What is a guaranteed maximum price (GMP) contract?

Think of it as cost-plus pricing with a safety net built in. You pay the builder for the actual cost of the work plus a set fee, but the total can never go above an agreed cap. The American Institute of Architects (AIA) defines it as a contract where "the contractor agrees to complete the project for a set price, with the maximum price being the agreed-upon cap." If the real cost comes in under that number, you usually keep most of the savings. If it runs over, the builder eats the difference. That cap is the whole point. It gives you the cost transparency of cost-plus with the spending limit of a fixed price.

How a GMP contract actually works

A GMP contract works by combining open-book pricing with a firm spending limit. The builder bills you for the real cost of labor, materials, and subcontractors, then adds a fee for overhead and profit. That part is cost-plus (you pay actual costs plus a markup). The GMP adds one rule on top: the sum of all those costs plus the fee cannot exceed the guaranteed maximum, no matter what.

The cap protects you from overruns. AIA puts it plainly: the owner's obligation "is limited to the agreed maximum amount, regardless of actual project costs," and "any expenses exceeding this ceiling become the contractor's responsibility." So if the GMP is set at $900,000 and the job ends up costing $940,000, you still pay $900,000, and the builder absorbs the extra $40,000. That is the guarantee in guaranteed maximum price.

Savings cut the other way and stay friendly to the owner. If the same project comes in at $860,000, you do not automatically pay the full cap. AIA notes that "if the final cost is less than the GMP, the client gets the benefit of those savings, while the contractor may retain a share, depending on the contract's terms." Many GMP contracts include a shared savings clause, where the under-budget amount is split on an agreed ratio, often heavily in the owner's favor. That split is one of the key terms to nail down before you sign.

How the GMP cap protects the owner

The GMP cap protects you in three concrete ways: it limits your downside, it keeps the books open, and it puts the cost risk on the party who controls the work. Each one matters on a custom home, where the budget is large and the unknowns are real.

• A ceiling on your spending. You know the worst case before construction starts. Your loan, your savings, and your nerves are all sized to a number that cannot be exceeded without your written approval through a change order.
• Open-book transparency. Because the base is cost-plus, you see the real invoices and the real subcontractor bids. You are not guessing at a builder's hidden markup, the way you might with a single lump-sum number.
• Risk lands on the builder. The party who orders the materials, schedules the trades, and runs the site is the party who pays if costs run past the cap. That lines up the incentive: the builder is motivated to buy smart and avoid waste, because overruns come out of the builder's pocket.

The cap is not a blank check against your own choices, though. If you change the plan, upgrade finishes, or add scope, the GMP rises by a change order that you approve. Honest overruns the builder cannot control, like a buyer-requested upgrade, move the cap. Overruns the builder should have planned for do not. Reading the change-order terms closely is how you keep the protection real.

GMP versus fixed-price versus cost-plus

The GMP sits between a fixed-price contract and a pure cost-plus contract, and that middle position is its main appeal. Each structure shifts cost risk differently, and the right one depends on how defined your plans are and how much transparency you want.

• Fixed-price (lump sum). You agree to one total for a defined scope. It gives the most certainty but the least transparency, since the builder's markup is baked in and hidden. Unknowns get padded into the price, so you may pay for risk that never happens. Best when the plans are fully complete.
• Pure cost-plus. You pay actual cost plus a fee, with no ceiling. It gives the most transparency but the least certainty, because the final number is open-ended. Best when scope is genuinely unknown and trust is high.
• GMP (cost-plus with a cap). You get the open books of cost-plus and a hard ceiling like a fixed price. You carry less risk than pure cost-plus and gain more visibility than lump sum.

For a deeper look at the two contract types a GMP blends, see cost-plus custom home contract and cost-plus versus fixed-price home contract. The AIA publishes standard GMP forms, such as the A102 owner-contractor agreement, that many Arizona builders adapt as a starting point.

What to confirm before you sign a GMP in Arizona

Before you sign a GMP contract in Arizona, pin down the cap, the savings split, the allowances, and the change-order rules in writing. A GMP only protects you if these terms are clear, and Arizona law already requires the contract to state the price. Under A.R.S. 32-1158, a residential construction contract must include "the total dollar amount to be paid to the contractor by the owner for all work to be performed under the contract, including all applicable taxes," along with the scope, the completion date, and the progress-payment schedule.

Check these points specifically:

• What sets the cap. The GMP should be tied to a defined scope, a finish schedule, and a complete set of plans. A cap built on vague plans is a soft cap that change orders will erode.
• The savings split. Confirm the ratio in writing. An owner-friendly split returns most of any under-budget savings to you.
• Allowances. Items priced as allowances (placeholder budgets for things like tile or fixtures not yet chosen) can blow past the cap if set too low. Make sure they are realistic. See what is a builder allowance and what happens if you go over.
• The fee. Know whether the builder's fee is a fixed dollar amount or a percentage, since a percentage fee can creep if scope grows.
• Change-order process. Require written, signed change orders before any work that moves the cap, so a verbal "while we're at it" cannot surprise you. Each change order should show its dollar effect on the GMP and update the running total, so you always know where the cap stands.

A GMP is one of the more owner-friendly ways to build a custom home, because it caps your exposure while keeping the pricing honest. It works best with complete plans and clear terms. This is a high-stakes contract on a large budget, so have your builder walk you through every clause, and confirm the cap, savings split, and allowances before you sign. For the full list of what belongs in any Arizona build contract, see what to include in a custom home contract.

How Jematell Homes helps

Transparency, a real license, and a written scope are how we work. Reach out and we will talk through how this applies to your specific lot and plan.

Sources

• AIA Contract Documents: Understanding Guaranteed Maximum Price Contracts
• AIA Contract Documents: Guaranteed Maximum Price (GMP) Contracts in Construction
• A.R.S. 32-1158 (Residential construction contract requirements)