Ontario's rule is the one most Canadians have heard of, and it is more useful than its reputation suggests. Under the province's consumer protection legislation, where a consumer agreement includes an estimate, the supplier cannot charge more than ten per cent above it — and if they do, the consumer "may require that the supplier provide the goods or services at the estimated price." That second half is the part that rarely gets quoted. It is not merely a ground for complaint; it is a right to hold the supplier to the estimate, set out in section 10 of the Consumer Protection Act, 2002.
The same section preserves the parties' ability to amend an estimate where the consumer requires additional or different work. That exception is precisely the door a tariff surcharge letter is trying to walk through, which is why what you sign next matters so much. Ontario has passed a newer consumer protection statute that will carry an equivalent ten per cent rule when it is proclaimed, but it is not yet in force, so the 2002 Act is the law that governs your renovation today.
Alberta takes the same idea and makes it considerably tighter. Under the province's Consumer Protection Act, it is an unfair practice to charge "more than 10%, to a maximum of $100, higher than the estimate given for those goods or services," unless the consumer expressly consented to the higher price before the work was supplied, or asked for additional or different work and agreed to amend the estimate. That wording, set out in section 6(2)(e) of the Act, is easy to skim past and startling once you do the arithmetic.
Because the allowance is capped at one hundred dollars, on a forty thousand dollar renovation the Alberta ceiling is one hundred dollars over the estimate — not four thousand. Ten per cent is the headline, but the dollar figure is what actually binds on any project of real size. An Alberta homeowner and an Ontario homeowner can hold the same signed estimate, face the same surtax on the same imported plywood, and be standing on quite different ground.