The assessment is the assessor's opinion of value typically as of January of the year of the assessment. Each municipality decides how often to re-access, although the state has some requirements that attempt to not allow them to fall too far out of line with current values. While prices were rising assessments were usually too low. As prices have come down, assessments have frequently been high, although somewhat distorted by the distressed sales. Most large cities use mass appraisals which have information about every property entered into a large data base. This mass appraisal automatically adjusts for differences between properties. Mass appraisals are not very good at evaluating condition or motivation in sales and therefore have a higher margin of error compared to an actual appraisal or broker's opinion. If you look at 2 properties in similar condition, in the same municipality, the assessment will usually do a reasonable job, showing the difference in value between the 2 properties which is the goal of the assessor. People in more expensive properties pay higher taxes. At the same time if one of those people is highly motivated, they may accept a lower price. You will rarely find 2 people with the exact some opinion of value or how to get there. It never hurts to get a second opinion, after all to error is human.