What an Over-Assessment Actually Costs You Per Year (Ontario)

An over-assessment sounds abstract until you convert it into dollars. Then it stops being abstract very quickly, because it isn't a one-time fee — it's a recurring annual charge that continues until either the province reassesses Ontario (no date announced) or you correct it.

This is the math, start to finish, using numbers you can pull off your own tax bill in two minutes.

The formula

Annual overpayment = (your assessment − fair assessment) × your effective tax rate

Two inputs, both easy to get:

Your effective tax rate. Ignore published rate tables — they vary by city, change annually, and often mix municipal and education portions inconsistently. Your own bill is the ground truth: take your total annual property tax and divide it by your assessed value. If you paid $5,480 on a $560,000 assessment, your effective rate is 0.98%. That one number captures your municipal rate, education levy, and any area charges, exactly as they apply to you.

Your assessment gap. This comes from comparing your assessment against genuinely similar nearby properties — the method is in our guide on how to tell if your MPAC assessment is too high. If your comparables cluster around $505,000 and you're assessed at $560,000, your gap is roughly $55,000.

Worked examples at different rates

Ontario's effective residential rates roughly span 0.7% (Toronto, with its huge tax base) to 1.5%+ (smaller cities like London). The same over-assessment costs very different amounts depending on where you live:

Over-assessed by

At ~0.7% (Toronto-like)

At ~1.0% (Ottawa/Peel-like)

At ~1.4% (London-like)

$30,000

~$210/yr

~$300/yr

~$420/yr

$55,000

~$385/yr

~$550/yr

~$770/yr

$80,000

~$560/yr

~$800/yr

~$1,120/yr

Two things jump out. First, a given percentage error hurts more in higher-rate cities — a 10% over-assessment in London costs roughly double what it costs in Toronto. Second, even the modest cases clear a few hundred dollars a year, which brings us to the real multiplier.

The freeze is the multiplier

Ontario hasn't run a province-wide reassessment since the January 1, 2016 valuation date, and none is scheduled. An over-assessment doesn't reset annually — it repeats. If your property came out of the 2016 model assessed $55,000 high at a 1.0% effective rate:

You've already overpaid roughly $550 × the number of years you've owned it since 2017 — potentially $5,000+ by now.

Every further year of the freeze adds another ~$550.

Past overpayments are generally not refundable through the standard review process. A Request for Reconsideration corrects your assessment going forward for the tax year in question — it doesn't claw back the last eight years. (Narrow municipal refund routes exist for gross clerical errors; see below.)

That last point is the strongest argument for checking now rather than someday. Every year you don't check an over-assessed property is money that's simply gone.

What correcting it is worth

Flip the math around. A successful review that removes a $55,000 over-assessment at 1.0% is worth:

~$550 in the first year, and

~$550 every subsequent year until the next reassessment, and

a correct baseline entering that reassessment, whenever it comes — the new model builds on accurate property details, not the old errors.

Against that, the cost side: filing an RfR with MPAC is free, and the evidence work — verifying your file, building a comparable table — is an evening. Even valuing your time aggressively, a genuine 10% gap pays for itself many times over in year one alone.

When the math says don't bother

Honesty cuts both ways. Run your numbers and skip the process if:

The gap is small. A $15,000 gap at 0.8% is $120/year. Mass appraisal has tolerance; so should you.

The gap is explainable. If your comparables lack your finished basement or premium lot, the "gap" may just be your property being worth more.

Your details check out and comps are genuinely mixed. If some similar homes are assessed above you and some below, you're inside normal model noise.

And one caution that belongs in every honest version of this article: verify your own recorded details before filing anything. If MPAC's file is missing an improvement — say, a basement you finished without a record — a review can theoretically move your assessment up, not down. Know what your file says first.

Frequently asked questions

Can I get back the years I already overpaid?

Generally no — the RfR process corrects the assessment for the current tax year forward. Municipalities do have narrow application processes (under the Municipal Act) for cancelling or refunding taxes in specific situations like gross and manifest clerical errors, typically limited to the current year and a short lookback with strict deadlines. Those are the exception, not the rule: for ordinary valuation inequity, the savings start when you fix it.

Does my mortgage lender's tax escrow change if I win?

Yes, eventually. Your municipality adjusts your bill after MPAC corrects the assessment, and your lender recalculates the monthly tax portion at its next escrow review. The savings are real either way — they just may take a cycle to show up in your monthly payment.

My effective rate calculation gives a different number than my city's published rate. Which is right?

Yours. The published headline rate often excludes education or area-specific levies. The bill you actually paid divided by your actual assessment is, by definition, what an assessment change is worth to you per dollar.

Is the $79 an appeal service charges worth it against these numbers?

Do the comparison yourself with your own gap and rate — that's the point of this article. For most genuinely over-assessed properties the annual savings are several multiples of any flat fee, and everything a service does can also be done free with your own time using the full DIY process. What you're ever paying for is compressed effort, not access.