Harris County Appraisal District (HCAD) values are much less reliable for older homes than for homes built after 1990. For homes built in 1990 or later, HCAD is typically within 8% of market value. However, the error rate is 20% for homes built before 1960, 18% for homes built before 1970, and 15% for homes built before 1990. Including all homes, HCAD’s valuation error rate is typically 14%. HCAD’s value is off by 20% or more for about 15% of all homes in Harris County. HOMES IN HARRIS COUNTY Year Built Typical HCAD Error Rate Before 1960 20% Before 1970 18 Before 1990 15 1990 or later 8% Harris County Appraisal District uses a process termed mass appraisal to value the 1.7 million tax parcels in Harris County. A licensed appraiser doing an appraisal for a home is usually accurate within 5 to 10%. Appraisal is not an exact science; it involves much judgment. Further, there is limited information available regarding the subject property and comparable sales even w...