Is a fixed-price contract always the safer choice?

Not necessarily. Fixed-price only protects you if the scope is genuinely complete and well-defined before signing. On an older home where hidden conditions are likely, a cost-plus contract with a guaranteed maximum price often gives you better real protection, since the contractor has no incentive to under-scope the bid to win it.

What is a guaranteed maximum price (GMP) contract?

A GMP is a hybrid: you pay actual cost-plus-fee like a cost-plus contract, but the contractor guarantees the total won't exceed an agreed ceiling. If actual costs come in under the cap, you typically split or fully receive the savings, depending on the contract terms.

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Contracts Guide · San Francisco

Fixed-Price vs. Cost-Plus: Which Contract Type Is Right for You?

By Solid Bay Construction SF | Updated 2026

The way your contract is structured affects your financial risk as much as the contractor you choose. Here's the real difference between the two most common structures, and where each one makes sense.

Fixed-Price (Lump-Sum) Contracts

You and the contractor agree on a total price for a defined scope of work before construction starts. The contractor absorbs the risk of cost overruns on things within their control (labor inefficiency, material waste), while you get budget certainty.

Pro: Predictable total cost — you know your number going in
Pro: The contractor is incentivized to manage the job efficiently, since savings benefit them (within an ethical, transparent process)
Con: Requires a detailed, finalized scope before signing — vague plans lead to expensive change orders later
Con: Contractors may build in more contingency into the price to protect themselves from unknowns, especially on older buildings

Cost-Plus Contracts

You pay the actual cost of labor and materials, plus a contractor fee (often 15–25% of costs, or a fixed fee). You see the real invoices; the contractor's margin is transparent.

Pro: Full transparency into actual costs — you're not paying for a padded contingency you never used
Pro: Works well when scope can't be fully defined upfront (older homes with unknown conditions behind walls)
Con: No cost certainty — your total isn't locked until the job is done
Con: Requires trusting the contractor's cost reporting; ask for a "guaranteed maximum price" (GMP) version, which caps your total exposure while keeping the transparency

Which One Makes Sense for Your Project

Situation	Better Fit
Well-defined scope, finished design	Fixed-price
Older home, significant unknowns behind walls	Cost-plus with a GMP cap
Fire or disaster rebuild with insurance involvement	Cost-plus (insurance carriers often prefer itemized actual costs)
Fixed budget that can't flex	Fixed-price

Whichever structure you choose, insist on a written change-order process — how additional costs get approved and documented — since that's where budget surprises actually happen, regardless of contract type.

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