How is assessed value determined for property taxes?

Last updated 2026-07-09

What does assessed value actually mean?

Assessed value is the dollar figure your local government assigns to your property to calculate your tax bill. It is not market value, not appraised value, and not what you paid. Those numbers point in different directions, and mixing them up is the most common mistake homeowners make before an appeal.

Here is the chain. The assessor estimates market value, multiplies it by the jurisdiction's assessment ratio, and that gives your assessed value. The county then applies the tax (mill) rate to that number to produce the bill. A $400,000 market value in a county with an 80 percent assessment ratio and a rate of 20 mills ($20 per $1,000) produces a $6,400 annual bill.

States set their own ratios by statute, and the spread is huge. California's Proposition 13 ties assessed value to the purchase price plus a 2 percent annual inflation cap, so a home bought in 1995 can carry an assessed value far below what it would sell for today ^[1]. Illinois assesses residential property at 33.33 percent of market value ^[2]. Georgia and Texas require assessment at 100 percent, meaning assessed value and market value should match.

The ratio decides how much your appeal is worth. If your state assesses at 100 percent and your assessed value beats what the house would actually sell for, you have a clean, documentable argument.

What are the three methods assessors use to determine value?

Every professional appraisal, for a bank or a tax office, draws from the same three methods. Knowing which one applies to your property tells you where the errors hide.

Sales Comparison Approach

This is what assessors use for nearly every single-family home. They find recent sales of similar properties (comparables, or comps), adjust for differences in size, age, condition, and features, and land on an indicated value. The whole thing rises or falls on how well the assessor picked and adjusted those comps. Stale sales, comps from the wrong neighborhood, or a failure to account for your dated kitchen all push your value too high.

For residential work, mass appraisal software like Tyler Technologies' iasWorld or CAMA (Computer Assisted Mass Appraisal) systems run hundreds of thousands of properties at once, often every year. The algorithms are not wrong on their own. They only know what you feed them. If your record says 2,400 square feet and the house is actually 2,100, the software is faithfully overvaluing you.

Cost Approach

The assessor estimates what it would cost to rebuild the structure today at current material and labor rates, subtracts depreciation for age and condition, then adds land value. This one shows up for new construction, unusual properties that rarely sell, and commercial or industrial buildings. Two errors dominate here. Bad replacement cost tables (assessors often lean on the Marshall & Swift/Boeckh cost manuals, which publish national averages that miss local labor markets) and too little depreciation. An older home with deferred maintenance or functional obsolescence (a one-bathroom ranch in a two-bathroom market) can be badly over-assessed if the depreciation was too thin ^[3].

Income Approach

Used almost only for income-producing properties: apartments, retail centers, office buildings. The assessor estimates gross potential income, subtracts vacancy and operating expenses to get net operating income (NOI), and divides by a capitalization rate to get value. A cap rate off by half a point can move a commercial assessment by hundreds of thousands of dollars. Own a small rental? Request the assessor's income and expense assumptions. They are public record in most states.

How does the assessment ratio affect your tax bill?

The assessment ratio turns estimated market value into taxable assessed value. Any state that does not assess at 100 percent uses a ratio, and those ratios swing wildly.

State	Assessment Ratio (Residential)	Source
California	Purchase price + 2%/yr cap (Prop 13)	Cal. Rev. & Tax. Code §51
Illinois	33.33% of fair market value	35 ILCS 200/9-145
Georgia	40% of fair market value	O.C.G.A. §48-5-7
Texas	100% of market value	Tax Code §23.01
New York (outside NYC)	Varies by assessing unit; often 1 to 3%	RPTL §305
Arizona	10% of full cash value (residential)	A.R.S. §42-15003
Colorado	6.95% of actual value (residential, 2023 to 2024 per HB23-1311)	C.R.S. §39-1-104.2
Florida	100% of just value; Save Our Homes caps annual increases at 3% for homesteaded property	Fla. Stat. §193.011

The ratio controls how much a market value cut actually helps you. In Georgia, a $20,000 reduction in estimated market value lowers assessed value by $8,000 (40 percent of $20,000). In Texas, that same $20,000 flows straight through at 100 percent.

States also run ratio studies to check whether assessors are hitting the statutory target. The International Association of Assessing Officers (IAAO) publishes standards for these studies, and many state equalization boards release annual results. If your county's median assessment ratio runs 15 percent above the statutory level and you can show it to a review board, you can win an equalization argument without ever proving your own market value ^[4].

For county-specific assessment details, see our guides for Maricopa property tax, Los Angeles County property tax, and San Diego property tax.

How does a mass appraisal system actually work?

Mass appraisal is not one appraisal done fast. It is statistical modeling that values thousands of properties at once using a standard set of characteristics. Your county enters data for every parcel (square footage, year built, bedrooms, bathrooms, garage, lot size, construction quality grade) into a CAMA system. The software builds regression models or comparable sales grids that estimate value from those traits and recent sales.

The IAAO defines mass appraisal as "the process of valuing a group of properties as of a given date using standard methods, employing common data, and allowing for statistical testing" ^[4]. The key word is "statistical." The model is tuned to be accurate for the average property in a neighborhood. It is systematically worse at unusual ones.

That is your opening. A layout problem (a bedroom with no closet counted as a bedroom), a condition problem (foundation issues, dated systems), or a location problem (backing to a highway, sitting in a flood plain, commercial use next door) that the algorithm never sees will push your value too high. None of those adjustments happen on their own.

Reassessment cycles vary by state and county. Some jurisdictions reassess every year. Cook County, Illinois reassesses by township on a three-year rotating cycle ^[5]. Others go every two, four, or ten years. Your cycle matters. A market that has fallen since the last assessment date is a real appeal argument. A market that has climbed means your assessment may be fair even if it looks high next to what you paid.

See how this plays out at the Cook County tax assessor tax bill guide.

What property characteristics does the assessor record, and why do errors matter?

Your assessor keeps a property record card (also called a field card or property data sheet) for every parcel. In most states you can pull yours online in under two minutes. The card usually lists:

• Gross living area (square footage above grade)
• Year built and effective age
• Number of bedrooms and full/half bathrooms
• Basement: finished or unfinished, square footage
• Garage: attached, detached, carport; number of spaces
• Deck, porch, pool, outbuildings
• Construction quality grade (usually a letter or number scale)
• Condition rating
• Lot size and zoning

Errors in any of these feed straight into your assessed value. Square footage is the most common error and the easiest to prove. Measure your home (exterior dimensions minus the garage, using ANSI Z765-2021 standards) and compare to the card. If the assessor shows 2,350 square feet and you measure 2,050, that is a documented 300-square-foot overstatement. At typical residential cost rates of $100 to $200 per square foot, that single error could mean a $30,000 to $60,000 over-assessment before the ratio even applies.

Condition and quality grade errors are harder to prove and potentially larger. If the assessor rates your home "Good" and you can document deferred maintenance, dated systems, or structural problems, dropping to "Average" on the assessor's own scale can cut assessed value hard. You need photos, contractor estimates, or inspection reports to make it stick.

For Georgia county assessors where these errors surface often, see Gwinnett County tax assessor, Cherokee County tax assessor, and Coweta County tax assessor for local appeal procedures.

How often does the assessed value change, and when does reassessment happen?

Reassessment timing depends on state law and sometimes county practice. Some states mandate annual reassessment. Others run multi-year cycles. A few, California most famously, freeze assessed value until a property sells or gets new construction, then reset to the sale price.

This table shows common reassessment cycles:

State	Reassessment Cycle	Statutory Basis
Texas	Annual	Tax Code §23.01
Georgia	Annual	O.C.G.A. §48-5-264.1
Illinois (Cook County)	Triennial by township	35 ILCS 200/9-220
California	Triggered by sale or new construction	Cal. Rev. & Tax. Code §51
New York	Varies by municipality; some annual, some not for decades	RPTL §302
Florida	Annual	Fla. Stat. §192.042
Colorado	Biennial (odd years)	C.R.S. §39-1-104

The practical upshot: if your county just finished a mass reassessment after a run of fast appreciation, assessed values can jump 20 to 40 percent in one notice cycle. That is when appeal volume spikes. Texas reassesses every year, which is part of why it sees some of the highest appeal rates in the country. Homeowners get a fresh number to fight annually.

Your appeal deadline is almost always tied to the notice date, not the tax bill date. In Georgia you have 45 days from the assessment notice ^[6]. In Texas the deadline is generally May 15 or 30 days after the appraisal notice is delivered, whichever is later ^[7]. Miss it and you forfeit your appeal rights for that year. Pull the notice date the day it arrives and calendar the deadline right then.

How do comparable sales affect your assessed value?

For residential property, comparable sales are the base of the assessor's market value estimate and the main tool you have to fight it. A comparable (comp) is a property that sold at arm's length (neither buyer nor seller under duress or related) and shares real traits with yours: neighborhood, size, age, style, condition.

Assessors pick comps using time windows (often 12 to 24 months before the assessment date), geographic filters (usually same subdivision or market area), and characteristic filters. In a thin market, they sometimes stretch time or distance further than they should and grab sales that do not really reflect your home.

When you build your own appeal, find comps that sold closer to the assessment date, in your actual neighborhood, more similar to your home than the assessor's picks, and pointing to a lower value. The assessor's office should hand you the list of comps they used, on the notice or on request. Line them up against yours.

Adjustments carry as much weight as the raw prices. If a comp sold for $380,000 but has an extra full bathroom and a finished basement your home lacks, the adjusted value of that comp for your home might be $335,000. The IAAO's adjustment guidelines and your county's published adjustment schedule govern which adjustments hold up ^[4].

The TaxFightBack DIY appeal kit walks through building a comp grid step by step, so you keep 100 percent of any savings instead of handing a contingency firm 30 to 50 percent of the first year's reduction.

What is equalization and how does it protect homeowners?

Equalization is how a state or county corrects systematic over- or under-assessment across jurisdictions or property classes. If the assessor hits 95 percent of market value on single-family homes but only 80 percent on condos, the state equalization board can apply a multiplier to the condo class to bring it back to the statutory level.

For a single homeowner, equalization matters two ways. If your county's overall level of assessment runs below the statutory ratio, you can end up owing more than intended because school districts set mill rates off the assessed value roll. And if you can show your assessed value is out of line with the prevailing ratio for your property class, some jurisdictions allow an "equalization appeal" on that basis alone.

The Illinois Department of Revenue publishes an annual report showing the median assessment-to-sale ratio for every county, then applies an equalization factor (the "multiplier") to bring each county's median to 33.33 percent ^[2]. In 2023, many downstate Illinois counties got multipliers above 1.0, meaning every property's assessed value moved up by statute. Cook County's multiplier is figured separately and shifts each three-year cycle.

Equalization data is public and badly underused. If your state publishes ratio studies, pull the data for your county and property class before your hearing. Showing that your property's assessment-to-sale ratio sits well above the countywide median for similar homes is strong evidence even without perfect comps.

Can exemptions reduce your assessed value or your tax bill?

Exemptions usually do not reduce your assessed value. They reduce the taxable slice of it. That distinction shapes your strategy. You can chase an assessment reduction (arguing the market value or ratio is wrong) and an exemption (arguing you qualify for a statutory break) at the same time. They are separate processes.

Common exemptions that shrink the taxable base:

Homestead exemption: Available in most states for a primary residence. Georgia's basic homestead exemption removes $2,000 from assessed value ^[8]. Florida's removes $25,000 from assessed value, plus another $25,000 on the portion of assessed value between $50,000 and $75,000 for non-school levies ^[9]. Texas offers a $100,000 homestead exemption on school district taxes as of 2023 under HB 5 ^[7].

Senior and disability exemptions: Many states stack extra exemptions on top of the homestead for taxpayers over 65 or with qualifying disabilities. These are often means-tested, though the income limits run higher than people expect.

Veterans exemptions: Available in all 50 states in some form, from a modest dollar reduction to full exemption for 100 percent disabled veterans.

Agricultural and conservation exemptions: If any part of your land qualifies as agricultural use, a preferential assessment rate may apply.

Exemption application deadlines usually fall in spring, often earlier than appeal deadlines. Miss the window and you wait a full year. Check your county assessor's website every January.

For local exemption details, see the Bibb County tax assessor and Madison County tax assessor guides, and Bexar County tax assessor for Texas homestead specifics.

What are the most common causes of an inflated assessed value?

This is the checklist to run before you decide whether to appeal.

Wrong square footage. Pull your permit history, your title insurance survey, and a tape measure. Compare to the property record card. Even a 5 to 10 percent overage is worth challenging.

Wrong property characteristics. The card might show a third bathroom that was never finished, a basement listed as finished when it is half done, or outbuildings you tore down years ago.

Poor comp selection. The assessor used sales from a different neighborhood, a different school district, or a very different time window.

No adjustment for condition. If you deferred maintenance, had major repairs, or live with a functional problem (odd floor plan, low ceilings, no primary bath for the price range), the mass appraisal model probably missed it.

A market that moved since the assessment date. In a flat or falling market, last year's model may still run on appreciation assumptions that no longer hold.

Wrong lot size or zoning. Less common, but a lot recorded at 0.75 acres that actually measures 0.55 acres inflates value, especially where the land schedule is acreage-based.

Before you pay an attorney or a contingency firm, spend two hours pulling your property record card, checking the square footage, and looking up five recent sales within a half-mile. That work costs nothing and tells you whether you have a case worth chasing.

How do you find out your assessed value and what the assessor used to set it?

Start at your county assessor's website. Every county in the country has one, and most now offer online property search where you can pull the full record card and recent sales history for free. Search by address or parcel number (the parcel number is on your tax bill).

What to request if the online portal is thin:

1. Property record card (field card) 2. Sales comparison grid or comparable sales list used in your assessment 3. Cost schedules if the cost approach was used 4. Income and expense assumptions if it is an income property

Under state public records laws, all of these are generally available to the owner. Some assessors charge a small fee for printed copies. Most give digital copies free. In Texas, the appraisal district must provide a copy of the appraisal records under Tax Code §41.461 if you request it at least 14 days before your hearing ^[7].

Once you have the data, cross-check your square footage against your original listing photos (MLS data is often more accurate than assessor records), your home inspection report, or a simple exterior measurement. Check every field on the card against what you know. Errors are more common than most homeowners expect. Researchers at the University of Chicago found systematic assessment regressivity in major U.S. cities, meaning lower-valued homes were often taxed at higher effective rates than higher-valued homes, partly from data quality problems in the underlying property records ^[10].

What should you do if your assessed value looks too high?

You have two parallel paths: the informal review and the formal appeal. Most jurisdictions offer both.

The informal review is a conversation with the assessor's office, often before the formal deadline, where you show your evidence and ask them to fix the record. It works best for clear data errors (wrong square footage, wrong bedroom count) or a comp argument that is hard to dispute. Many assessors correct obvious errors without a formal hearing because it saves everyone time.

The formal appeal goes to an independent review board: a Board of Equalization, an Assessment Review Board, an Appraisal Review Board (in Texas), or a similar body depending on your state. You file a written protest by the deadline, present evidence at a hearing, and the board rules. In most states you can then appeal that decision to the state Tax Court or circuit court if you disagree.

For the hearing, build a packet with:

• The assessor's property record card with errors highlighted
• Your own measurement or documentation showing the correct data
• Three to five comparable sales printed from the MLS or the assessor's own sales database, with a grid showing adjusted values
• Photos documenting condition issues
• Any inspection reports, contractor estimates, or appraisals that back your position

You do not need an attorney for most residential appeals. The process is built to be accessible. The TaxFightBack DIY appeal kit puts this evidence in the exact format review boards expect, so you walk in prepared instead of improvising.

For Texas-specific procedures, Bexar County tax assessor and the Lake County property tax guide cover the ARB process in detail.

Frequently asked questions

Is assessed value the same as market value?

No. Market value is what your home would sell for between a willing buyer and seller. Assessed value is the figure your government uses to calculate taxes. In most states, assessed value is a percentage of market value set by the assessment ratio, which runs from 10 percent in Arizona to 100 percent in Texas and Georgia. They only match if the state assesses at 100 percent and the assessor's estimate tracks market reality.

How do I find my property's assessed value?

Go to your county assessor's website and search by address or parcel number. The parcel number is printed on your tax bill. Most counties provide a full property record card online for free, showing assessed value, the market value estimate, and the characteristics the assessor used. If the site is unclear, call the assessor's office and ask for your property data card.

Can I lower my assessed value, and how long does that take?

Yes. File a formal appeal by the deadline on your assessment notice, typically 30 to 45 days from the notice date. Informal reviews with the assessor can take two to four weeks. A formal hearing before a review board usually happens within 60 to 90 days of filing, though backlogged counties like Harris County, Texas can take longer. If you win, the reduction usually applies to the current tax year's bill.

What is an assessment ratio and how does it affect what I owe?

The assessment ratio is the percentage of market value used to set your taxable assessed value. Arizona residential property uses 10 percent, Illinois uses 33.33 percent, Georgia uses 40 percent, and Texas uses 100 percent. A lower ratio means the same market value produces a lower assessed value and a lower bill, all else equal. High-ratio states often set lower mill rates to compensate, so comparing bills across states is rarely simple.

How often is my property reassessed?

It depends on your state and county. Texas and Georgia reassess annually. Illinois' Cook County uses a triennial cycle by township. California reassesses only when a property sells or gets significant new construction, then locks assessed value to the purchase price plus a 2 percent annual cap. Colorado reassesses in odd-numbered years. Check your state statute or assessor's website to find your cycle.

What is a homestead exemption and does it affect assessed value?

A homestead exemption reduces the taxable portion of your assessed value for a primary residence. It does not change the assessor's market value estimate. Georgia's basic homestead exemption removes $2,000 from assessed value. Florida removes up to $50,000 depending on value tier. Texas removed $100,000 from school district assessed value as of 2023. Apply through your county assessor's office, usually by a spring deadline.

What information does the assessor use to calculate my home's value?

Assessors enter square footage, year built, bedroom and bathroom count, basement finish, garage type, deck, pool, lot size, construction quality grade, and condition rating into a mass appraisal system. The software models those traits against recent sales in your area to produce an estimated market value. Errors in any input flow straight into your assessment. Pull your property record card and verify every field against what you know.

How are assessed values determined for rental properties?

Assessors typically use the income approach for income-producing properties. They estimate gross potential rental income, subtract a vacancy allowance and operating expenses to reach net operating income, then divide by a capitalization rate to get a value. A 0.5 percent cap rate error on a small apartment building can shift assessed value by tens of thousands of dollars. Request the assessor's income, expense, and cap rate assumptions under your state's public records law.

Can a new home sale reset my assessed value higher?

In most states, yes. The sale is evidence of market value, and many assessors treat a recent sale as a trigger to update the assessment. In California, Prop 13 explicitly resets assessed value to the purchase price when ownership changes. In Texas and Georgia, the assessor weighs all available market evidence including recent sales, so buying at a high price can prompt a higher assessment at the next cycle.

What happens if I miss the appeal deadline?

You generally lose your appeal rights for that tax year and cannot get a retroactive reduction. The deadline is set by state statute, and review boards almost never waive it except for documented failure to receive notice. Mark the deadline the day the assessment notice arrives. In Georgia it is 45 days from the notice date. In Texas it is typically May 15 or 30 days after notice delivery, whichever is later.

Do I need an attorney or appraiser to appeal my property taxes?

For most residential appeals, no. Review boards hear self-represented homeowners all the time. You need organized evidence: a corrected property record card, three to five comparable sales with adjusted values, photos, and any condition documentation. An independent appraisal ($300 to $600) strengthens a complex case or one headed to court, but it is not required at the first level. Contingency firms take 30 to 50 percent of first-year savings; doing it yourself costs nothing.

What is a CAMA system and does it make mistakes?

CAMA stands for Computer Assisted Mass Appraisal. It is the software assessors use to value thousands of properties at once by modeling property traits against market sales. It is statistically accurate for typical properties and systematically worse at unusual ones: homes with condition problems, odd layouts, or traits the model does not capture well. That is why pulling your property record card and checking the input data is the first step in any appeal.

Can my assessed value go down without me appealing?

Yes, if market values in your area fall between reassessment cycles and the assessor's model reflects the drop. In states that reassess annually, a falling market should produce lower assessments on its own, though the adjustment can lag by a cycle. In states with multi-year cycles or purchase-price caps like California, a market decline may not lower your assessed value at all unless you file an appeal claiming market value has dropped below your assessed value.

Is there a difference between assessed value and appraised value on my tax notice?

Yes, in most states the notice shows both. Appraised value (or market value) is the assessor's estimate of what your property would sell for. Assessed value is that number multiplied by the assessment ratio. Your tax bill runs off assessed value. In states that assess at 100 percent the two match. In Arizona, the full cash value (appraised) might be $400,000 and the assessed value $40,000 (10 percent ratio).

Sources

1. California State Board of Equalization, California Revenue and Taxation Code Section 51: California's Proposition 13 ties assessed value to purchase price plus a maximum 2% annual inflation cap under Rev. & Tax. Code §51
2. Illinois Department of Revenue, Property Tax information: Illinois assesses residential property at 33.33% of fair market value under 35 ILCS 200/9-145 and applies annual equalization factors by county
3. International Association of Assessing Officers (IAAO), Standard on Mass Appraisal of Real Property: Cost approach methodology including depreciation schedules and the use of cost manuals such as Marshall & Swift/Boeckh are described in IAAO mass appraisal standards
4. International Association of Assessing Officers (IAAO), Standard on Ratio Studies: The IAAO defines mass appraisal as 'the process of valuing a group of properties as of a given date using standard methods, employing common data, and allowing for statistical testing' and publishes ratio study standards
5. Cook County Assessor's Office, Reassessment Schedule: Cook County, Illinois reassesses property on a triennial rotating cycle by township under 35 ILCS 200/9-220
6. Georgia Department of Revenue, Property Tax Appeals: Georgia property owners have 45 days from the date of the assessment notice to file an appeal under O.C.G.A. §48-5-311
7. Texas Comptroller of Public Accounts, Property Tax: Texas protest deadline is May 15 or 30 days after the appraisal notice is delivered, whichever is later, under Tax Code §41.44; appraisal district must provide records under §41.461; homestead exemption raised to $100,000 for school taxes by HB 5 (2023)
8. Georgia Department of Revenue, Property Tax Exemptions: Georgia's basic homestead exemption removes $2,000 from assessed value for a primary residence under O.C.G.A. §48-5-44
9. Florida Department of Revenue, Property Tax Exemptions: Florida's homestead exemption removes $25,000 from assessed value plus an additional $25,000 for the assessed value between $50,000 and $75,000 for non-school levies under Fla. Stat. §196.031
10. Avenancio-León & Howard, 'The Assessment Gap: Racial Inequalities in Property Taxation,' Quarterly Journal of Economics, 2022: University of Chicago researchers found systematic assessment regressivity in major U.S. cities where lower-valued homes were assessed at higher effective rates, partly due to data quality issues in property records
11. Arizona Department of Revenue, Property Tax: Arizona assesses residential property at 10% of full cash value under A.R.S. §42-15003
12. Colorado Department of Local Affairs, Division of Property Taxation: Colorado residential assessment rate was set at 6.95% of actual value for 2023–2024 under C.R.S. §39-1-104.2 per HB23-1311; reassessment occurs in odd-numbered years under C.R.S. §39-1-104