GMP vs lump sum vs cost-plus contracts

Picking a contract pricing model is a risk decision before it is a price decision. The same $5M building can be bought four different ways, and each way moves the cost overrun risk, the contingency, and the incentive to find savings to a different party. As the estimator, your job in preconstruction is to match the pricing model to how well the scope is actually defined, then price it honestly so the owner signs off knowing what they hold and what you hold. Below is how the four common models compare, what each party carries, and where the open-book line falls.

What are the four main contract pricing models?

There are four pricing structures you will quote on most US commercial work.

Lump sum, also called stipulated sum, is a single fixed price for a defined scope. You take off the drawings, price the work, add overhead and profit, and commit to a number. If the job costs less, you keep the difference. If it costs more, you eat it.

Guaranteed maximum price (GMP) sets a ceiling the owner will not exceed for a defined scope. You bill actual cost plus a fee up to that cap. Costs below the cap are an underrun, usually shared between owner and contractor through a savings clause. See GMP for the mechanics.

Cost-plus reimburses your actual cost of the work plus a fee. The fee can be a fixed dollar amount, a percentage of cost, or a percentage with a not-to-exceed cap. Pure cost-plus with a percentage fee and no cap is the most owner-exposed structure of the four.

Unit price pays a fixed rate per unit of work installed, for example $14 per cubic yard of structural fill or $9 per linear foot of curb. The owner pays for the quantities actually placed, measured in the field. It fits work where the design is set but the quantity is genuinely unknown until you dig.

Most negotiated commercial projects today are GMP. Hard-bid public work is usually lump sum. Cost-plus shows up on fast-track and heavily owner-directed jobs. Unit price is common in sitework, earthwork, and utilities, and it routinely lives as a line-item structure inside a larger lump sum or GMP contract.

Who carries the risk under each model?

Risk allocation is the whole conversation. Here is who holds what.

Under lump sum, the contractor carries cost risk. Once the price is set, every dollar of overrun is yours, and every dollar of underrun is yours. The owner carries scope risk: if the drawings are incomplete and you have to build something the documents did not show, that is a change order and the price moves. This is why lump sum punishes thin documents. You will load contingency and markup to cover what you cannot see, and the owner pays for that uncertainty whether or not it materializes.

Under cost-plus, the owner carries cost risk. You are reimbursed for what the work actually costs, so an overrun lands on the owner's side of the ledger. Your exposure is limited to running the job efficiently enough to protect your reputation and your fee. With a percentage fee and no cap, your fee actually grows as costs grow, which is the incentive misalignment owners worry about.

Under GMP, risk is split at the cap. Below the guaranteed maximum, the owner carries cost risk the same as cost-plus. Above the cap, you carry it the same as lump sum. The savings clause then gives you a reason to push costs down, because you share in the underrun rather than just handing it all back.

Under unit price, the contractor carries the risk of the rate being right and the owner carries the risk of the quantity. You guarantee your unit cost is correct; the owner accepts that final quantities, and therefore the final total, are not known at signing.

What is the open-book vs closed-book distinction?

Open-book means the owner can see your actual costs: invoices, payroll, subcontractor billings, equipment rates, the works. Closed-book means they see a price and nothing behind it.

Lump sum is closed-book by nature. The owner agreed to a number; how you spend against it is your business, and your margin is yours to protect. They do not get to audit your buyout.

Cost-plus and GMP are open-book by nature, because the owner is reimbursing real cost and has to be able to verify it. That visibility is the trade. The owner sees your numbers, and in exchange they take on more of the cost risk. A clean GMP defines exactly what counts as cost of the work versus what is covered by the fee, names the allowable markup rate on changes, and states how contingency gets drawn and who owns what is left. Sloppy definitions here are where GMP disputes start.

Unit price is partly open: the rates are agreed and visible, but the rate buildup behind each unit is usually yours.

When does each contract type fit?

Match the model to the maturity of the documents and the owner's appetite for risk.

Use lump sum when the scope is fully designed and you can take off complete drawings with confidence. Hard-bid public work, tenant improvements off finished construction documents, and well-defined building shells fit here. The owner gets price certainty and a clean number to finance against. They pay for that certainty through the contingency you build into the price.

Use GMP when the owner wants a ceiling but the design is not fully complete, which describes most negotiated commercial work. GMP lets you start while drawings finish, gives the owner a cap to underwrite, and the open-book structure plus the savings split keeps incentives roughly aligned. It is the workhorse of negotiated delivery for a reason.

Use cost-plus when speed matters more than price certainty and the scope is genuinely unknowable at the start. Emergency restoration, early site mobilization ahead of design, and heavily owner-directed work fit. Owners should pair it with a fixed or capped fee so your compensation does not balloon with cost.

Use unit price when the design is set but the quantity is not, the classic case being earthwork, undercut, rock removal, and utilities. Nobody knows how much unsuitable soil is down there until the excavator hits it, so you price the rate and measure the field.

Worked example: GMP with an 80/20 savings split

Take a $5,000,000 job bought as a GMP with an 80/20 savings split in the owner's favor. The guaranteed maximum price is $5,000,000. You build the work and the final actual cost, including your fee, comes in at $4,700,000. These figures are illustrative.

The underrun is the difference between the cap and what you actually spent:

• $5,000,000 GMP minus $4,700,000 actual cost = $300,000 savings.

The savings clause splits that $300,000 80/20 in the owner's favor:

• Owner share: $300,000 times 0.80 = $240,000.
• Contractor share: $300,000 times 0.20 = $60,000.

So the owner is credited back $240,000 of the underrun, and you keep $60,000 as your share of the savings. The owner's final payment is the actual cost plus your savings share:

• $4,700,000 actual cost plus $60,000 contractor savings share = $4,760,000 paid by the owner.

The owner spent $4,760,000 against a $5,000,000 cap, so they saved $240,000 versus the ceiling they underwrote. You earned your base fee plus a $60,000 bonus for bringing the work in under the cap. Both parties came out ahead of the worst case, which is exactly what the savings clause is designed to produce.

Worked example: the same job as lump sum

Now buy the same job as a lump sum at a fixed $5,000,000, and assume your actual cost still comes in at $4,700,000. These figures are illustrative.

Lump sum is closed-book, so there is no savings clause and no split. The owner owes the contract price regardless of what the work cost:

• Owner pays $5,000,000, the full lump sum.

The contractor keeps the entire difference between the contract price and actual cost:

• $5,000,000 contract price minus $4,700,000 actual cost = $300,000 retained by the contractor.

Compare the two structures on identical cost performance. Under GMP, the owner paid $4,760,000 and you kept $60,000 of the underrun. Under lump sum, the owner paid $5,000,000 and you kept all $300,000. The owner pays $240,000 more under lump sum for the same delivered building, because in a fixed price they have no claim to the savings, while you carried the full cost overrun risk to earn it. That is the core trade between the two models, stated in dollars.

For reference, the same scope as cost-plus with an illustrative 8 percent fee on the $4,700,000 actual cost would run $4,700,000 plus $376,000 fee, or $5,076,000, with no cap protecting the owner if cost had run higher. The structure you choose changes the price the owner pays even when the work is identical.

How does AI assist the estimate without taking the sign-off?

The pricing model only protects the owner if the number behind it is right, and that number comes from the takeoff. This is where Ruh fits. Ruh reads the drawings, runs the quantity takeoff, and prices the work against the contractor's own price book rather than generic regional averages. It produces a structured, line-item basis you can drop into a lump sum proposal or a GMP schedule of values. The estimator reviews the assumptions, the inclusions and exclusions, the contingency, and the markup, then signs off. The machine does the repetitive measurement and pricing; the human owns the judgment and the commitment. That division is the point, because the signature on a GMP or a lump sum is a promise no model should be allowed to make for you.

Choose the contract by reading the documents in front of you. Complete drawings and a price-certain owner point to lump sum. Incomplete drawings and an owner who wants a ceiling point to GMP with a clean savings clause and a tight definition of cost of the work. Genuine unknowns in quantity point to unit price, and genuine unknowns in scope point to a capped-fee cost-plus. Whichever model you land on, the discipline is the same: define the scope as far as the documents allow, put the contingency where the risk actually sits, write the open-book terms so nobody argues about them later, and make sure the price you sign reflects the work you can see. The structure decides who carries the risk. Your estimate decides whether anyone gets hurt by it.