What is a cost-plus custom home contract and how does it work?

Instead of one locked-in number, a cost-plus custom home contract bills you for the real cost of building your home, plus a separate fee for running the job. You cover the actual price of labor, materials, subcontractors, and permits as the bills come in, and on top of that you pay the builder either a flat fee or a percentage markup, commonly 10 to 20 percent. The National Association of Home Builders describes this method as basing the charge on "the amount of time and labor it will take to construct your home, plus a percentage markup on all material that goes into your home." The defining trait is transparency. You see the true cost of the work, usually backed by invoices and receipts, rather than one fixed number set before construction starts. That openness is the trade-off for taking on the risk that costs can rise.

How the billing actually works

In a cost-plus contract, the builder bills you in stages for the real costs they paid, then adds their agreed fee. There is no single locked price. The total is whatever the home actually costs to build, plus the builder's compensation on top.

The "cost" side covers the hard costs of the job. That means materials like lumber, concrete, windows, and fixtures, the labor of the crews, the bills from licensed subcontractors for plumbing and electrical, the permit fees, and the equipment rentals. Under Arizona's B residential license, the builder must subcontract trades like electrical and plumbing to contractors licensed in those trades, and those subcontractor invoices flow through as a cost to you.

The "plus" side is the builder's fee. This is set two ways. A percentage fee adds a markup, often 10 to 20 percent, on top of every cost. A fixed fee is a flat dollar amount agreed up front, so the builder's pay does not climb as costs climb. Many owners prefer the fixed fee, because it removes the builder's incentive to let costs grow.

Billing usually happens monthly or by draw. The builder submits the period's costs with backup, you pay that amount plus the matching fee, and the cycle repeats until the home is done. A construction lender funding the build will require the same documentation before releasing each draw, so the paperwork serves double duty.

One more piece is common on larger custom homes. Some cost-plus contracts add a guaranteed maximum price, a cap the total cannot exceed. With that cap in place, you keep the open-book benefit and the savings on any item that comes in low, but you are protected if costs run high. We cover that hybrid in the cost-plus versus fixed-price comparison, since it sits between the two.

What "open book" means and why receipts matter

Cost-plus is an open-book contract, which means the builder shows you the actual invoices and receipts behind every charge. This is the heart of the method and the main reason owners choose it. You are not trusting a number. You are seeing the real cost of your home as it is spent.

In practice, each billing comes with backup. You should see subcontractor invoices, material receipts or supplier statements, and a clear line showing the fee added on top. A good cost-plus builder gives you a running budget that compares estimated costs to actual costs, line by line, so you can see where the project stands.

This transparency cuts both ways. You get the benefit of any savings. If lumber prices drop or a subcontractor comes in under estimate, you pay the lower real cost, not a padded bid. The NAHB notes a fixed bid may carry "contingency markups" the builder builds in to protect against surprises, and a cost-plus contract strips those out.

The catch is that open book only protects you if you actually read the books. Ask up front exactly what documentation you will receive and how often. Put it in writing. A builder who resists sharing invoices is the wrong fit for a cost-plus job, because the whole structure depends on seeing the real numbers.

Estimates, allowances, and how the budget gets set

A cost-plus contract starts with an estimate and uses allowances for choices you have not made yet, but neither one caps your final price. The estimate is the builder's best projection of the total. Allowances are budgeted dollar amounts set aside for items like flooring, cabinets, or appliances before you pick the exact product.

Here is how allowances work. Say the contract sets a 15,000 dollar allowance for kitchen cabinets. If you choose cabinets that cost 20,000 dollars, you pay the extra 5,000 dollars, plus the fee on that amount. If you choose cabinets that cost 12,000 dollars, you save the difference. Allowances let the contract move forward before every finish is selected, which is common on a custom home where decisions happen during the build.

This is why a cost-plus estimate is a target, not a guarantee. If your selections run above the allowances, or if the build hits a surprise like rock excavation on a Rio Verde or Cave Creek lot, the cost rises and so does your bill. A careful builder sets realistic allowances so the estimate reflects the home you actually want, not a stripped-down version that looks cheaper on paper.

Watch the allowance numbers closely before you sign. Lowball allowances are the most common way a cost-plus estimate looks attractive and then climbs during construction. Compare each allowance to the real cost of the finishes you actually want, and adjust them up front so the estimate is honest.

What Arizona requires in the contract

Arizona law requires your cost-plus contract to be in writing and to spell out the price terms, even though the final cost is not fixed. A.R.S. 32-1158 lists what a residential contractor's written contract must contain, and it applies to cost-plus jobs.

The statute requires the contract to 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 dollar amount of any advance deposit" and "the dollar amount of any progress payment and the stage of construction at which the contractor will be entitled to collect progress payments." For a cost-plus job, the contract should show how the fee is calculated, how costs are documented, and when each progress payment is due.

A.R.S. 32-1158 also requires the contract to name the contractor, the contractor's business address and license number, the owner and jobsite, the contract date, the estimated completion date, and a description of the work. The owner's right to file a complaint with the Registrar must be "prominently displayed in the contract in at least ten-point bold type." Confirm the builder's B license is active with the Arizona Registrar of Contractors before you sign.

Cost-plus works best with a builder you trust, because you are funding the real cost as it happens. Used well, it gives you a clear window into where your money goes and lets you control selections during the build. At Jematell Homes we use written, open-book agreements so you see the actual cost of your custom home, with the fee stated plainly up front. If price certainty matters more to you than transparency, compare cost-plus against a fixed-price contract before you decide.

Building with Jematell Homes

We are a licensed, family-owned Arizona custom home builder. If you are planning a custom home in Scottsdale, Rio Verde, or the greater Phoenix metro, we are happy to walk through your project.

Sources

• NAHB: How Do Builders Charge for Their Services?
• A.R.S. 32-1158 (Contract requirements; provision of documents)
• Arizona Registrar of Contractors (License Classifications)