the building directory purpose and scope of service classifications for construction management firms.

Cost-plus appears most often in emergency response work, renovation projects with unknown existing conditions, or highly specialized construction where competitive bidding is impractical. Federal disaster recovery contracts and confidential government facility projects are recurring cost-plus environments.

Unit price dominates civil and infrastructure work: highway construction, sewer and water main installation, site grading, and similar projects where quantities cannot be fixed in advance. The Federal Highway Administration (FHWA) and state departments of transportation use unit price schedules as the standard bid format for road and bridge projects.

Decision boundaries

Selecting a contract type is a risk allocation decision, not simply an administrative choice. The following boundaries apply across project types:

1. Design completeness: Lump sum requires ≥95% complete documents. GMP can proceed at 60–70% design completion. Cost-plus operates with minimal design definition.
2. Owner cost certainty: Lump sum provides maximum owner certainty. GMP provides ceiling certainty with savings-sharing upside. Cost-plus provides no ceiling unless negotiated.
3. Schedule urgency: Cost-plus and GMP allow construction to begin during design. Lump sum requires design completion before bid, adding 6–12 weeks to preconstruction on complex projects.
4. Scope stability: Frequent owner-directed scope changes favor cost-plus or GMP; each change order under a lump sum contract is a discrete renegotiation with dispute potential.
5. Quantity uncertainty: Projects with variable subsurface or field conditions use unit price to avoid loading contingency into lump sum bids.
6. Public procurement law: Federal and state agencies are often statutorily required to use competitive sealed bidding (lump sum) unless they document justification for alternate contract types under applicable procurement codes.

The how to use this building resource reference explains how contract type classification intersects with contractor licensing tiers and project delivery method documentation requirements tracked across the construction sector.

References

• Federal Acquisition Regulation (FAR) Part 16 — Contract Types, 48 CFR Part 16
• FAR Part 31 — Contract Cost Principles and Procedures
• AIA Contract Documents — A133, A134, A101 Standard Forms
• Engineers Joint Contract Documents Committee (EJCDC) — C-700 Standard General Conditions
• International Code Council (ICC) — International Building Code
• Federal Highway Administration (FHWA) — Construction Contract Administration