What Is Factored Base Year Value
Factored Base Year Value is your property's assessed value from a specific base year, adjusted annually by an inflation factor capped at 2% per year. This calculation limits how much your assessment can increase annually, even if your property's actual market value rises faster. The factor applies to the original base year value, not to the previous year's factored amount, which is why it's called "factored" rather than compounded.
This mechanism exists in states like California (Proposition 13), Illinois, and others that implement assessment caps. Your property gets reassessed to full market value only when it transfers ownership or undergoes significant improvements. Until then, the factored calculation controls annual increases. For example, if your base year value was $400,000 in 2010 and the 2% factor has been applied consistently, your factored value in 2024 would be roughly $600,000, even if comparable sales show your property worth $850,000.
How It Affects Your Assessment
The factored value directly determines the amount subject to your property tax rate. If your factored base year value is $600,000 and your tax rate is 1.25%, you pay $7,500 annually. A neighboring property that sold recently might have a current assessed value of $800,000 on the same tax rate, paying $10,000 despite similar market conditions. This gap widens over time and explains why assessment appeals often target the base year value itself rather than the annual factor application.
When challenging your assessment, you need to establish what the correct base year value should have been. If assessors made errors in determining that original value, correcting it now impacts all subsequent factored calculations. This is where comparable sales data becomes critical in board of review hearings. Presenting three to five recent arm's length sales of similar properties in your neighborhood directly contests whether the base year was accurate.
Factored Value vs. Market Value
The gap between your factored base year value and current market value creates what tax professionals call "assessment resistance." In markets with rapid appreciation, this gap grows substantially. A property assessed at $600,000 (factored) but worth $950,000 (market) has a significant assessment ratio of 63%, meaning you're paying tax on only 63% of fair market value. Most states aim for assessment ratios between 95% and 105%, so ratios below 80% indicate you're underassessed relative to others in your area.
Underassessment across an entire class creates tax base erosion for local governments. Some jurisdictions conduct reassessment cycles to address this. When reassessment happens, your property reverts to full market value appraisal, and the factoring begins anew from that higher base. Until then, your annual factored increases remain capped at the statutory limit, regardless of market conditions.
Using This in Board of Review Appeals
- Challenge the base year value itself using recent comparable sales within 6 to 12 months of the base year date. Historical sales data proves what your property was actually worth when that base was established.
- Request the assessor's valuation methodology for that base year. If they used the cost approach on an older estimate of construction costs, or sales comparison approach on insufficient comps, documentation gaps strengthen your case.
- Calculate your property's current assessment ratio. Present it alongside neighborhood averages to show disproportionate treatment. If assessments of similar properties in your area average 92% and yours is 65%, that disparity is persuasive evidence.
- Document any exemptions you qualify for that should reduce the taxable portion, as these affect the calculation before the annual factor is applied.
- Bring evidence of property condition changes since the base year that reduce value, such as environmental issues, easements, or zoning restrictions. These modify the appropriate base year value downward.
Common Questions
If my property's market value drops, does my factored base year value decrease? No. The 2% cap is typically one-directional, meaning it limits increases but does not mandate decreases. However, if you suffer a major loss from disaster, disease (agricultural), or decline in income (agricultural), some jurisdictions allow downward appeals. You must file during the appeal window, usually 30 to 60 days after the assessment notice.
When does my property get reassessed to current market value? Transfer of ownership triggers full reassessment in cap states. Some improvements above a threshold percentage of value also trigger reassessment. Otherwise, factoring continues indefinitely unless the state implements a countywide reassessment. Check your assessor's website for your jurisdiction's specific reassessment policy.
Should I appeal my factored value or the 2% factor itself? Appealing the factor rate is nearly impossible since it's set by state law. Instead, appeal the base year value to the board of review. If you can reduce the base value from $400,000 to $350,000, all subsequent factored amounts decrease proportionally, saving you money for years.
Related Concepts
- Base Year Value is the foundation that the annual factor is applied to.
- Assessment Cap is the statutory limit, typically 2%, that controls annual increases in factored value.