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. Texas protest deadline is May 15, or 30 days after the appraisal district mails your Notice of Appraised Value (whichever is later), per Tex. Tax Code §41.44. Texas is a NON-DISCLOSURE state — sale prices are not public — so the over-assessment-via-sale check cannot run; use comparable appraised values and the equity argument (§41.43(b)(3)).
Honest null: Collin 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 Collin County appeal page → Confirm the official deadline →against the official Collin County / TX deadline (collincad.org · Tex. Tax Code §41.44). These are public deadline facts, not legal or tax advice — confirm your exact date with the Appraisal Review Board (ARB) and consult a professional before filing.
More for Collin County: Collin appeal deadline · how to lower Collin taxes.