You're over-assessed when the County's value is higher than what your home would actually sell for — and then you're over-paying every year until you appeal. Utah appeals to the County Board of Equalization are due September 15, or 45 days after a value-change notice (whichever is later), per Utah Code §59-2-1004. Utah is a NON-DISCLOSURE state — sale prices are not public — so the over-assessment-via-sale check cannot run; argue from comparable assessed values and any appraisal you commission.
Honest null: Salt Lake County is in a non-disclosure state — sale prices aren't public — so no service (including us) can prove your over-assessment from a sale comparison. Instead, argue from comparable assessed values (the equity argument) and any independent appraisal. The deadline above still applies and is the thing not to miss.
Most owners who win use a contingency property-tax appeal firm — it costs nothing unless they cut your bill, then takes a share of the first-year saving. About 95% of over-assessed owners never appeal, so the saving is usually left on the table.
Open the official Salt Lake County appeal page → Confirm the official deadline →against the official Salt Lake County / UT deadline (saltlakecounty.gov · Utah Code §59-2-1004). These are public deadline facts, not legal or tax advice — confirm your exact date with the County Board of Equalization and consult a professional before filing.
More for Salt Lake County: Salt Lake appeal deadline · how to lower Salt Lake taxes.