Fixed-Price vs. Cost-Plus: How to Read a Builder's Contract

The contract you sign determines who carries which risks for the next 18 months. Fixed-price and cost-plus are two fundamentally different models. Most clients don't fully understand the difference until month nine, when it matters most. Here's the difference, in plain English, ahead of time.

Fixed-price (sometimes called "lump-sum" or "guaranteed maximum price")

The builder commits to a total price for a defined scope. They quote you $2.4M for the project. Whatever it actually costs them, your number is $2.4M (plus any change orders you authorize).

Pros for you:

• Cost certainty — you know your number on day one
• Builder absorbs cost overruns
• Builder is motivated to manage trades, prevent rework, and finish on time
• Easier to finance (banks love fixed-price contracts)

Cons for you:

• Scope must be defined in detail before signing (full CDs, specs, schedule)
• Mid-project changes are expensive — the builder treats change orders as profit centers
• Builder may cut corners on items not visible to you (cheaper subs, lower-spec hardware) to preserve margin
• Tougher to negotiate post-signing because the price assumes scope

Cost-plus (sometimes "time-and-materials" or "cost-plus-fee")

You pay actual construction costs plus a fee — either a flat fee or a percentage (typically 12-20%) of the construction cost. Whatever the project actually costs, you pay that, plus the fee.

Pros for you:

• Full transparency — you see every receipt, every sub bid, every cost
• Flexibility to make changes mid-project without renegotiation
• Builder has less incentive to cut corners (they're not protecting margin)
• Better fit for renovations or projects with unknowns

Cons for you:

• No cost certainty up front
• Builder has less incentive to manage costs tightly (their fee is fixed regardless)
• Requires high trust and active client involvement
• Harder to finance (banks want fixed-price)
• Easy for "cost-plus" to drift into "way more than expected" without a strong process

Fixed-price is for projects where the scope is locked. Cost-plus is for projects where the scope is going to evolve. Pick the one that matches your project's reality, not the one that sounds safer.

The hybrid: Guaranteed Maximum Price (GMP) with shared savings

The contract style we use most often for new custom homes. The builder commits to a maximum number — say $2.4M — but tracks actual costs transparently like cost-plus. If actual costs come in lower, savings are shared (often 80% to client, 20% to builder).

Best of both:

• Cost certainty (you can't pay more than the GMP)
• Transparent accounting (you see actual costs)
• Aligned incentive (builder benefits from saving you money)
• Flexibility to make scope adjustments mid-project

Banks like it because there's a max. Clients like it because there's visibility. Builders like it because the upside is shared. Worth asking your builder if they'll do this.

Language to look for in either contract

1. Scope definition. The contract should reference specific construction documents and a specification book. "Per attached drawings dated [date]" should appear. Anything vague will be expensive.
2. Change order process. Written change orders with cost, schedule impact, and your signature required before work begins. No verbal change orders. No retroactive changes.
3. Allowance items list. Every line item where a placeholder budget is being used (cabinets, lighting, plumbing fixtures, etc.) should be explicitly listed with the allowance amount. Be wary of allowances above 15% of total scope.
4. Substitution clauses. Can the builder substitute equivalent materials without your sign-off? Limit this aggressively — you want to approve any substitution above $500.
5. Schedule and liquidated damages. Target completion date plus consequences for delay (typically $500-$2,000/day). Most builders resist this; the good ones agree to reasonable terms.
6. Warranty terms. Standard is 1-year fit-and-finish, 2-year mechanical, 10-year structural. Anything less is below industry standard.
7. Lien waivers. Each draw should require lien waivers from subs and material suppliers. Protects you from sub-tier disputes.
8. Insurance and bonding. Builder's risk insurance, general liability, workers' comp. Bonding is optional but valuable for large projects.

The conversation that decides which model is right

Sit with the builder for two hours. Walk through:

1. How detailed is the spec book right now? (Determines if fixed-price is feasible)
2. How likely is significant scope change during construction? (Pushes toward cost-plus or GMP)
3. How comfortable is the builder with transparent accounting? (If they resist, fixed-price is the only fit)
4. How does the builder handle unknowns in renovations vs. new construction? (Different answers tell you about their process)
5. Will they walk through their last three completed projects' actual costs vs. originally budgeted costs? (The honest builders will. The ones who won't are signaling something.)

The contract type matters less than the contract clarity and the builder honesty. A great builder on a cost-plus delivers a better project than a sketchy builder on fixed-price. Hire well first, structure the contract second.

— Margaret Larsen, COO. Eighteen years guiding clients from first conversation through groundbreaking — budgets, contracts, permits, financing. Get the free Ultimate Home Building Checklist for the field-tested list we walk every Angel home through.