Fixed Price vs Cost-Plus Contract for Idaho Custom Homes

A fixed-price (or lump-sum) contract sets a total price for the completed home. The builder agrees to deliver the finished product for that price regardless of what it actually costs them to build it. Cost overruns are the builder's problem, not yours — as long as you haven't changed the scope of work.

For lenders, fixed-price contracts are straightforward to underwrite. The loan amount is based on the contract price plus soft costs and contingency. There's no ambiguity about what the project costs.

A cost-plus contract pays the builder for their actual costs — materials, labor, subcontractors — plus a fixed fee or percentage markup. You see exactly what everything costs, and the builder's profit is transparent. If materials come in under budget, you benefit. If they come in over, you pay more.

Cost-plus contracts are common in Idaho for high-end custom homes where the scope is evolving, or where the owner wants maximum transparency and control over material selections.

Fixed-price contracts are easier to finance. The lender knows the maximum project cost at closing, can set the loan amount accordingly, and the draw schedule aligns cleanly with the contract milestones. Most Idaho construction lenders prefer fixed-price contracts.

Cost-plus contracts require more documentation and ongoing monitoring. The lender needs to see detailed cost breakdowns, and the loan amount may need to be set conservatively to account for potential cost increases. Some lenders require a cost-plus contract to include a guaranteed maximum price (GMP) to provide a ceiling on the borrower's exposure.

Regardless of contract type, include these protections in your Idaho construction contract: