The choice of GMP vs cost-plus is not a pricing preference. It is a decision about which party holds the cost overrun. A construction contract is a risk-allocation instrument before it is a payment schedule, and the form you sign decides who eats the difference between the budget and the bill. A guaranteed maximum price caps the owner's exposure and pushes overruns above the cap onto the contractor. A cost-plus contract with no ceiling reimburses actual cost and leaves the owner holding every dollar of escalation. The contract does not reduce the risk. It decides who carries it.
The thesis is simple. Owners who read the contract as a price sheet miss its real function. The signature line is where development risk gets assigned.
Key Takeaways
GMP vs cost-plus is a risk-allocation decision, not a pricing style. A guaranteed maximum price caps the owner's cost and shifts overruns above the cap to the contractor; a cost-plus contract with no cap leaves the overrun with the owner.
Under the AIA A102-2017, the standard cost-plus-with-GMP form, the contractor commits to a maximum price and cannot bill the owner above it unless a change order adds scope, per AIA Contract Documents.
Savings below the cap are a negotiated fill-in. Representative splits run from all savings to the owner to a 75/25 or 50/50 division between owner and contractor, set in the agreement, not assumed.
A contractor's contingency sits inside the GMP and only the contractor may draw it, a structure owners should scrutinize, per a Peckar and Abramson analysis published by ConsensusDocs.
A 12 percent hard cost overrun on a 30 million dollar budget costs the owner nothing above a signed GMP cap and 3.6 million dollars under an uncapped cost-plus contract.
What is the difference between GMP, cost-plus, and lump sum contracts?
The three contract types differ in one variable that matters most: who absorbs the gap between estimated and actual cost. A lump sum fixes the price and puts overrun risk on the contractor. A cost-plus contract reimburses actual cost plus a fee and puts the risk on the owner. A guaranteed maximum price is the hybrid, an open-book cost-plus with a hard ceiling.
A guaranteed maximum price contract, documented in standard form by AIA Contract Documents as the A102-2017 and by ConsensusDocs as the 230 owner-constructor agreement, works on the cost of the work plus a fee, capped at a negotiated maximum. The owner sees every invoice, which a closed lump sum hides, but is protected from cost above the cap, which a bare cost-plus does not provide. That combination is why GMP dominates on complex commercial development where the drawings are not final when construction has to start.
Dimension | Lump sum | Guaranteed maximum price | Cost-plus (no cap) |
|---|---|---|---|
Price certainty for owner | Fixed price | Capped at the GMP | Open, no ceiling |
Who bears the overrun | Contractor | Contractor above the cap, owner below | Owner |
Savings if under budget | Contractor keeps | Shared or to owner, per contract | Owner keeps |
Cost transparency | Low, closed price | High, open book to the cap | High, open book |
Owner's development risk | Low on cost, rigid on scope | Moderate, capped | Highest |
The table is the whole argument in one frame. Lump sum trades transparency for certainty. Cost-plus trades certainty for transparency. GMP is the attempt to hold both, and it does, up to the cap. Above the cap, the contractor is exposed. Below it, the owner captures savings. The negotiation is entirely about where that cap sits and what can move it.
Who bears the cost overrun under GMP vs cost-plus?
Under a guaranteed maximum price, the owner bears no overrun above the cap unless a change order raised the ceiling for added scope. Under cost-plus with no cap, the owner bears the full overrun because the contract reimburses whatever the work actually costs. Same escalation, same crews, same material invoices: the contract, not the market, decides the loser.
Work the numbers with stated inputs. A developer sets hard costs at 30 million dollars, and both contracts are priced off the same estimate. During construction, pricing and productivity push the actual cost of the work to 33.6 million dollars, a 12 percent overrun, with no owner-driven scope change to justify a change order.
Under a signed GMP: the contractor is bound to the 30 million dollar cap. The owner pays 30 million. The contractor absorbs the 3.6 million above the cap.
Under uncapped cost-plus: the owner reimburses the actual cost of the work plus the fee. The owner pays 33.6 million plus fee, absorbing the full 3.6 million.
The overrun is identical. The owner's exposure is 0 versus 3.6 million dollars, decided by which contract was signed. On a project underwritten to a thin development spread, that 3.6 million is the difference between a completed deal and a capital call. This is the same mechanism that makes construction cost overruns run past the contingency line: the overage lands on total project cost, and under cost-plus it lands entirely on the owner. The hard costs most exposed to escalation are exactly the line items the cap is meant to fence.
How do savings and contingency work under a GMP contract?
Below the cap, savings and contingency decide who benefits when the project comes in under budget. If the final cost of the work plus fee is less than the guaranteed maximum price, the difference is savings. Who keeps it is a negotiated term, not a default. The contractor's contingency, a reserve inside the GMP, is a separate pool the owner should watch closely.
AIA Contract Documents describes savings plainly: if a project priced at a 10 million dollar GMP is completed for 9 million, the 1 million can be split or passed entirely to the owner depending on the terms. The AIA A102-2017 makes the split a fill-in provision. Representative structures range from all savings to the owner to a 75/25 or 50/50 division, and the number is worth negotiating, because a generous contractor share turns a conservative estimate into contractor profit.
Contingency is where an owner's attention pays for itself. A contractor's contingency, typically set as a representative 2 to 5 percent of the cost of the work, sits inside the GMP and, when drafted properly, only the contractor may draw it. An owner's contingency, often in the range of 5 to 10 percent, sits above the contract for owner-directed changes. A Peckar and Abramson analysis published by ConsensusDocs warns that a loosely drafted contingency clause lets a contractor spend the reserve without the notice, backup, and approval controls that keep it honest, which quietly erodes the savings an owner expected to share. The quotable line for an owner: a GMP cap is only as strong as the contingency clause beneath it.
Which contract shifts the most development risk onto the owner?
Cost-plus with no cap shifts the most development risk onto the owner, because it converts every dollar of escalation, rework, and productivity loss into an owner reimbursement. GMP shifts the least among the open-book options, capping the owner's downside while preserving transparency. Lump sum caps cost too, but buys that certainty by hiding the contractor's numbers and pricing in a risk premium.
The ranking depends on how finished the design is when construction starts. On a fully drawn project, a lump sum is defensible because the contractor can price the risk it is taking. On a fast-track development where the GMP is set on incomplete documents, the owner is asking the contractor to cap a number neither party can see fully, and the contractor prices that uncertainty into the cap or into the contingency. This is why entitlement and design risk that lives before you break ground should be resolved before the GMP locks: the more unknowns remain, the higher the cap, or the weaker the cap's protection.
The lender reinforces the choice. A construction loan funds to the budget the owner signed, and a lender underwriting a capped GMP treats the owner's cost exposure differently from an open cost-plus. The contract you choose does not only allocate risk between owner and contractor. It shapes the terms the capital stack will offer against the project.
Frequently Asked Questions
Does a GMP contract fully protect the owner from cost overruns?
No. A guaranteed maximum price caps the owner's cost only for the defined scope. A change order for added or changed scope raises the cap, and owner-directed changes are billed on top. The cap protects against escalation and productivity overruns within the agreed scope, not against the owner expanding the project.
Who keeps the savings if a project comes in under the GMP?
It depends on the contract. The AIA A102-2017 makes the savings split a fill-in provision, so the parties negotiate it. Representative structures range from all savings to the owner to a 75/25 or 50/50 division between owner and contractor. Nothing is default; read the clause before assuming the owner keeps it.
Is cost-plus ever the right choice for an owner?
Yes, when speed and flexibility outweigh cost certainty, such as an urgent project or one with a scope that must evolve during construction. Cost-plus gives full transparency and avoids a risk premium baked into a cap, but the owner accepts the overrun risk. Many owners convert to a GMP once the design is defined enough to cap.
Conclusion
GMP vs cost-plus is the clearest example of a truth that development contracts obscure: the paper does not lower the risk, it assigns it. A guaranteed maximum price caps the owner's exposure and moves overruns above the cap onto the contractor, at the cost of a risk premium in the number and a contingency clause that must be drafted tightly. A bare cost-plus gives transparency and flexibility and leaves the owner holding every dollar the market adds. The right choice tracks design completeness: cap what you can see, and do not sign an open reimbursement for what you cannot. An owner who reads the construction contract as a risk instrument prices the deal correctly. An owner who reads it as a payment schedule finds out who held the overrun after the concrete is poured.