To do that, they usually rely on comparables, or the price that homes similar to yours sold for within the last year. This will take into account location, how many bedrooms and bathrooms the home has, and any other amenities it might include, such as a fenced-in yard or an accessory dwelling unit. If you’ve added a backyard shed or sunroom to the home in the last year, you’ll see that reflected in the estimate.
Source: https://www.washingtonpost.com/home/2021/10/30/appeal-your-property-tax-assessment/