The GMP Trap: What a Guaranteed Maximum Price Contract Doesn't Guarantee
The word "guaranteed" is doing a lot of work in "guaranteed maximum price."
It sounds like protection. It sounds like a ceiling that keeps you safe. And for a lot of commercial GCs — especially ones moving into GMP work from a lump-sum background — that word creates a dangerous assumption: that the GMP is the owner's problem, a cap that protects them, and that everything underneath it is comfortably within reach.
That assumption costs firms real money. Because a guaranteed maximum price guarantees exactly one thing — a maximum the owner won't pay above. It guarantees nothing about whether you make money getting there. The GMP is a ceiling on the owner's exposure. It is not a floor under your margin.
Here's what the "guarantee" actually covers, what it doesn't, and where GCs lose money inside a contract they thought was protecting them.
What a GMP Actually Is
A guaranteed maximum price contract sets a not-to-exceed number for the project. The owner agrees to reimburse the contractor for the actual cost of the work plus a fee, up to the GMP. If the project comes in under the GMP, the savings are typically shared or returned to the owner depending on the contract. If it comes in over — that's the part people miss.
If the project exceeds the GMP, the contractor absorbs the overage. Every dollar above the guaranteed maximum comes out of the contractor's pocket. That's what the "guarantee" means: you've guaranteed the owner they won't pay more than the cap, and you've taken personal responsibility for anything beyond it.
So the GMP isn't a number that protects you. It's a number you're promising not to exceed — on a project where costs are uncertain, design may be incomplete, and the market won't sit still. The protection runs toward the owner. The risk runs toward you.
Where the Margin Actually Lives: The Contingency
In a GMP contract, the buffer between a profitable project and an overage is the contingency — the amount built into the GMP to cover the unknowns. And how that contingency is defined, controlled, and accessed is where GMP projects are won or lost.
The questions that determine whether your contingency protects you:
Whose contingency is it? There's often a distinction between the contractor's contingency and the owner's contingency. Knowing which covers what — and who has to approve a draw against each — matters enormously when costs start moving.
What can it be used for? A contingency that can only be tapped for narrowly defined conditions isn't much of a buffer. One that covers a realistic range of unknowns is.
How big is it, really? A contingency sized for a fully designed project is very different from one sized for a project that's still developing. If the design isn't complete when the GMP is set, the contingency has to carry that uncertainty — and it's often too thin to do it.
A GMP with a poorly structured contingency is a fixed-price contract wearing a more comfortable name. You've taken on the overage risk without building in the buffer to survive it.
The Trap: Setting a GMP Against Incomplete Design
The most expensive mistake in GMP work is committing to the number too early — before the design is developed enough to price accurately.
This happens constantly, especially on progressive design-build and negotiated work where there's pressure to lock in a GMP to get the project moving. The design is at 60%, or 75%, and everyone wants a number. So a GMP gets set against a design that still has significant decisions unmade. Every one of those unmade decisions is a risk you've just guaranteed.
When the design completes and the details land harder than assumed, there's no one to pass the difference to. The owner has their cap. The subs have their contracts. You have the gap. And "the design wasn't finished when we priced it" is not a defense — it's a description of the mistake.
The discipline that protects you is knowing what you're assuming when you set the number, documenting it explicitly, and making sure your contingency and your qualifications account for everything the incomplete design leaves open.
Where GMP Projects Quietly Bleed
Beyond the big structural risks, GMP contracts leak margin in smaller, steadier ways that add up:
Cost substantiation. GMP work is cost-reimbursable up to the cap, which means you have to substantiate costs. Weak cost tracking and documentation means disputed costs, delayed payments, and reimbursements you can't fully collect.
The savings-share giveback. If your contract shares savings with the owner, every dollar of contingency you don't spend may be partly returned. That's fine — but it means sloppy estimating that inflates the GMP doesn't actually help you, and tight management of real costs is what drives your fee.
Scope gaps between subs. Same as any project — the work that falls between trade scopes becomes your cost. But in a GMP, that cost eats your contingency directly, moving you closer to the cap with every gap.
Change management. Distinguishing a genuine owner-driven change (which can adjust the GMP) from a cost that's properly inside the existing scope (which can't) is constant work. Get it wrong and you absorb changes that should have raised the cap.
The Real Protection Isn't in the Contract Type
Here's the throughline: a GMP doesn't protect your margin, and neither does a lump sum, and neither does cost-plus. The contract type determines where the risk sits and how it moves. It does not determine whether you make money. That's decided by the quality of the work you do before the number is set.
Setting a defensible GMP requires the same disciplines that protect margin on every project, applied with even more rigor because the overage risk is yours: realistic assumptions, documented qualifications, a properly sized and clearly governed contingency, tight scope coverage so nothing falls through the cracks, and the judgment to know when the design isn't developed enough to commit to a number yet.
That's preconstruction work. And it's the difference between a GMP that earns your fee and one that quietly turns into a fixed-price contract you didn't mean to sign.
That's what Stable Ground Consulting does — bringing 25 years of commercial construction judgment to the decisions that determine whether a contract protects you or exposes you. Go/No-Go reviews, buyout coverage, and scope letter development, on 24–48 hour turnarounds.
If you've got a GMP coming together and you want to pressure-test the number — the assumptions, the contingency, the scope coverage — before you sign it, let's talk.
