Mass Appraisal Process & Reports


Understanding Mass Appraisal

The Residential Division of the Snohomish County Assessor's Office is responsible for appraising a variety of property types including land, single family residences, manufactured homes, duplexes, and triplexes in the residential neighborhoods.

The county uses a "mass appraisal" process to appraise the more than 254,000 residential properties each year. "Mass appraisal" is the processes of valuing large numbers of properties as of a given date, using standard methods, employing common data, and allowing for statistical testing. Standard procedures are used to collect property data, analyze data, apply the results of the analysis, and report the results. Computer programs apply market derived land rates by neighborhood and property type. Computer programs apply building costs and depreciation factors calibrated to local market conditions using sales data, by neighborhood, building style, grade of construction, and building condition.

Statistical methods are used in producing the appraised values and testing the appraisal results. Fairness and equity is maintained by comparing the appraised values to actual sales prices on a countywide basis, regional basis, and neighborhood by neighborhood, a process known as a "ratio study."

Summarize the Results

Mass appraisal reports summarize the results of the "ratio studies." The reports include a summary of the change in value from year to year, ratio study statistics (level of assessment and measures of uniformity), market changes over time (graphic), a map of the study area, a value change summary abstract (change by property group), and neighborhood profile statistics. Ratio performance statistics and histograms for all of the properties in the study area with a break out for the single family residences (the most predominate property type in the county) and in the neighborhood reports the sales used to produce the report are listed.