How do I challenge my property assessment?
File a request for reconsideration with the provincial assessment authority before the deadline on your notice. It is free. Your evidence is sold prices of comparable properties as of the legislated valuation date, not today's market.
Assessments are produced by mass appraisal and errors are common: wrong square footage, a finished basement you do not have, the wrong lot size, a comparison to a renovated house.
Start by requesting the property details the authority holds for your address and checking them line by line. A factual error is the easiest correction to win.
The substantive argument is that comparable properties sold for less than your assessed value as of the legislated valuation date. That date is fixed by statute and is not today, so recent sales are only relevant if they support value as of that date.
The request for reconsideration is free and has a deadline printed on your notice. If it fails you can appeal to the provincial assessment review board, which charges a fee.
Worth doing because it compounds: an over-assessment costs you every year until it is corrected, and a successful correction can also be applied to prior years in some circumstances.
Next questions
Are property taxes different between GTA cities?
Substantially. Each municipality sets its own rate against the same provincial assessment, and the spread across the GTA is wide enough to change your monthly carrying cost by hundreds of dollars on the same price.
Ontario
How do property tax bills work after I buy?
Settle two things in your first month: what your municipality's own payment schedule is, and whether your lender is collecting property tax with your mortgage payment. Either you pay the municipality directly or your lender remits it, and assuming wrong creates arrears.
Ontario