The 10% Rule: How Far a Contractor Can Legally Exceed a Written Estimate in Ontario
A written estimate is worth more than a number. It is worth a legal ceiling — if it is attached to the contract.
In short
Under Ontario’s Consumer Protection Act, if a written estimate is part of your contract, the final price cannot exceed it by more than 10 per cent — unless you agree to a new price and sign a change to the contract.
Charged more than that without agreeing? You can demand the price be adjusted back to the estimate. Not negotiated down. Adjusted.
The whole protection turns on four words most people skip: part of the contract. An estimate emailed separately, or scribbled on the back of a card, is a number. An estimate attached to the agreement is a ceiling.
What does 10% actually look like?
Work it out before you sign, because this is the number you will be arguing about if anything goes wrong.
| Written estimate | Legal ceiling | Anything above this |
|---|---|---|
| $1,000 | $1,100 | You may demand $1,000 |
| $4,500 | $4,950 | You may demand $4,500 |
| $8,000 | $8,800 | You may demand $8,000 |
| $15,000 | $16,500 | You may demand $15,000 |
| $30,000 | $33,000 | You may demand $30,000 |
On a $15,000 kitchen, the difference between knowing this and not knowing it is $1,500 — and quite often a good deal more, because a contractor who intends to overrun rarely stops at eleven per cent.
Why “part of the contract” is the whole sentence
Ontario’s own guidance puts it plainly: make sure that any written estimate you receive is part of the contract, so that the ten per cent rule will apply.
That is an instruction to the consumer, and it is the step almost everyone misses. The usual sequence is a detailed estimate by email, then a short contract that says something like “kitchen renovation as discussed” and a total. At that point the email is background reading and the contract is the agreement.
Fixing it takes one sentence. Ask for the estimate to be attached to and referenced in the contract — “as per attached written estimate dated 12 October” — and initial it. A contractor who objects to that has told you something useful for free.
When is a higher price legitimate?
When you asked for something different. The rule does not stop a consumer and a business agreeing to amend the estimate where you require additional or different goods or services — and it should not.
Renovation genuinely uncovers things. Rotten subfloor under old tile, knob-and-tube behind a wall, a drain that turns out to be lead. A contractor who finds one of those and stops to discuss it is doing the job properly.
The distinction that matters is when the conversation happens. A change agreed and signed before the extra work is done is an amended contract. The same change presented on the final invoice is not an amendment; it is a bill you did not agree to, and the ten per cent ceiling still applies to it.
What should a written estimate contain?
Enough detail that each line can be checked on its own. A single total is difficult to dispute later and impossible to compare against another quote today.
What to do if the final bill is too high
Work through it in order, and keep everything in writing from the first message.
Red flags
What to do next
Before you sign anything, ask one question: will the written estimate be attached to and referenced in the contract? That sentence is the difference between a number and a legal ceiling, and it costs nothing to ask for.
Then compare renovation and construction companies in Ontario on Ontario24 and get estimates from two or three. Ontario24 is a classified directory — we help you find and compare businesses, and we do not vet them; our guide to choosing a contractor and our safety tips cover what else to ask.
For the rest of your rights on that contract — the ten-day cooling-off period, the one-year window for a misleading sale, and what happens if the work never starts — read our guide to cancelling a contract in Ontario. If the job needs approval, our building permit guide covers whose job that is.
Before you call anyone, it helps to know what this job should cost and what the next one will. What things cost in Ontario in 2026 puts 75 service prices in one table.
Frequently asked questions
Ten per cent. If a written estimate forms part of your contract, the final price cannot exceed it by more than ten per cent unless you agree to a new price and sign a change to the contract. Charged more without agreeing, you may demand the price be adjusted.
The protection attaches to a written estimate that is part of the contract. That is why getting the estimate in writing, and making sure it is included in the agreement rather than sitting beside it, is the step that turns a number into a right.
You can file a complaint with the Ministry of Public and Business Service Delivery and Procurement, which enforces the Consumer Protection Act. Keep the written estimate, the contract, the invoice and any messages about changes — a complaint with documents attached is a different thing entirely.
Yes, and that is fair. The rule does not prevent a consumer and a business agreeing to amend the estimate where you require additional or different goods or services. What it prevents is the price moving past ten per cent without your agreement.
Enough detail that each line can be checked: labour, materials, permits, disposal and taxes shown separately rather than as one total. A single number with no breakdown is difficult to dispute later and difficult to compare against another quote today.
In everyday use people mix the terms, but what matters legally is what your contract says and whether the written estimate is part of it. Ask for the figure in writing and ask that it be attached to the agreement, whichever word the business uses.
Ontario’s guidance states that where there is a dispute over unclear or vague wording, the law will interpret it in favour of the consumer. That does not make a vague contract a good idea, but it does mean ambiguity is not automatically the business’s advantage.
Sources
The ten per cent rule, the requirement that the estimate form part of the contract, the right to demand adjustment, and the treatment of arbitration clauses and invented tax charges all come from the Government of Ontario’s guidance “Your rights when signing or cancelling a contract”, last updated 1 April 2025, under the Consumer Protection Act. Checked 17 September 2026. Ontario passed a new Consumer Protection Act in December 2023 that comes into force on proclamation — check the province’s current page before relying on any figure here. General information only, not legal advice. Ontario24 is a classified directory and is not a party to your contract.