By law, the assessor’s value on your property is presumed to be correct. That presumption can be overcome only by the presentation of clear, cogent, and convincing evidence, which is evidence showing that it is highly probable that the assessor made a mistake. To prove your case, present sales of properties that are similar to yours. The sales should be as close to the assessment date as possible.
You can find data on property sales at your county assessor’s website, or by consulting local real estate agents. Organize the data you find about the comparable sales you intend to present by using the Board of Tax Appeal’s Sales Grid.