Supplementary assessments on new Ontario homes

You closed on a new home, moved in, and started paying property tax on your bills. Months later, an envelope from MPAC arrives, followed by a tax bill that seems to go back in time. If this is you, you're not alone, and there's a reason for it.

Why it happens

When a home is newly built, the property on the assessment roll is often still the vacant lot, or a lot with a partly built house. The tax you pay at first is based on that. Once the house is finished and occupied, MPAC adds the value of the house to the roll part-way through the year. That addition is a supplementary assessment. If something should have been on the roll for an earlier period but was missed, MPAC can also issue an omitted assessment.

Your municipality then sends a supplementary tax bill for the difference, usually back to the date the house was finished or occupied. Because the paperwork takes time, the bill can arrive many months after you moved in and cover a long stretch at once.

MPAC reported assessing about 81,000 new homes in 2025, so these notices are a routine part of buying new in Ontario. They're still a shock if nobody warned you.

Is the supplementary bill a mistake?

Usually not. The tax is real, because you now own a finished house, not a lot. What can be wrong is the value MPAC assigned, or the date it applied from. Those are what you can question.

How to read a supplementary notice

Look for:

• The issuance date. This starts your window to ask MPAC to reconsider. It's the date printed on the notice, not the day it arrived.
• The assessed value added for the house.
• The period the assessment covers.
• The property details: floor area, storeys, basement, garage, lot.

Your window: 120 days from the issuance date

To ask MPAC to reconsider a supplementary or omitted assessment, file a Request for Reconsideration (RFR) within 120 days after the issuance date on the notice. A deadline that lands on a weekend or holiday moves to the next business day. Source: Assessment Act, R.S.O. 1990, c. A.31, s. 39.1(3) (notice under s. 35(1) for omitted/supplementary assessment: 120 days after the issuance date on the notice)

For a home, the RFR has to come first. You can appeal to the Assessment Review Board only after MPAC has decided your RFR, or failed to decide in time. Source: Assessment Act, R.S.O. 1990, c. A.31, s. 40(3), 40(5) para 1-2; s. 39.1(8) (MPAC notice for omitted/supplementary RFR due 180 days after the request — no extension)

How to tell whether the value is right

New subdivisions are a good place to check, because your neighbours' homes were often built by the same builder, at the same time, to similar plans. For new Ontario homes, we compare with homes in the same subdivision built within 2 years of yours. If your model is assessed noticeably higher per square foot than the same model two doors down, that's worth asking about. So is a floor area that doesn't match your builder's plans.

If there's an error in the facts

If MPAC's facts are wrong, for example an unfinished basement recorded as finished, include that in your RFR with proof: builder plans, the agreement of purchase and sale, photos. There's also a separate municipal route for errors in a supplementary assessment. An application under section 358 of the Municipal Act can be made up to December 31 of the second year after the assessment was made. Source: Municipal Act, 2001, S.O. 2001, c. 25, s. 358(1)(b), (3.1) (error in an assessment under s. 33 or 34 of the Assessment Act: on or before December 31 of the second year following the year in which the assessment was made); Legislation Act, 2006, s. 89(1)

Paying the bill in the meantime

Asking MPAC to reconsider doesn't pause the tax bill. If you're struggling with a large retroactive amount, ask your municipality about payment arrangements. If the value comes down later, the municipality adjusts your account. Any tax refund or credit goes to you, the owner, not to us.

Why we're careful in Ontario

An issue that has been settled or decided generally can't be argued again later in the same assessment cycle, so we only file when the case is strong.

You may be fairly assessed. If you are, we'll tell you.