An assessment is the value placed on a property by the assessor’s office as a basis to determine the property tax due. The assessor typically does not complete an appraisal to determine this, nor are they bound by the same requirements that appraisers are. They usually do not do an interior inspection to take into account the condition of the property or to confirm measurements the 2nd or 3rd stories.
For example, if there is a 2 story foyer or family room, they would not know this and would inadvertently inflate the GLA. Assessments may often be outdated as well, since they are not typically done on a yearly basis.
An appraisal, on the other hand, is completed by a state licensed or certified appraiser and is reflective of the current market value of the property. The appraiser takes into account every detail of the entire property, inside and out, along with current market conditions and sales to determine a value. The assessed value is not a factor that is considered by an appraiser.