The contract structure chosen at the start of a custom build does more to determine the project's financial trajectory than almost any other early decision. It sets the incentive structure for both owner and contractor. It defines what flexibility exists for design evolution. It establishes who carries the risk of cost variation, and on what terms.
Most owners come to this decision with strong impressions but limited information. Cost-plus is sometimes characterized as a contractor's dream and an owner's exposure. Stipulated sum is sometimes characterized as the safer choice, full stop. Neither characterization is correct.
The question is not which contract is better. The question is which contract is right for the specific project, the specific owner, and the specific scope.
The Two Models, Plainly
Cost-Plus
The owner reimburses the contractor for the actual costs of the work, plus a fee for the contractor's management, overhead, and profit. The fee is most commonly a percentage of project costs (typically 15-20% on residential custom work), though fixed-fee and hybrid structures exist. The books are open: the owner has the right to see every invoice, every payment, and every accounting record that supports the costs being billed.
The contractor's profit comes from the management fee, not from the spread between estimated cost and actual cost. The owner carries the risk of cost variation directly. The contractor carries the risk of executing the work without overrunning the schedule.
Stipulated Sum
The owner agrees to pay a fixed total price for a defined scope of work. The contractor is responsible for delivering that scope at that price. If the contractor's actual costs come in below the contract price, the contractor keeps the difference as profit. If actual costs exceed the contract price, the contractor absorbs the loss.
The books are typically closed. The owner sees what they agreed to pay; the contractor's internal cost structure is the contractor's business. The contractor carries the risk of cost variation. The owner carries the risk that the scope they defined will not match what they actually want by the end of the project.
When Cost-Plus Works
Cost-plus is the right contract when several conditions converge:
| Condition | Why It Matters |
|---|---|
| Scope Will Evolve | Custom builds often refine specifications as construction progresses. Cost-plus accommodates this without constant change-order negotiation. |
| Quality Drives the Decision | The contractor's fee is fixed; quality decisions do not affect their margin. Removes incentive to substitute or compress. |
| Custom Finishes & Heritage | When scopes cannot be priced with confidence at contract execution, cost-plus prevents the contractor from padding to absorb the unknown. |
| Audit-Comfortable Owners | Cost-plus requires owner attention. The books must be read. Owners who are comfortable with this discipline capture the model's full benefit. |
| Strong Existing Relationship | Cost-plus rewards trust and punishes its absence. An owner with independent representation can extract real value from the model. |
The pattern across these conditions is consistent. Cost-plus works when the owner is positioned to engage with the project's financial reality, either personally or through representation. It does not work when the owner expects to write cheques against summaries without examining the underlying documentation.
When Stipulated Sum Works
Stipulated sum is the right contract when a different set of conditions hold:
| Condition | Why It Matters |
|---|---|
| Scope Is Fully Defined | When drawings are complete and the work can be priced with confidence by competing contractors, stipulated sum produces price discovery and locks in cost certainty. |
| Budget Drives the Decision | When the primary constraint is total project cost (financing limits, hard caps), stipulated sum provides the certainty cost-plus cannot. |
| Financing Constraints | Construction lenders often require fixed-price contracts for residential financing. The contract type is a function of the financing. |
| Schedule Certainty Matters | Stipulated sum contracts typically include schedule commitments with liquidated damages. Cost-plus rarely does. |
| Limited Owner Bandwidth | Owners who cannot or will not invest the time in cost-plus oversight are better served by stipulated sum, even at higher total cost. |
Stipulated sum works when the owner needs a contract that protects them from variability they cannot manage themselves. The contractor's risk premium, built into the price, is the cost of buying that protection.
The Management Fee Question
This is the conversation most contractor-side material avoids. We will not.
On a cost-plus residential custom build in Quebec or Ontario, management fees typically range from 15% to 20% of project costs. The fee covers the contractor's project management time, site supervision, office overhead, financial management, contract administration, insurance, and profit. The percentage is not arbitrary. It reflects the actual cost of running a competent custom construction business.
Where the Number Comes From
A custom residential build of meaningful scale ($1.5M-$3M in construction value) requires, conservatively, the equivalent of one full-time project manager for 12-18 months, plus part-time site supervision, plus office support for accounts payable, draw production, change orders, and client communication. The fully-loaded cost of that team, including overhead and a reasonable profit margin, lands in the 15-20% range.
Contractors operating below that range are typically: cross-subsidizing from other projects, understaffing the management function, or compensating through markups on materials and subtrades that may not be transparent. None of these is a sustainable model. Owners who pressure for fees below the market range are often pressuring the contractor toward one of these compensating mechanisms, the consequences of which the owner will absorb anyway.
What the Fee Covers
Project management time and site supervision. Pre-construction planning, scheduling, sequencing. Subcontractor procurement, contract administration, payment management. Financial management - draw production, accounts payable, holdback tracking, reconciliation. Change order pricing, negotiation, documentation. Client communication, design coordination, decision facilitation. Office overhead - insurance, software, administrative staff, financing costs. And profit.
What the Fee Does Not Cover
Direct project costs (labour, materials, subcontracts) - these are reimbursed separately at cost. Site-specific costs (temporary services, fencing, equipment rental) - typically reimbursed at cost or stated markup. Permitting fees and professional fees - typically pass-through items.
"A contractor accepting a 10% fee on a project that requires 17% to deliver well is a contractor who must, structurally, recover the difference somewhere. The recovery typically appears in three places: padded subcontractor pricing, inflated material costs, and reduced supervision. The cheapest fee is rarely the cheapest project."
The Hybrid: Stipulated Sum with Allowances
The third option, less commonly discussed, is a hybrid structure. The bulk of the project is priced on a stipulated sum basis. Specific scopes that cannot be priced confidently at contract execution are carved out and treated as allowances, billed at cost with a stated markup. Common allowance scopes include custom cabinetry, countertops, plumbing fixtures, lighting, landscaping, and any heritage or restoration work where conditions are unknown.
This structure captures benefits from both models. The stipulated sum portion provides cost certainty on the scopes that can be priced. The allowance portion accommodates the scopes where pricing is not realistic. The contract typically defines a budget for each allowance, with mechanics for handling overruns.
The hybrid works well when the project has clearly separable scopes - a defined construction scope plus a smaller, less defined finishing scope - and when both parties accept that allowance scopes will require a cost-plus mentality even within a stipulated sum contract. It does not work well when the allowance scopes are large enough to dominate the project, at which point the contract is functionally cost-plus with extra paperwork.
The Decision Test
The contract decision usually resolves with five questions:
01 Is the scope fully defined? If yes, stipulated sum is available. If no, cost-plus or hybrid is the realistic answer.
02 Does the financing structure require a fixed price? If yes, this is decisive. Stipulated sum or hybrid.
03 How important is cost certainty relative to quality? Certainty dominates: stipulated sum. Quality dominates: cost-plus.
04 Does the owner have bandwidth or representation to oversee an open-book project? If yes, cost-plus is viable. If no, stipulated sum protects from a model the owner cannot manage.
05 What is the relationship with the contractor? Trust high, relationship established: cost-plus rewards it. Unknown contractor or transactional relationship: stipulated sum's structural protections are more valuable.
The right contract is the one that matches the project's actual conditions. The wrong contract is the one chosen on price-of-fee alone, without examining the operating conditions that will determine whether either model can succeed.
LSPP Solutions advises owners on contract structure, contract review, and the negotiation of cost-plus, stipulated sum, and hybrid construction agreements.
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