By Rahul Kumar, HCRA-Licensed Custom Home Builder & Real Estate Broker · Golden Brick Homes · August 2026 · 10-minute read
One of the first questions I get from clients once we’ve found a lot and started talking about what to build is: “What kind of contract should I be signing with my builder?”
And I get it — you’ve just committed to a significant piece of land in South Oakville. You want to know what this project is going to cost before you start, not twelve months in when the invoices have outrun the original quote by $300,000.
The honest answer is that neither fixed price nor cost-plus automatically protects you. What protects you is understanding how each model works, where each one hides risk, and what the contract actually says — because the label on the document and the commercial reality of the deal are often very different things.
Here’s what I’ve learned from being on both sides of this: as a builder, and as a broker who has helped clients navigate the process from lot acquisition through to move-in.
How a fixed-price contract works
In a fixed-price (also called “lump sum”) contract, you and your builder agree on a total cost before construction begins. In theory, that’s your number. The builder carries the risk that materials cost more than they estimated, that trade labour runs over, that they misread the scope. If they get it wrong, that’s their problem — not yours.
In practice, it’s more complicated than that.
A fixed-price contract is only as certain as its scope definition. If the design isn’t complete, the specs aren’t locked, and the selections aren’t finalized when you sign — and on a custom home, they rarely are — then every gap in the scope becomes a future change order. And change orders on a fixed-price contract are where budgets go to die.
Builders know this. A builder quoting a fixed price on an incomplete design will almost always protect themselves with one or both of the following:
- Inflated allowances — placeholder numbers for items not yet selected (more on this below)
- Broad exclusions — line items simply left out of the contract, to be priced later
The opening number looks reassuring. The final number — after change orders, allowance overruns, and excluded-item surprises — is a different conversation.
When fixed-price makes sense
Fixed-price works well when drawings are substantially complete, selections are made or nearly made, site conditions are understood, and you need budget certainty for financing. It lets you compare builders on an apples-to-apples basis, and it gives your lender a number they can work with for construction mortgage purposes.
How a cost-plus contract works
In a cost-plus contract, you pay the builder’s actual costs — labour, materials, subcontractors, permits, site expenses — plus a fee. That fee is either a fixed dollar amount or a percentage of total costs, typically ranging from 10% to 20% depending on the builder and scope.
The upside: you see exactly where the money is going. The builder isn’t padding estimates to protect their margin, because they’re billing you their real costs. And if the design evolves during the build — which on a genuinely custom home it often does — you’re not triggering a change-order war every time you decide to move a wall.
The downside: you don’t know what the final number is until the project is done. And the incentive structure is backwards — the more the build costs, the more the builder earns in fees, if the fee is percentage-based. You need a builder you trust absolutely, and you need to be actively engaged in reviewing monthly cost reports.
The hidden risk most homeowners miss
The biggest danger in cost-plus isn’t overspending — it’s an undefined “cost of work.” If the contract doesn’t explicitly state what counts as a reimbursable cost and what doesn’t, you can end up paying:
- Builder markup on subcontractor prices that already include the trade’s own overhead and profit
- Fee calculated on the HST-inclusive amount rather than pre-tax cost
- Site supervision billed hourly as a separate cost item, while the fee nominally already covers “management”
- Corrections and rework caused by builder error
As a practical rule: if a cost-plus quote comes in significantly lower than a fixed-price quote for the same project, the gap almost certainly lives in missing or underestimated line items that will be billed to you once the project starts.
When cost-plus makes sense
Cost-plus is the right structure when the design will genuinely evolve, site conditions are uncertain, and you have the bandwidth to monitor monthly cost forecasts actively. It’s also more honest about what a complex custom build actually involves — reality often doesn’t fit neatly into a fixed-price box.
The allowance problem
Whether your contract is fixed or cost-plus, allowances are where most South Oakville custom home budgets quietly collapse. Understanding them is non-negotiable.
An allowance is a budget placeholder for an item that hasn’t been fully selected or specified at the time of signing. Common allowance categories include kitchen cabinetry and millwork, countertops, flooring, tile, plumbing fixtures and faucets, lighting, appliances, and cabinet and door hardware.
The problem is almost always the number. A builder working to win your contract may set a flooring allowance at $8 per square foot installed. If your taste runs to engineered hardwood at $16 to $22 installed — and on a 5,000 square foot home in South Oakville it usually does — that allowance is off by $40,000 to $70,000 before you’ve picked a single board.
Multiply that across eight allowance categories and you have found your “budget overrun.”
What to check before you sign
Every allowance in your contract should specify:
- Quantity — square footage, fixture count, linear feet. Not just a lump dollar figure.
- Type — supply-only, or installed with labour included.
- Product standard — “mid-range” means nothing. Ask for a specific product example at the stated budget.
- Markup on overruns — if your selection costs more than the allowance, what percentage does the builder add to the difference?
- Credit mechanism — if you come in under the allowance, does the saving flow back to you in full?
If your contract can’t answer all five of those questions for each allowance line, you don’t yet know what you’re signing.
Change orders: the other budget killer
Every project has change orders. On a custom home, the question isn’t whether you’ll have them — it’s whether they’re disciplined.
A change order is any modification to the original scope: a window moved, a ceiling height increased, an extra circuit added, a kitchen layout revised after framing has started. On a fixed-price contract, each change alters the contract price and the schedule. On a cost-plus contract, the actual cost plus fee applies to whatever was added or changed.
What kills budgets isn’t any individual change order — it’s the cumulative effect of a dozen small ones, none of which felt significant at the time.
The discipline that protects you
Regardless of contract type, insist on this rule: no work proceeds on any change until it is described in writing, priced in writing, and approved by you in writing. No verbal approvals, no “we’ll sort it out later,” no “it’s just a small thing.” A builder who pushes back on this discipline is a builder whose change-order process needs scrutiny.
Also watch for change-order pricing that doesn’t reflect the original contract rate. If your fixed-price contract used $150 an hour for carpenter labour and change orders suddenly come in at $195, that’s a contract discussion worth having before work starts.
The hybrid approach
In practice, many well-structured custom home contracts are hybrids — and for good reason. On our builds the approach is to be clear about what’s known and what isn’t, and price accordingly:
- Fixed price on defined scope — the structure, the mechanicals, and anything with firm specs and drawings.
- Unit pricing on variable quantities — excavation and grading, where soil conditions dictate depth and haul, priced per cubic metre rather than a lump sum that either over- or under-estimates reality.
- Owner-directed selections handled transparently — you choose, the bill reflects your choice, and the markup is clearly stated in advance.
This structure gives you predictability where the scope is knowable, and honesty where it isn’t.
Ontario legal context: what the rules say
A few Ontario-specific points worth knowing before you sign anything.
The 10% holdback. Ontario’s Construction Act requires a statutory 10% holdback on every progress payment, retained until the lien period expires. This applies to both fixed-price and cost-plus contracts. As of January 2026, annual holdback release rules and new notice requirements are in effect — make sure your contract addresses these correctly, or they can create payment disputes mid-build. The same holdback shapes how your lender releases funds: see how a construction mortgage actually works.
Prompt payment rules. Contractors must receive payment for proper invoices within 28 days. Owners must issue a prescribed non-payment notice within 14 days if disputing an invoice. These rules apply regardless of contract type.
The Consumer Protection Act. In some qualifying consumer agreements — and not all custom home contracts automatically fall under this — a contractor cannot charge more than 10% above a written estimate without written authorization for the additional work. Whether it applies to your specific contract is a legal question. Confirm with your real estate lawyer before signing.
The questions to ask any builder before signing
Whether you’re evaluating a fixed-price or a cost-plus contract, these questions cut to what actually matters:
- What drawings and specifications were used to produce this number, and are they attached to the contract?
- What is explicitly excluded? Ask for the exclusions list in writing — every builder has one.
- What are the allowance amounts, and what exactly do they cover: supply-only, or installed?
- What is the change-order process, and what pricing rate applies to it?
- How is the fee calculated — pre-tax or post-tax? Does it apply to subcontractor markups?
- How are discounts, rebates and returned materials handled? Do they reduce my cost?
- How often will I receive a budget-versus-actual cost report?
- What are my audit rights on cost-plus invoices?
A builder who can’t or won’t answer these questions clearly before you sign is telling you something important.
What this looks like in practice: 546 Fourth Line
Our current project — a 5,300 square foot custom home with pool, cabana and elevator on a 60 by 272 foot lot in South Oakville — is a useful illustration.
A home of this complexity, with a highly customized finish program, doesn’t fit neatly into a pure fixed-price box. Some scope elements are precisely defined and priced accordingly. Others — particularly the interior finish selections, which are still being refined by the client — are structured with realistic allowances at a finish level appropriate for the project, with clear overage and credit terms. The client knows exactly what they’re signing up for in each category before we’ve broken ground.
That transparency is the standard every custom home contract should meet.
The bottom line
Neither fixed price nor cost-plus is inherently safer. What matters is the quality of the scope definition behind the number, the realism of the allowances, the discipline of the change-order process, and the clarity of the fee calculation.
A thoughtful hybrid contract, reviewed by your lawyer, with a builder you’ve vetted and trust, is worth more than any label on the cover page.
And the number the contract is built on has to be right in the first place. If you haven’t modelled the real all-in cost yet, start with what it actually costs to build a custom home in Oakville — and check whether the HST rebate applies to you before you count it in.
Free contract read
Send me the contract structure. I’ll tell you what it protects.
Fixed price and cost plus each shift risk somewhere, and the detail that matters is usually buried in the allowances, the change order clause and the holdback language. Tell me what you have been offered and I will tell you plainly what it does and does not cover.
You do not have to send the document itself if you would rather not. Describing the structure and the payment schedule is usually enough for a useful answer.
Rahul Kumar — Broker, Royal LePage Certified Realty · HCRA-Licensed Custom Home Builder, Golden Brick Homes. General information, not legal advice. Have your real estate lawyer review any construction contract before you sign.
And before you compare two contracts at all, be careful comparing the headline rates attached to them. Cost per square foot hides more than it reveals — including whether the basement and garage were counted.
Frequently asked questions
What is the difference between a fixed-price and a cost-plus building contract?
In a fixed-price or lump-sum contract you and your builder agree a total cost before construction begins, and the builder carries the risk of materials or labour costing more than estimated. In a cost-plus contract you pay the builder’s actual costs — labour, materials, subcontractors, permits and site expenses — plus a fee, which is either a fixed dollar amount or a percentage of total costs, typically 10% to 20%.
Which contract is safer for a custom home?
Neither is inherently safer. A fixed price is only as certain as its scope definition, and on an incomplete design the gaps become change orders. Cost-plus is transparent but you do not know the final number until the project is done. What protects you is the quality of the scope definition, the realism of the allowances, the discipline of the change-order process and the clarity of the fee calculation.
What is an allowance in a building contract?
An allowance is a budget placeholder for an item that has not been fully selected or specified when you sign — typically cabinetry, countertops, flooring, tile, plumbing fixtures, lighting, appliances and hardware. Allowances are where most custom home budgets quietly collapse, because the placeholder figure is often set well below what the owner actually chooses.
What should every allowance in my contract specify?
Five things: the quantity in square feet, fixture count or linear feet rather than a lump dollar figure; whether it is supply-only or installed; a specific product example at the stated budget rather than a vague standard like “mid-range”; the markup the builder applies to any overrun; and whether a saving flows back to you in full if you come in under.
What is a typical cost-plus builder fee in Ontario?
The fee is either a fixed dollar amount or a percentage of total costs, typically ranging from 10% to 20% depending on the builder and the scope. What matters as much as the percentage is how it is calculated — whether it applies to pre-tax or HST-inclusive amounts, and whether it is charged on subcontractor prices that already include the trade’s own overhead and profit.
What is the 10% holdback on an Ontario construction contract?
Ontario’s Construction Act requires a statutory 10% holdback on every progress payment, retained until the lien period expires. It applies to both fixed-price and cost-plus contracts. As of January 2026, annual holdback release rules and new notice requirements are in effect, and a contract that does not address them correctly can create payment disputes mid-build.
What is a hybrid construction contract?
A hybrid contract prices each part of the project according to how well it is known: a fixed price on defined scope such as structure and mechanicals, unit pricing on variable quantities such as excavation and grading where soil conditions dictate depth and haul, and owner-directed selections handled transparently with the markup stated in advance.
Still at the lot-finding stage?
If you’re wondering whether your lot can support the home you’re imagining — or what a contract on it should realistically look like — that’s where this conversation starts. I hold both licences: a licensed broker and an HCRA-licensed, Tarion-backed builder. You can verify any of that yourself — here is how to check if a builder is licensed in Ontario, and the two licences a contract home actually requires.
Book a free 30-minute consultationCall or text (647) 772-1642
Related reading: find your lot and build · building lots in South Oakville · selling a lot in Oakville · what a custom home really costs all-in · how a construction mortgage works · the $130,000 HST rebate.
This article is general information about how construction contracts are structured. It is not legal advice, and I am not a lawyer. Contract terms, construction law and consumer protections depend on the specific language of your agreement. Have your real estate lawyer review any construction contract before you sign it. Rahul Kumar — Broker, Royal LePage Certified Realty, and principal of Golden Brick Homes, an HCRA-licensed and Tarion-backed builder in Oakville.
