A guaranteed maximum price (GMP) is one of the most commonly used and most commonly misunderstood structures in commercial and multifamily construction. Owners often treat the GMP number as a fixed price, when it’s actually a ceiling built on top of a cost-plus foundation, with contingency, shared savings, and exclusions that determine how much protection that ceiling actually provides.
This guide breaks down what a project GMP is, how a guaranteed maximum price proposal gets built, what it typically includes and excludes, and what an owner should evaluate before signing one.
What GMP Means in a Construction Contract
A guaranteed maximum price contract is a cost-plus agreement with a cap. The contractor is reimbursed for the actual cost of the work (labor, materials, subcontractors, and general conditions) plus a fee, up to an agreed-upon maximum. If costs run over that ceiling, the contractor absorbs the difference. If costs come in under it, any savings are typically shared between owner and contractor according to a formula negotiated in the contract.
That structure is what distinguishes a GMP from the two contract types it sits between:
| Structure | Who absorbs cost overruns | Cost transparency |
|---|---|---|
| Lump sum / fixed price | Contractor, for the entire contract value | Limited; owner sees the price, not the cost breakdown |
| GMP | Contractor, above the ceiling only | Open-book, with a negotiated cap |
| Cost-plus (uncapped) | Owner | Fully open-book |
The GMP’s defining feature, the ceiling, is also its most negotiated element. A GMP is only as protective as the definitions underneath it: what counts against the cap, what contingency exists to absorb normal cost growth, and how savings are shared if the project comes in under budget.
How a Guaranteed Maximum Price Proposal Is Built
A GMP proposal isn’t a single number pulled from a rough estimate. It’s assembled from several distinct components, typically once design has reached a defined level of completeness:
- Direct construction costs: subcontractor bids for each trade scope, priced against a substantially complete set of construction documents.
- Self-performed work costs: any scope the general contractor performs directly rather than subcontracting.
- General conditions: the contractor’s on-site staffing, temporary facilities, equipment, and project management costs for the duration of construction.
- Contingency: a reserve built into the GMP to cover unforeseen conditions and normal cost growth within the defined scope — this is addressed in more detail below.
- Fee: the contractor’s overhead and profit, structured as either a fixed amount or a percentage of cost.
The proposal is typically prepared once design documents reach roughly 80–90% completion. A GMP set against an earlier design stage carries more risk for both parties. The contractor prices wider contingency to cover the uncertainty, and the owner pays for that uncertainty whether or not it materializes.
Ironside prepares GMP proposals from real subcontractor buyout, not a factored estimate, which is why our preconstruction process pushes for design completeness before a guaranteed number goes on paper.
What’s Typically Included and Excluded
A GMP’s ceiling only covers what’s explicitly defined in the contract. Owners evaluating a proposal should confirm what’s included:
- Generally included: hard construction costs, subcontractor work, general conditions, the contractor’s fee, and construction contingency for the defined scope.
And what’s commonly excluded, unless the contract states otherwise:
- Owner-furnished items and equipment not part of the base building scope.
- Design fees and other soft costs handled outside the construction contract.
- Permit fees, unless specifically included in the GMP.
- Owner’s contingency, held separately for scope changes and design decisions the owner controls.
- Costs from owner-directed changes, which typically trigger a formal GMP amendment rather than being absorbed within the existing ceiling.
A GMP proposal that doesn’t spell these exclusions out explicitly is the most common source of disagreement mid-project, not because either party acted in bad faith, but because “the GMP covers it” and “the GMP doesn’t cover it” were never clearly defined for the same list of items.
Contingency and Shared Savings Inside a GMP
Contingency is the mechanism that makes a guaranteed maximum price actually guaranteed, and it typically exists in two layers:
- Contractor’s (or GC) contingency, generally 2–5% of construction cost, held by the contractor to absorb bid errors, productivity shortfalls, and trade-level issues within the defined scope. This contingency is fully at risk to the contractor. Unused amounts are not typically returned to the owner.
- Owner’s contingency, generally 5–10% of construction cost, held separately to fund scope additions, design changes, and conditions that fall outside the contractor’s defined risk. This should be explicitly owner-controlled, released through a formal change order process.
Most GMP contracts also include a shared savings provision: if actual costs come in under the guaranteed maximum, the difference is split between owner and contractor according to a negotiated formula, rather than returned entirely to the owner. This detail matters more than it first appears because the contingency is priced into the GMP whether or not it’s used, an owner who assumes 100% of any unused contingency returns to them may be underestimating the effective cost of the ceiling they negotiated.
When a GMP Structure Makes Sense
A GMP tends to fit best when:
- Design is substantially complete before construction needs to start, giving the contractor a reliable basis to price the ceiling.
- The owner or a capital partner requires a firm cost cap as a condition of financing or investment underwriting.
- The project is large or complex enough that open-book cost visibility matters, but an uncapped cost-plus structure carries too much owner-side risk.
- The project has enough schedule flexibility to allow a genuine preconstruction and buyout process before the number is finalized. A GMP rushed against incomplete documents undermines the reliability of the guarantee itself.
Fast-track projects that need to start site work before design and permitting fully resolve are often better served by an early cost-plus phase, converting to a GMP once design reaches the completeness the structure depends on.
What to Evaluate Before Signing a GMP Proposal
Before accepting a guaranteed maximum price proposal, an owner should confirm:
- What design completeness the GMP was priced against, and whether that basis is documented in the contract, not just assumed.
- How contingency is structured and controlled. Contractor’s contingency versus owner’s contingency, and who releases each.
- What happens to unused contingency at project close-out, and how shared savings are calculated.
- What’s explicitly excluded from the GMP, cross-checked against the owner’s own project budget to confirm nothing falls into a gap between the two.
- What triggers a GMP amendment, and what documentation is required to support one, this is where most GMP disputes originate.
A GMP proposal that answers all five of these clearly is a meaningfully different risk than one that states only the final number.
The Risk of Treating a GMP as a Fixed Price
The most common mistake owners make with a guaranteed maximum price is treating it as equivalent to a lump sum contract. It isn’t. A GMP protects against cost growth within its defined scope and contingency. It does not protect against scope the owner adds later, design decisions made after the GMP is set, or conditions explicitly excluded from the contract. Proactive scope definition before the GMP is finalized, and disciplined change management after it, are what determine whether the guarantee holds in practice or gets eroded by amendment after amendment.
Build Your Vision with Ironside
Ironside Building is a full-service Southern California general contractor based in Irvine, providing clarity, craftsmanship, and accountable construction management on commercial and multifamily projects. Our preconstruction team builds GMP proposals from real subcontractor pricing and a clearly defined scope, and we walk owners through exactly what a proposed guaranteed maximum price does and doesn’t cover before it’s signed.
If you’re evaluating a GMP proposal or preparing to start preconstruction on a project, schedule a consultation with our team and we’ll review the structure with you line by line.