A cost-plus contract pays you back what the build actually costs, plus an agreed fee for running it. For custom home builders, it’s often the cleanest way to price a project when the plans aren’t final, the client is still choosing finishes, or the site holds surprises nobody can price yet.
The trade-off is that cost-plus only works if your books are open and accurate. Clients can see every invoice, every hour and every markup. That transparency is what earns their trust, and it’s also where most cost-plus disputes start.
This guide covers how cost-plus contracts work on a custom home, the main fee structures, a worked example, how to set your fee and what to put in the contract so the job runs smoothly.
What is a cost-plus contract?
A cost-plus contract is an agreement where the client reimburses the builder for the actual cost of the work (labor, materials, subcontractors and equipment) and pays a separate fee on top. The fee covers the builder’s overhead and profit. It’s set either as a percentage of costs or as a fixed amount.
In a fixed-price (lump-sum) contract, you quote one number and carry the risk if costs rise. In a cost-plus contract, the client carries most of the cost risk, and you agree to show them exactly where the money goes.
You’ll also see it called:
- Cost of the work plus a fee. This is the wording the American Institute of Architects uses in its standard forms.
- Cost-plus pricing or cost-plus billing. These describe the invoicing side.
- Open-book contract. This highlights the transparency requirement.
When custom home builders use it
Cost-plus fits best when the final scope can’t be priced with confidence on day one. Common cases:
- Design-build projects where construction starts before every drawing is finished
- High-end homes with many client selections and allowances still open
- Difficult sites: slopes, unknown soils, rock, remote access
- Clients who expect to change things as they see the house take shape
- Volatile material prices, where a lump sum would need a large contingency
If the plans are complete and the client mainly wants a firm number, a fixed-price contract is usually the better fit. Our cost plus vs fixed price guide compares the two side by side.
How a cost-plus contract works on a custom home
A cost-plus project still starts with an estimate. The difference is that the estimate is a target, not a price. Here’s the usual sequence:
- Build a detailed estimate. Price the home line by line from the current drawings and specs, with allowances for anything not yet selected. Clients will judge you against this number, so make it realistic.
- Agree on what counts as cost. The contract defines which expenses are reimbursable and which come out of your fee. This is the most important part of the agreement (more on it below).
- Agree on the fee. Choose a percentage, a fixed fee or a hybrid, and decide whether there’s a price cap.
- Set a billing rhythm. Most builders invoice monthly or at each draw. Each invoice lists the costs incurred in that period, with backup, plus the fee earned on them.
- Track every cost to the job. Supplier bills, subcontractor invoices, labor hours and equipment all get coded to the project and to a budget line.
- Share the backup. The client (or their lender) reviews receipts, timesheets and sub invoices before paying. Many contracts give the client the right to audit your job records.
- Compare actuals to the estimate. Report cost-to-date against budget at every draw, so changes and overruns show up early, not at closeout.
- Reconcile at the end. Final invoice, final accounting, and any shared savings or cap adjustments.
Types of cost-plus contracts
Most residential cost-plus contracts are a cost-plus-percentage or cost-plus-fixed-fee deal, sometimes with a guaranteed maximum price on top. The fee structure changes who carries risk and what each side is motivated to do.
| Type | How the fee works | Risk on cost overruns | Best for | Watch out for |
|---|---|---|---|---|
| Cost plus percentage | A % of total cost, often 10–25% on custom homes | Client | Early-stage designs, many open selections | Your fee grows as costs grow, so clients may question your incentive to save |
| Cost plus fixed fee | A flat dollar fee set at signing | Client (builder’s fee doesn’t move) | Defined scope with some unknowns | Big scope changes need a fee adjustment clause |
| Cost plus with guaranteed maximum price (GMP) | % or fixed fee, but total price is capped | Builder above the cap | Clients and lenders who need a ceiling | Needs near-complete drawings, and you price in contingency for the cap |
| Cost plus with incentive or shared savings | Base fee plus a bonus (or split) for finishing under target | Shared | Repeat clients, strong relationships | Agree clearly how the target is set and adjusted |
Cost plus percentage
The most common form for custom homes. If costs are $800,000 and your fee is 15%, you bill $920,000. It’s simple to explain and automatically scales your fee as the client upgrades. The downside is perception: some clients worry you profit more when the house costs more.
Cost plus fixed fee
You agree a fee in dollars, say $110,000, regardless of final cost. This removes the incentive question, which makes it easier to sell to cautious clients. You’ll need a clause that adjusts the fee when the client adds major scope, or a 1,500 sq ft addition ends up earning you nothing extra.
Guaranteed maximum price (GMP)
A GMP caps the total the client will pay. Costs above the cap come out of your fee or your pocket, unless they’re caused by approved change orders. Contracts often include a shared-savings clause that splits any money left under the cap. The AIA A102 form covers cost plus a fee with a GMP. The A103 form covers cost plus a fee with no GMP.
GMP sounds safe for the client, but it only works once drawings are close to complete. Otherwise you’re pricing a lump sum under another name, with less margin.
Cost-plus contract example
Here’s how the numbers work on a 3,400 sq ft custom home. The builder’s estimate puts the cost of the work at $820,000, and the fee is 15%.
A monthly draw
In month 3, the builder invoices the costs incurred that month, with receipts and timesheets attached:
| Line item | Cost |
|---|---|
| Framing subcontractor | $48,500 |
| Lumber and sheathing | $31,200 |
| Window package deposit | $18,000 |
| Site supervisor, 160 hrs × $55/hr | $8,800 |
| Telehandler rental | $3,400 |
| Dumpster and portable toilet | $1,100 |
| Cost of the work this period | $111,000 |
| Builder’s fee (15%) | $16,650 |
| Amount due | $127,650 |
How the fee type changes the final bill
Now say the project ends at $900,000 in actual costs. The client approved $50,000 in upgrades through change orders, and the excavator hit rock that added $30,000.
| Cost plus 15% | Cost plus fixed fee | Cost plus fixed fee with GMP | |
|---|---|---|---|
| Fee terms | 15% of cost | $123,000 (15% of the estimate) | $123,000, total capped at $960,000 |
| Cap after $50,000 in approved change orders | None | None | $1,010,000 |
| Cost of the work | $900,000 | $900,000 | $900,000 |
| Client pays | $1,035,000 | $1,023,000 | $1,010,000 |
| Builder’s fee actually earned | $135,000 | $123,000 | $110,000 |
Under the percentage deal, the rock and the upgrades raised the builder’s fee by $12,000. Under the fixed fee, the client saved that $12,000. Under the GMP, the builder absorbed the $13,000 that went over the cap. That’s why GMP contracts depend on how well unknowns like soil conditions are excluded or priced in up front.
Reimbursable costs vs. what your fee covers
Most cost-plus disputes come from one question: was that a cost of the work, or was it supposed to come out of the fee? Settle it in writing before you break ground.
The usual split looks like this. Your contract can move items between columns, as long as it says so clearly.
| Usually reimbursable (cost of the work) | Usually covered by the fee (overhead and profit) |
|---|---|
| Materials and supplies delivered to the job | Office rent, utilities and admin staff |
| Subcontractor invoices | Owner and estimator salaries |
| Field labor at agreed rates, with burden | Company vehicles not dedicated to the job |
| Site supervisor time, if stated in the contract | Accounting, software and general insurance |
| Equipment rental and small tools bought for the job | Marketing and sales costs |
| Permits, inspections and utility fees | Profit |
| Temporary facilities: power, toilets, fencing, dumpsters | Costs to fix your own defective work |
| Builder’s risk insurance and job-specific bonds | Fines or penalties caused by the builder |
| Cleanup and debris hauling |
The gray areas to settle up front:
- Project manager and superintendent time. Is it billed by the hour as a job cost, or included in the fee? Either works, but say which.
- Labor burden. If you bill your crews at a loaded rate (wages plus payroll taxes, workers’ comp and benefits), list the rate or the burden percentage.
- Your own equipment. Set a rate schedule for company-owned machines, so you aren’t billing retail rental rates for equipment you already own.
- Rework. Decide who pays when something has to be redone, and whether it depends on whose mistake it was.
- Discounts and rebates. Most contracts pass supplier discounts through to the client. Say whether early-payment discounts count too.
How to set your cost-plus fee
Cost-plus fees on custom homes typically run 10–25% of the cost of the work (Landmark Services, CostToBuildHouse). Larger homes and larger builders tend to sit lower in that range. Smaller projects and boutique builders sit higher. Don’t pick a number because competitors use it. Work it out from your own costs:
- Add up your annual overhead. Include everything the fee must cover: office, admin and management salaries, insurance, vehicles, software and marketing. Say it’s $300,000.
- Estimate your annual volume. Total cost of the work you expect to bill in a year. Say $3,000,000.
- Work out your overhead rate. $300,000 ÷ $3,000,000 = 10%. That’s your break-even fee.
- Add your target profit. If you want profit equal to 8% of cost, your fee is 18%.
- Adjust for what’s billed as a job cost. If supervision and project management are reimbursable, your fee can be lower than if they come out of the fee.
Two more things to decide:
- Is the fee the same on everything? Some builders charge a lower fee on big pass-through items like appliances or client-supplied fixtures, or a minimum fee if the client buys materials directly.
- When is the fee billed? Most builders bill it with each draw, proportional to costs incurred. Billing it all up front is rare and makes clients nervous.
Pros and cons of cost-plus contracts
Cost-plus protects your margin but costs you more admin. Here’s how it looks from each side of the table.
For the builder
| Pros | Cons |
|---|---|
| Your fee is protected when costs rise or the site surprises you | Heavy documentation: every receipt, timesheet and sub invoice |
| No need to pad the bid with large contingencies | Clients can question any line item |
| Easier to start before plans are 100% final | Cash flow depends on fast, accurate draw invoices |
| Upgrades and changes don’t eat your margin | Less upside: you can’t earn more by buying well, unless there’s a savings split |
| Builds trust with clients who want to see the numbers | Weak cost tracking shows up quickly and hurts your reputation |
For the client
| Pros | Cons |
|---|---|
| Full visibility into what the house really costs | No firm final price, unless there’s a GMP |
| No hidden contingency built into the price | Budget can creep with selections and changes |
| Freedom to change selections as the build progresses | More invoices to review |
| Savings on materials go to them, not the builder | Requires trust in the builder’s cost control |
| Often a lower total than a heavily padded lump sum | Some lenders want more paperwork for cost-plus draws |
What to put in a cost-plus contract
A good cost-plus contract answers every money question before it comes up. Use a standard form as your starting point, such as the AIA A102/A103 or the short-form A105 for smaller residential projects, or your state builders’ association template. Then have a construction attorney review it for your state. Make sure it covers:
- Definition of the cost of the work. A detailed list of reimbursable costs and a matching list of non-reimbursable ones.
- Fee. Percentage or fixed amount, what it applies to, and when it’s billed.
- Labor rates. Hourly rates by role, or how burden is calculated.
- Estimate or budget. Attach it, and state that it’s an estimate, not a guaranteed price.
- GMP terms, if any. The cap, what raises it (approved change orders), what’s excluded, and how savings are split.
- Allowances. Amounts for open selections and what happens when a client goes over or under.
- Change orders. How changes are requested, priced and approved in writing, and how they affect the fee or cap. A change order template helps keep this consistent.
- Billing and payment. Draw schedule, backup required with each invoice, payment terms, and retainage if any.
- Audit rights. How long the client can review your job records, and how.
- Budget reporting. How often you’ll report cost-to-date against the estimate, and the threshold for flagging an overrun.
- Rework and warranty. Who pays to correct defective work during construction and after.
- Discounts and rebates. Whether they’re passed through to the client.
- Termination. What’s owed if either side ends the contract early.
How to run cost-plus jobs without disputes
On a cost-plus job, your bookkeeping is part of the product. The builders who do it well tend to follow the same habits.
Code every cost on the day it happens. A receipt that sits in a truck for three weeks becomes a line nobody can explain at draw time. Assign each bill and timesheet to a job and a budget line as it comes in.
Send invoices clients can follow. Group costs by phase or budget line, attach the backup, and show the fee as its own line. A client who can read the invoice in five minutes pays faster.
Report budget vs. actual at every draw. Show original estimate, approved changes, cost-to-date and projected final cost. Clients get upset by surprises, not by overruns they saw coming two months earlier.
Put every change in writing before work starts. On cost-plus, it’s tempting to skip change orders because “they’ll pay for it anyway.” Don’t. A signed change order is your proof that the client approved the extra cost, and it adjusts the fee or GMP.
Manage allowances in the open. Show the client where each selection stands against its allowance as they choose. Allowance overages are the most common source of budget creep on custom homes.
Give the client a window into the job. A client portal where they can see invoices, selections, change orders and progress photos cuts down on calls and on doubt.
Run cost-plus projects in Buildern
Cost-plus contracts only work when cost tracking keeps up with the job. Buildern keeps the costs, the fee and the client’s view in one place:
- Cost-plus invoicing. Turn bills and timesheets into cost-plus invoices with your markup applied, alongside fixed-price jobs.
- Bills and job costing. Log supplier and subcontractor bills against the project budget as they arrive, and see budget vs. actual at any point.
- Change orders and allowances. Get client approval on changes and track each selection against its allowance.
- Client portal. Clients see invoices with clear cost breakdowns, approve changes and follow progress without calling you.
- Payment tracking. Follow invoice status, partial payments and retainage from one dashboard, and sync with your accounting software.
Try Buildern free and send your next cost-plus draw in minutes.
Frequently Asked Questions
What is a cost-plus contract in simple terms?
The client pays what the project actually costs, plus a fee to the builder for overhead and profit. The builder shows the client receipts and records for every cost.
What is a typical cost-plus percentage for a custom home builder?
Most custom home builders charge 10–25% of the cost of the work. The right number depends on your overhead, your annual volume and whether supervision is billed as a job cost or covered by the fee.
Is cost-plus or fixed price better for custom homes?
Cost-plus suits projects with incomplete plans, many open selections or uncertain site conditions. Fixed price suits projects with complete drawings and clients who want a firm number. See our cost plus vs fixed price comparison for more.
Who pays for mistakes in a cost-plus contract?
It depends on the contract. Most cost-plus contracts make the builder pay to fix their own defective work or their subcontractors’ work, rather than billing it as a cost. Spell this out in the rework clause.
Does a cost-plus contract have a maximum price?
Not by default. If the client needs a ceiling, add a guaranteed maximum price (GMP). The builder then absorbs costs above the cap, except for approved change orders.
Can a builder mark up subcontractor invoices on a cost-plus job?
Only as the contract allows. Usually the builder’s fee applies to all costs, including subcontractor invoices, and there’s no separate markup. Charging both a markup and the fee on the same invoice is a common cause of disputes.
