The Property-Tax Bill That Arrives After Closing: Why Commercial Buyers Need a Supplementary-Assessment Holdback
A newly built or renovated commercial property is taxed in two passes. At closing the building may not be on the tax roll yet — so a year or two later, MPAC catches up and a bill lands, backdated to the day you took title. If your agreement is silent, that bill is yours alone.
My commercial deal just closed. Why did a property-tax bill arrive a year later, backdated to the day I took title?
Because a newly built or renovated property is assessed in two passes, and the catch-up bill can reach backward. At closing your lawyer only prorated the tax bill that already existed; but MPAC frequently assesses the land first and the building later. Under Ontario’s Assessment Act, a supplementary assessment (s.34) covers the current year, and an omitted assessment (s.33) can reach back to the current year plus the two preceding years — up to three years of tax billed to you in one shot. MPAC issues these only when assessed value rises by 5% of the current assessment or by $10,000 or more — a bar a freshly completed commercial building almost always clears.
Sources: Ontario Assessment Act, s.33 (omitted) and s.34 (supplementary); Municipal Property Assessment Corporation (MPAC), Supplementary and Omitted Property Assessments (ontario.ca / mpac.ca, accessed July 2026)
I am Arthur Zhao. On the commercial side, the buyers who get blindsided by property tax are rarely the ones acquiring a stable older building with a clean, predictable bill. They are the ones buying something freshly built or substantially renovated.
The story usually runs like this. You close on a new commercial unit, your lawyer prorates the tax bill on the closing statement down to the day, and you assume the tax page is turned. Then a year — sometimes two — later, an MPAC catch-up bill arrives, backdated all the way to the day you took title. The reason: at closing the municipality was really only taxing the land. The building was not on the roll yet.
This is a different animal from the tax proration on your Statement of Adjustments, and I draw that line first. The real question is who pays this late-arriving bill, how big it can get, and why experienced buyers park a tax holdback in the agreement to cap the risk before it ever shows up.
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ℹ️This piece explains an evergreen Ontario assessment mechanism and how the risk is allocated in a contract, to help you see the problem before you negotiate. It is not tax or legal advice for any specific deal. The holdback amount and clause for any commercial closing should be drafted by your real estate lawyer against the actual property and municipality.
This is not the tax proration on your Statement of Adjustments
The proration your lawyer runs at closing handles a bill that already exists — the current year’s tax, split by the number of days each side owned the property. It divides a known number.
Supplementary and omitted assessments are a different thing entirely: at the moment of closing, the tax on the building has not been generated yet. MPAC assesses it later, the municipality bills it later, and that bill applies retroactively. The proration cannot capture it, because when the adjustment was calculated it did not exist. That is precisely why a standard statement of adjustments does not protect you here.
Why a new building gets taxed in two passes
Once you see the mechanism, you understand exactly what the holdback is guarding against. The key fact: land and building do not hit the tax roll at the same time.
Pass one: you are billed on the dirt
Pass two: the building appears (supplementary assessment, Assessment Act s.34)
The reach-back: omitted assessment (s.33, current year plus two prior years)
ℹ️Why hammer on the two-year reach-back? Because it defines the length of your risk window. The longer MPAC takes to assess a completed building, the more years stack into a single catch-up bill — so both the holdback amount and the long-stop date should be set to the worst case, not to an optimistic “it should be assessed by next year” assumption.
💡 The trigger (per MPAC): MPAC issues a supplementary or omitted assessment only when the increase in value hits 5% of the current assessment or $10,000 or more. That threshold might spare minor residential tweaks, but a commercial building rising from nothing carries hundreds of thousands — often millions — in structure value. It clears the 5% bar almost by definition. For new commercial construction, the catch-up bill is not a question of if, only of when, and how many years back.
Who eats the retroactive bill? Silence in the APS means the buyer does
Here is the pivot: the supplementary or omitted bill goes to whoever is the registered owner when it is issued — regardless of who held title during the years being reached back into. The bill often lands a year or two after closing, by which point the seller has taken their money and gone, possibly out of the country.
If the Agreement of Purchase and Sale says nothing, a bill that reaches back into the seller’s ownership — even into the construction period — lands squarely on the current owner: you. Want to chase the seller for their share? Without a contract clause, you have no lever. This is the gap that has to be closed in the agreement, before closing.
🚨The worst-case script: you left the holdback out of the APS, the bill arrives 18 months after closing reaching back into the construction period, it is six figures, and the seller has pulled their money, is out of the country, and will not return a call. You are the registered owner, so the tax is yours — and without a clause you do not even have a party to make a claim against. This is not a scare story; it is a recurring gap in new-construction commercial closings.
The fix: a supplementary-assessment holdback in the APS
The logic is simple: at closing you do not hand the seller the full price. A defined amount stays in your lawyer’s trust account, earmarked to absorb the late bill. When it arrives, it is paid from that fund; if it never arrives (or a deadline passes), the balance is released to the seller. Three steps I watch when I negotiate this clause.
Step one: write the holdback into the APS
Step two: size it
Step three: define the release — triggers and a long-stop date
⚠️Do not set the long-stop date short. Buyers often want to close things out and get the seller their balance, so they set the release a few months after closing — but s.33 lets an omitted assessment reach back the current year plus two, and MPAC’s reassessment routinely lags a year or more. Set the release too early and the funds are gone before the bill ever arrives, making the holdback a formality.
💡 The one-line framing of a holdback: it is not a price cut and it is not distrust of the seller. It is a timing mismatch — a bill generated after closing but reaching back before it — assigned by contract to the party that actually benefited during that period. Framed that way in negotiation, sellers accept it far more readily.
Two execution details buyers miss
First, supplementary and omitted bills are, per the City of Toronto, generally due in two installments and cannot be placed on the pre-authorized tax payment plan — so you cannot assume they will be quietly auto-debited like a regular bill. You have to watch the cash flow deliberately.
Second, the retained funds must sit in the lawyer’s trust account with a written purpose — not with the seller on a promise to reimburse you later. The bill often surfaces a year or two out, and by then the seller’s willingness and even reachability are entirely outside your control. Keeping the money with your own lawyer is the only version of this strategy that actually works.
✅With this built in, your position becomes: the bill arrives and it is paid from the retained funds, out of no extra pocket of yours; the bill never comes and the long-stop date passes, and the balance releases to the seller. The risk shifts from a landmine that detonates on some unknown date after closing to a priced, pre-defused line item in the contract — which is exactly the certainty the holdback is there to buy.
Frequently Asked Questions
Can this retroactive catch-up bill hit me on a resale (not new-build) commercial property too?
The risk is much lower, but not zero. Supplementary and omitted assessments mainly target properties with recent construction, additions, or major renovations that MPAC has not yet put on the roll. Buying an older building whose value has been stable for years, you almost never see a surprise bill. But if the seller recently added space, changed the use, or put idle area into service and it has not hit the roll, the same reach-back can be triggered — so whether the property was worked on recently is your first signal on whether a holdback is warranted.
Exactly how many years back can the bill reach?
Under Ontario’s Assessment Act, a supplementary assessment (s.34) covers the current year; an omitted assessment (s.33) can capture value left off the roll for the current year plus the two preceding tax years — so a single bill can reach back up to three years. The longer a completed building goes before MPAC assesses it, the more years stack up and the larger the one-time bill.
Is there a dollar threshold, or will any small change trigger a bill?
There is a threshold. Per MPAC, a supplementary or omitted assessment is issued only when the increase in assessed value is 5% of the current assessment or $10,000 or more. Small changes often fall below that line. But a commercial building rising from nothing adds hundreds of thousands in value and clears 5% almost by definition — so on new commercial construction, the bill is effectively a certainty.
How much should the tax holdback be?
There is no fixed figure; you size it to the property and the municipality. The method: multiply the building’s likely assessed value × your municipality’s effective commercial tax rate × the number of years the assessment can reach back (up to the current year plus two), then add a buffer. Because the reach-back runs up to three years and MPAC lags, the retained amount on a new commercial building is often five or six figures, not a token few thousand. Have your real estate lawyer run the real numbers against the actual rate and assessment basis.
Who holds the retained money at closing, and when does the seller get it?
It should be held by the buyer’s lawyer in a trust account, with its purpose written into the APS — dedicated to any supplementary or omitted tax arising for periods before the closing date, including the construction period. The usual release: when the bill arrives, it is paid straight from the retained funds and any surplus goes to the seller; if a defined long-stop date passes with no assessment, the full balance releases to the seller. Set that date long enough to cover MPAC’s reach-back and lag.
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