The Regressive Map

A property assessment system can appear accurate overall and still systematically over-assess lower-value homes. Nationwide research based on millions of residential transactions has found widespread evidence of this pattern — known as assessment regressivity. In the average jurisdiction studied, homes toward the bottom of the price distribution were assessed at substantially higher ratios of assessed value to market value than homes toward the top. The important question for local government is therefore not simply "are our assessments accurate?" but "are they equally accurate across property values, neighborhoods and market areas?"

Assessment inequity rarely appears as a single jurisdiction-wide number. It shows up in patterns tied to neighborhoods, market areas, property characteristics, reassessment cycles, model performance, sales data and appeal behavior — which is where geography becomes decisive. This report sets out the mechanism, the independent evidence for its scale, one documented reform (Cook County) and one contested case (Detroit), the IAAO measures that already exist (COD, PRD, PRB), and a four-quarter uniformity program a jurisdiction can start with the sales data it already holds.