Cost-Plus or Fixed Price: Which Model Should You Use on Your Jobs?
There are two main ways to sell a job: a firm price agreed upfront (fixed price), or real costs plus a markup (cost-plus, also called "time and materials"). Most contractors always use the same model — the one they learned — without asking whether it fits the job in front of them. Yet the wrong choice is expensive either way: on a fixed price, surprises come out of your pocket; on cost-plus, a distrustful client disputes every hour. Here's how to choose, and how to bill cleanly in both cases.
1. The two models, plainly
Fixed price: you commit to a price for a defined scope. The client gets certainty; you carry the estimating risk. If you're efficient, the extra margin is yours — the client doesn't get to know what the job actually cost you. If you got it wrong, the loss is yours too.
Cost-plus: you bill your real costs (labour, materials, subcontractors), plus a markup agreed upfront — typically an "administration and profit" line of 10 to 25% depending on the type of work. The overrun risk shifts to the client; in exchange, they see your costs. Transparency isn't optional in this model: it's the product you're selling.
Keep the underlying logic in mind: fixed price sells certainty, cost-plus sells transparency. The right model is the one the job actually needs.
2. When fixed price wins
Fixed price is the right tool when you can estimate tightly:
- The scope is clear and closed. Precise plans, material choices locked in, no dark corners behind the walls.
- You've done this job ten times. A standard bathroom, a roof, siding: your cost history makes the estimate reliable — and your efficiency becomes your margin.
- The client is a homeowner who needs a number. For most residential clients, price certainty is a condition of signing.
Two conditions keep a fixed price from turning against you: a detailed line-by-line quote that draws the boundary of the scope, and documented, approved extras whenever the client steps outside it. In Quebec, under a fixed-price contract, the Civil Code doesn't let you claim more than the agreed price for the work covered by the contract — the only relief valve is a separate agreement on added work. A fixed price without extras management is a fixed price that leaks.
3. When cost-plus wins
Cost-plus is the right tool when estimating would be a lottery:
- The scope is uncertain. Renovating a century-old building, hidden framing, plumbing of unknown vintage: nobody knows what's behind the wall until it's open.
- The emergency leaves no time to estimate. Water damage, insurance work, taking over a job from another contractor.
- The client wants to decide as they go. Materials not chosen yet, decisions that will evolve during the work.
In those situations, a fixed price forces you to pad the number with contingency to protect yourself. Result: either you lose the contract to a bolder competitor, or you win it and pray. Cost-plus removes the lottery: the client pays what it really costs, you get paid for what you really do.
4. The traps of cost-plus (and how to close them)
The model has a bad reputation with clients — "the contractor has no incentive to work fast." That distrust is managed through structure:
- The markup is announced upfront, in writing. The "administration and profit" percentage is negotiated at signing, not on the first invoice. And remember that markup has to cover your overhead and your profit — if you don't know your overhead, go back to calculating your real hourly rate first.
- Every cost must be justifiable. Under cost-plus you have to account for what you bill: precise time records per project, supplier invoices, subcontractor invoices. A cost-plus documented as you go gets paid without discussion; a cost-plus reconstructed from memory at month's end is a negotiation on every invoice.
- Invoice often. Weekly or biweekly. The client who sees costs coming in regularly stays confident; the one who gets a big surprise invoice after six weeks calls their lawyer.
- Offer a ceiling if the client hesitates. The "guaranteed maximum price" is the classic compromise: real costs plus markup, but never beyond an agreed amount. The client is reassured, and you keep the model's transparency.
5. The hybrid, in practice
On many jobs, the right model isn't a binary choice:
- Fixed price on the known, cost-plus on the unknown. The kitchen at a fixed price; opening the load-bearing wall on cost-plus, with the written agreement before the first swing of the sledgehammer.
- Allowances inside the fixed price. A provisional amount for undecided items ("tile allowance: $2,500"), adjusted to real costs against invoices.
- Extras at a pre-agreed rate. Even in a fixed-price contract, the agreement can set in advance the hourly rate and materials markup that will apply to added work — the extras conversation becomes a calculation, not a negotiation.
The real referee: your actual costs
Whatever the model, the deciding data is the same: what the job actually costs you, hour by hour. On cost-plus, that's what you bill — you need it to invoice. On fixed price, that's what tells you whether your estimate was good — you need it to learn. The contractor who doesn't track real costs is flying blind in both models; the one who does picks the right model job by job, with eyes open.
At Vantage, cost-plus mode is built into quotes: you enter your lines at real cost, set the markup, and the client sees the cost subtotal, the "Administration and profit (X%)" line and taxes — the model's transparency, without a spreadsheet. On approval, the invoice automatically carries the markup line. And GPS punch clocking assigns your crew's hours to the right project, so the real profitability of every job — fixed price or cost-plus — calculates itself. Try it free — 30 days, every feature, no credit card.