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Lump sum, cost-plus and design-build are not three competing versions of the same thing. Lump sum and cost-plus describe how the contractor is paid; design-build describes how design and construction are contracted. A design-build project can use either a lump-sum price or a cost-plus arrangement, including a negotiated guaranteed maximum price (GMP).
The right choice depends on how complete the scope is, who can manage uncertainty, how much price transparency the owner needs, how much design control is important, and which procurement rules apply. The contract language—not the label alone—determines what happens when quantities change, work is added, costs rise or the schedule moves.
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At a glance
| Name | Best for | Key spec | Checked |
|---|---|---|---|
| Lump sum (stipulated sum) | Defined scope, drawings, specifications and quantities that can be priced competitively | One agreed amount for the defined work; changes, allowances and exclusions can alter the amount | November 2026 |
| Cost-plus without GMP | Projects that must start while design or scope is still developing | Defined allowable costs plus a fee, with no stated maximum in the payment form | November 2026 |
| Cost-plus with GMP | Early contractor involvement with a negotiated ceiling for covered work | Cost of work plus fee up to a GMP, subject to inclusions, exclusions, allowances and change terms | November 2026 |
| Design-build | Owners seeking one entity responsible for coordinating design and construction | Delivery method, not a price type; may use lump sum, cost-plus or another agreed basis | November 2026 |
What do these contract terms actually describe?
Lump sum and cost-plus are payment bases. They answer: “How is the contractor compensated, and who carries the risk that the work costs more or less than expected?” Design-build is a delivery method. It answers: “How are design and construction services procured and placed under contract?”
In traditional design-bid-build, the owner commonly has one contract with the architect and another with the contractor. In design-build, design and construction are combined under one owner contract. The payment basis remains a separate decision. The AIA/AGC primer on project delivery terms and AIA’s contract relationship diagrams illustrate why delivery and pricing should be selected and drafted together, but not confused.
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That distinction prevents a common mistake: assuming that design-build automatically means a fixed price, or that cost-plus automatically means an uncapped budget. Either assumption can be wrong. The agreement, incorporated conditions, governing law and project facts control.
What is a lump-sum construction contract?
A lump-sum, or stipulated-sum, contract sets one agreed price for a defined scope of work. The contractor generally takes more estimating and delivery-cost risk within that scope, while the owner gains a known contract sum at award.
Price certainty is conditional, not absolute. The contract must say exactly what the sum includes and excludes, how allowances are reconciled, how changes are valued, and when the contractor receives more time or money. A fixed number does not erase differing site conditions, owner-directed changes, design revisions or work that was never included.
When lump sum works well
- Drawings and specifications are sufficiently complete to define the work.
- Quantities and site information are reliable enough for bidders to price with comparable assumptions.
- The owner wants a clear baseline for budgeting and financing.
- Competitive bids can be evaluated against the same scope and instructions.
Where lump sum becomes risky
If quantities are uncertain or likely to change substantially, bidders may add a contingency premium, make different assumptions or exclude work. That can produce an apparently firm price but extensive change-order negotiations later. For U.S. federal construction, the Federal Acquisition Regulation (FAR) Part 36 says, “Generally, firm-fixed-price contracts shall be used to acquire construction,” while also allowing lump-sum, unit-price or combined pricing. FAR guidance prefers lump sum to unit prices except in circumstances such as uncertain or significantly changing quantities. That federal rule applies to federal acquisitions, not automatically to private or non-U.S. projects.
Clauses that make a lump sum workable
- Scope hierarchy: identify which document controls if drawings, specifications and schedules conflict.
- Exclusions and assumptions: list work, taxes, permits, utilities, temporary facilities and owner-furnished items that are outside the sum.
- Allowances: state the assumed amount, markup treatment and reconciliation method when actual selections differ.
- Schedule of values and milestones: connect progress payments to verifiable work rather than front-loaded billing.
- Change procedure: require written authorization, define pricing methods and state how overhead, profit and time extensions are calculated.
- Different site conditions and price escalation: explain notice deadlines and whether specified risks are shared, capped or excluded.
What does cost-plus mean?
A cost-plus contract reimburses defined project costs and pays the contractor a fee. The owner carries more exposure to actual allowable costs, so the contract’s cost definitions, records and approval controls are central.
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“Cost” should not mean every amount the contractor chooses to spend. The agreement should identify reimbursable labor, materials, equipment, subcontractors, insurance, supervision, home-office expenses and other items, along with excluded costs and required documentation. It should also state whether the fee is a fixed amount, a percentage of cost, or another formula.
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Without a GMP, the payment form has no stated maximum for the covered work. This can be useful when the owner needs construction to proceed while design, investigations or procurement are still developing. It also leaves the owner with the greatest exposure if costs increase.
Controls normally include open-book accounting, periodic cost reports, audit rights, approval thresholds, competitive-procurement requirements, staffing rules, a contingency budget and a process for authorizing scope changes. A fee percentage can create unintended incentives if the contract does not address self-performed work, subcontractor markups and savings.
Cost-plus with a GMP
A GMP is a negotiated ceiling for the costs covered by the agreement; it is not automatically part of every cost-plus contract. The cap only works as written. The parties must define whether it includes contingency, allowances, design completion, escalation, taxes, insurance, owner changes and other categories.
Important questions include who controls the contingency, whether buyout savings reduce the GMP, whether savings are shared, how unused allowances are credited and what happens if the ceiling is exceeded. Some GMPs exclude owner-directed changes or identified unforeseen conditions, so a “maximum” may not be a maximum for the entire project.
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AIA’s contract-document families provide examples: A101 is an owner-contractor stipulated-sum agreement; A102 is cost of the work plus a fee with a GMP; and A103 is cost of the work plus a fee without a GMP. These are examples of form structures, not substitutes for project-specific legal review or confirmation of the current edition. The AIA/AGC delivery primer and AIA design-build team checklist also emphasize defining compensation, incentives, savings and risk allocation.
What is a design-build contract?
Design-build places design and construction under one contract with the owner. The design-builder may be a single firm or a team that combines a contractor and design professional. This can create one primary point of responsibility for coordination, but it does not guarantee a fixed final cost, a particular schedule or protection from poorly defined requirements.
The owner must state performance requirements, functional criteria, quality expectations, review and approval rights, design standards and commissioning obligations. The contract should identify who is responsible for design decisions, delegated design, permitting, errors and omissions, professional liability, intellectual property and record documents.
Design-build can be priced as lump sum, cost-plus without a GMP, cost-plus with a GMP or another negotiated arrangement. Therefore, “design-build versus lump sum” is not an apples-to-apples comparison: one term describes delivery and the other payment.
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Potential advantages and trade-offs
- Integrated coordination: design and construction participants work under one contractual structure, which can reduce interface disputes when responsibilities are clearly drafted.
- Early planning: the team may advise on constructability, procurement and sequencing while design develops.
- Owner control: the owner gives up some direct contracting separation and must establish meaningful design-review gates and performance criteria.
- Risk clarity: a single contract does not remove risk; it reallocates responsibility, and ambiguous requirements can still generate claims.
Do not promise that design-build is always faster or cheaper. The official guidance reviewed for this comparison does not establish a universal percentage advantage. Schedule overlap, early procurement and coordination benefits depend on the project and the quality of the contract administration.
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What is the difference between lump sum and cost-plus?
Lump sum gives the owner an agreed amount for defined work, with the contractor generally carrying more cost risk inside that scope. Cost-plus exposes the owner to actual allowable costs plus a fee, unless a GMP or other cap limits the covered exposure.
| Issue | Lump sum | Cost-plus |
|---|---|---|
| Budget visibility at award | High for the defined scope | Lower without a cap; a GMP improves visibility only within its stated boundaries |
| Owner’s cost exposure | Changes, exclusions, allowances and specified adjustments remain exposed | Actual allowable costs remain exposed, subject to any GMP and exclusions |
| Information needed before pricing | Detailed scope, drawings, specifications and quantities | Can start with less-complete design, but requires stronger cost controls |
| Administration emphasis | Scope interpretation, progress valuation and change orders | Cost accounting, audits, approvals, reports and fee calculations |
Can a design-build project be lump sum or cost-plus?
Yes. Design-build is compatible with either payment basis. An owner might use design-build with a lump sum after performance requirements and design development are sufficiently mature, or use cost-plus with a GMP to involve the builder before the scope is fully resolved.
The contract should coordinate the two dimensions: identify the design deliverables and review gates, then state how design changes affect the price, fee, GMP, contingency and schedule. It should also explain whether the design-builder may propose alternatives, how owner approvals are documented and who bears the consequences of late decisions.
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Use the following questions before selecting a form or requesting bids.
- How complete is the scope? If drawings, quantities or site data are uncertain, a fixed price may include a large risk premium or invite disputes over what was included.
- Who can manage the uncertainty? Assign each risk to the party best able to estimate, control, insure or mitigate it. Moving risk on paper does not make the underlying uncertainty disappear.
- How much cost transparency is required? Cost-plus requires definitions, records, audit rights and approval controls. A GMP adds a ceiling only to the categories and circumstances stated.
- How much design control does the owner need? Separate design-bid-build contracts preserve a direct architect relationship. Design-build integrates the team but requires precise owner performance requirements and review rights.
- Is schedule overlap valuable? Early contractor involvement can support constructability reviews, long-lead procurement and sequencing, but schedule results are project-dependent rather than guaranteed.
- How will changes and savings be treated? Decide valuation methods, approval authority, contingency control, shared savings, fee adjustments and time extensions before work begins.
- What selection method is permitted? Low-bid, best-value and qualifications-based selection have different implications, and public procurement rules may restrict which method is available.
- Which local rules and funding conditions apply? Federal acquisitions follow FAR requirements; Scottish public projects use guidance such as the Scottish Government contract-selection chapter. Private and other international projects require their own legal and procurement review.
What should be compared before signing?
Compare the actual contract package, not just the heading on the first page. At minimum, review:
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- Owner requirements, drawings, specifications, geotechnical information and surveys.
- Definitions of contract sum, cost of work, fee, GMP, contingency and allowance.
- Document precedence and responsibility for design completion or coordination.
- Payment applications, retainage, stored materials and schedule-of-values rules.
- Change-order notice, pricing, authorization and emergency-work procedures.
- Different-site-condition, escalation, force-majeure and delay provisions.
- Insurance, bonds, warranties, indemnities and professional-liability coverage.
- Inspection, testing, substantial completion, commissioning and closeout requirements.
- Audit rights, record retention, subcontractor competition and conflict-of-interest controls for cost-plus work.
- Termination, suspension, dispute escalation, mediation, arbitration or litigation provisions.
For a design-build team, also compare qualifications, prior experience with the project type, design-responsibility matrix, owner review periods, intellectual-property rights and the process for resolving design comments. For a GMP, compare the cap’s inclusions, exclusions, allowance reconciliation, savings treatment and overrun responsibility line by line.
What is the practical conclusion?
Choose lump sum when the work is defined well enough to price fairly and the parties can administer changes clearly. Choose cost-plus when uncertainty makes a reliable fixed price impractical, adding a GMP only when its ceiling, exclusions and controls are genuinely understood. Choose design-build when integrated design-and-construction responsibility fits the owner’s needs—but select its payment basis separately.
There is no universally cheapest, fastest or safest structure. A sound decision matches scope maturity, risk capacity, transparency, design control, schedule needs, qualifications and applicable procurement rules, then records those decisions in detailed contract language.
For complex projects, have construction counsel and the design and cost professionals review the complete agreement, incorporated conditions and scope documents before execution. A familiar form name can provide a framework, but it cannot resolve an unclear scope or an unallocated risk by itself.
Frequently Asked Questions
Is a GMP the same as a fixed-price contract?
No. A GMP is a ceiling for the cost categories covered by the agreement, while a fixed-price contract sets a stipulated amount for defined work. Owner changes, allowances, exclusions, escalation and unforeseen conditions may sit outside the GMP unless the contract says otherwise.
Can the parties change from cost-plus to lump sum during construction?
They can agree to amend the contract, but the amendment should establish a new scope baseline, pricing assumptions, treatment of prior costs, open changes, contingencies, schedule effects and responsibility for unresolved design information. The change should be signed before the new pricing basis governs future work.
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Who is responsible for design errors in design-build?
Responsibility depends on the design-responsibility matrix, professional standard of care, owner criteria and governing law. A design-builder may be responsible for design it controls, while the owner can retain responsibility for defective performance requirements or owner-directed changes; the contract should state these boundaries expressly.
Are unit-price contracts an alternative to lump sum and cost-plus?
Yes. Unit pricing pays an agreed rate for measured quantities and is useful when quantities are uncertain or expected to change. It can also be combined with lump-sum items, but the measurement rules, quantity verification and adjustment thresholds must be written into the contract.
