The contract form determines who carries the risk that the house costs more than anyone expected. Everything else about the choice follows from that.
| Fixed price | Cost-plus | Cost-plus with GMP | |
|---|---|---|---|
| Who carries estimating risk | Builder | Client | Builder above the ceiling, client below it |
| Price certainty | High, for the defined scope | Low until the project is complete | Capped |
| Cost transparency | Low — you see a price, not the costs behind it | High — you see every invoice | High |
| Builder incentive | To build efficiently; margin improves if costs come in low | Neutral to inverted where the fee is a percentage | To stay under the ceiling |
| Best suited to | Well-defined scope, complete drawings, conventional construction | Renovations, unknown site conditions, evolving design | Custom homes where the design is settled but details are not |
| Main failure mode | Thin allowances and heavy change orders erode the certainty you paid for | Costs drift with nobody carrying consequence | Ceiling set too high to constrain anything |
Fixed price
The appeal is obvious: one number. The limitation is that the number only covers the scope defined in the documents, and on a custom home the documents are rarely complete enough to define everything.
The practical consequence is that fixed-price custom contracts frequently arrive at a final cost well above the contract price, through allowances and change orders rather than through any dishonesty. A builder pricing a fixed contract also has to include contingency for their own risk, which you pay for whether or not it is needed.
- Works best when the drawings and specification are genuinely complete before pricing.
- Demands close attention to the allowance schedule, which is where the certainty leaks.
- Ask what contingency the builder has included and whether unspent contingency returns to you. Usually it does not.
Cost-plus
You pay documented costs plus a fee. It is transparent and it handles uncertainty gracefully, which is why it dominates renovation work and any project with unknown existing conditions.
The structural weakness is the incentive. Where the fee is a percentage of cost, a more expensive project pays the builder more. Most builders do not exploit that, but a contract should not depend on goodwill.
- Prefer a fixed fee over a percentage where you can negotiate it — it removes the inverted incentive entirely.
- Define precisely what counts as a cost: are supervision, site vehicles, small tools, warranty reserve and office overhead inside the cost or inside the fee?
- Require monthly documentation with invoices attached, not a summary.
- Agree an approval threshold above which a cost needs your sign-off before it is incurred.
- Ask whether supplier rebates and volume discounts are passed through to you.
Cost-plus with a guaranteed maximum price
Costs are open-book and billed as incurred, but the total is capped. Below the cap you pay actual cost; above it the builder absorbs the overrun. For custom homes with a settled design and unsettled details, this is usually the most balanced arrangement available.
- Ask how the GMP was calculated and what contingency sits inside it.
- Establish exactly which events allow the GMP to be adjusted — client changes almost always do, and the list should be short and specific.
- Negotiate savings sharing below the cap. A split gives the builder a reason to pursue efficiencies rather than simply spending to the ceiling.
- Check that the ceiling is tight enough to constrain anything. A GMP set 25% above a realistic estimate is a fixed-price contract with extra paperwork.
Whichever form you choose, ask: under exactly which circumstances can this price change without my written approval? Get the answer as a short, specific list. A long or vague answer means the form you think you signed is not the one you have.