How to find comparable sales to lower your property tax assessment

Last updated 2026-07-09

What are comparable sales and why do they matter for property taxes?

Comparable sales, called comps, are recorded sale prices of homes meaningfully similar to yours in size, age, location, and condition. Every state that assesses at market value has to anchor its numbers to actual market evidence, and comps are the most direct form of that evidence. When your assessor says your home is worth $480,000, they are supposed to be pointing at homes like yours that sold near that price around the assessment date.

Get the comps wrong, or cherry-pick ones that flatter a high value, and your tax bill runs hundreds or thousands of dollars too high every single year.

Here's the part the contingency firms don't advertise: comparable sales are public record. You can pull the same raw data the assessor used. And you can find comps that tell a different story.

Most residential assessments lean on the sales comparison approach as the primary or check valuation method ^[1]. The assessor picks a handful of sales, makes dollar adjustments for square footage, lot size, bedroom count, garage, and condition, then lands on an indicated value for your property. There's real subjectivity baked into that. Which sales get chosen, and how the adjustments get made, are both places where errors creep in.

You don't need to out-appraise the assessor. You need to show the board that the right comparable sales say your home is worth less than the assessed value. Three or four clean comps that bracket a lower number usually shift the argument your way.

How do I find comparable home sales without paying anyone?

Five free sources cover almost every situation, and using more than one is smart because each has blind spots.

Your county assessor's website. Most assessors now run a public parcel search where you pull the sale history for any address in the county. Start there. You can search by neighborhood or subdivision, filter by sale date, and in many jurisdictions export the results to a spreadsheet. Some counties, like Cook County in Illinois, publish their entire residential sales file as a downloadable dataset ^[2].

County deed or recorder databases. Every real estate transfer in the U.S. gets recorded as a deed. Most county recorders post deed images and sale prices online at no charge. A few states, notably Kansas, Montana, and others, are non-disclosure states where sale prices aren't public, which narrows your options but doesn't close them off ^[3].

Zillow, Redfin, and Realtor.com sold tabs. These sites pull from MLS data and county records. Zillow's sold listings go back at least two years in most markets and include photos, which matter when you're arguing condition. Filter by sold date, then narrow by square footage range and property type. The addresses are real and the prices come from recorded deeds.

Your county's GIS mapping tool. Many counties run a geographic information system where you click parcels on a map, see sale history, and pull lot size and year built. This is the tool to reach for when you want to search by physical proximity instead of ZIP code.

FHFA's House Price Index. The Federal Housing Finance Agency publishes quarterly House Price Index data at the metro and county level ^[4]. It won't hand you individual sale prices, but it tells you what your market did in the months before the assessment date. If prices dropped 8% between January and October and your assessment uses a January value, that's context you can put in front of a board.

What makes a comparable sale actually comparable?

This is where most DIY appeals fall apart. People grab a nearby sale and call it a comp without asking whether a board member will buy the logic. Apply these filters before anything else.

Proximity. Closer is better. Within a quarter mile is ideal in a dense suburb. In rural areas you may have to reach a mile or more. Same subdivision or neighborhood is the strongest play because location adjustments are hard for assessors to argue with.

Sale date. Most states value your property as of a specific lien or assessment date. In California, that's January 1 ^[5]. In plenty of states it's January 1 of the tax year. Your comps should have sold in the 12 months before that date, and the 6 months right before it carry the most weight. Older sales need time adjustments and hand the assessor room to push back.

Size. Square footage is the single biggest price driver for houses. Rule of thumb: stay within 15 to 20% of your finished square footage. A 2,000 sq ft comp for a 3,200 sq ft house is a stretch even after adjustments.

Physical similarity. Same property type (single-family to single-family, never a condo). Similar bedroom and bathroom count. Similar lot size in markets where lots vary. Similar age or era of construction. Similar condition, which is harder to prove, but MLS photos help.

Arm's-length transaction. A legal term meaning buyer and seller were unrelated and neither was under duress. Foreclosures, estate sales between relatives, and deals that bundle personal property into the price are NOT arm's-length. Most state assessment statutes exclude non-arm's-length sales from the comparable pool ^[6]. The assessor will toss them, and so should you, or you'll look like you didn't do your homework.

Filter	Ideal standard	Acceptable outer limit
Proximity	Same subdivision or 0.25 mi	1 mile (suburban), 3 miles (rural)
Sale date	Within 6 months of assessment date	Within 12 months; 24 months with time adjustment
Square footage	Within 10%	Within 20%
Lot size	Within 10-15%	Within 25%
Age	Within 10 years	Within 20 years
Property type	Identical (SFR to SFR)	Non-negotiable: must match
Transaction type	Arm's-length only	No exceptions

Aim for three comps minimum, four or five if you can get them. Boards trust a pattern more than a single outlier.

How do I find comparables the assessor already used?

Look at the assessor's work before you build your own set. In most jurisdictions you have the right to request the assessor's appraisal card or valuation record for your property, and sometimes for the comps they leaned on.

The assessment notice you got may already list the comparable sales the assessor picked. If it doesn't, file a public records request (state open-records laws vary, and many people call it a FOIA request out of habit) for the property record card and any sales grid the assessor prepared ^[7]. This step matters because it tells you three things:

1. Which sales they actually used. 2. What adjustments they made and how big those adjustments were. 3. Whether any of their comps are non-arm's-length or otherwise wrong.

If they used a sale from two years ago while several sales from eight months back show lower values, that's a real argument. If they added $30,000 for a bathroom when the market doesn't support that number, that's a real argument. You can only make these points if you know what they did.

Some assessors post their whole sales analysis online as part of the mass appraisal documentation. The International Association of Assessing Officers (IAAO) sets the professional standards for mass appraisal and publishes guidance on acceptable sale ratio statistics ^[8]. An assessed value sitting above the 95th percentile of the local sales ratio study is a red flag worth documenting.

How do I adjust comparable sales to account for differences?

Perfect comps don't exist. Every sale you find differs from your property somehow, and you have to account for those differences through adjustments, or the hearing officer waves your comps away as apples to oranges.

The concept is simple. You're asking: what would this sale price have been if the comp were identical to my property? A comp sold for $350,000 but has an extra bathroom yours lacks, so you subtract something from $350,000. A comp is 200 square feet smaller, so you add something.

Picking the right dollar amount is the hard part. Three ways to do it:

Matched pairs analysis. Find two sales identical except for one feature (one has a garage, one doesn't) and the price gap tells you the market's dollar value for that feature. This is the most defensible method, but it needs a large sales dataset to find true pairs.

The assessor's own adjustment schedule. Many counties publish the cost tables or adjustment grids they use internally. If the assessor values a bathroom at $8,000 in your market, use $8,000. Their own numbers shut down the argument that yours are made up.

Regression or mass appraisal data. Download enough sales and a simple linear regression of price on square footage gives you a per-square-foot adjustment rate that holds up statistically. No statistics degree required. Excel's LINEST function does it.

For most DIY appeals, matched pairs applied to the differences that actually move price (size, bathrooms, garage) is plenty. Don't adjust for every feature. Total adjustments over 25% of the sale price look shaky no matter how you justify each one.

What are the most common mistakes people make with comps?

Using active listings instead of closed sales is the number one mistake. A listing price is an asking price. It is not a sale. Boards of equalization and assessment review boards will not accept active listings as evidence of value because they don't show what a buyer actually paid ^[9]. Only closed sales count.

Second mistake: using distressed sales without noticing. Foreclosures, REO (bank-owned) dispositions, and short sales often sell at steep discounts that don't reflect the open market. One foreclosure in your comp set and the assessor flags it immediately, dragging your credible comps down with it.

Third: reaching outside your neighborhood when better sales sit inside it. Boards want to see you understand the local market. Six sales in your subdivision and you present one from across town? Someone will ask why.

Fourth: ignoring condition. A renovated kitchen and new roof can add $20,000 to $40,000 or more in some markets. If your comp has those features and your home doesn't, and you present the unadjusted price as your home's value, that's a dishonest comparison. The assessor will call it out and the board will agree.

Fifth: too few comps, or comps that all land right at the assessed value instead of clearly below it. Your comps have to show a pattern. Three sales averaging to your assessed value do nothing for you. You need three or more that bracket a value well below it.

How do comparable sales differ from Zillow estimates?

The Zestimate is an automated valuation model (AVM). It's a statistical guess built from public records and user-reported data, not an appraisal and not a sale. Boards of equalization in nearly every state reject a Zestimate as evidence of value ^[9]. Don't use it.

What Zillow IS good for is finding actual comparable sales. The sold listings tab shows real closed sale prices sourced from MLS feeds and county deed records. Those are genuine market transactions, and you can cite each one with its address, date, and recorded price.

Redfin's sold data works the same way and often carries more detailed MLS characteristics than Zillow in markets where Redfin has a strong brokerage. Neither site replaces pulling the deed from the county recorder to confirm the price, especially on older sales, but they're solid starting points.

Want something closer to a formal appraisal without hiring one? The assessor's own comparable sales analysis (the sales grid from the property record card) carries far more weight before a board than any AVM output.

What if I'm in a non-disclosure state where sale prices aren't public?

Thirteen states are full or partial non-disclosure states as of 2024, including Alaska, Idaho, Kansas, Louisiana, Mississippi, Missouri, Montana, New Mexico, North Dakota, Texas, Utah, Wyoming, and parts of Indiana ^[3]. In these states, sale prices on recorded deeds don't show up in public databases.

That limits your options. It doesn't kill your appeal. Four moves:

Ask your assessor directly. Assessors in non-disclosure states still need sale data to run mass appraisal, so they collect it through sales questionnaires, permit data, and other channels. Many will share their sales file or comparable sales analysis if you ask during your appeal.

Use a real estate agent. Agents with MLS access can pull closed sales for you. The MLS itself isn't public, but an agent can produce a Comparative Market Analysis (CMA) listing actual sold prices. A CMA from a licensed agent, presented at your hearing with the agent's credentials attached, carries more weight than your own research in these markets.

Hire a fee appraiser for comp research only. Some appraisers take a limited engagement, delivering comparable sales research and a sales adjustment grid without a full narrative appraisal, for $200 to $400. Contesting a $4,000 a year tax bill? That's often money well spent.

Focus on ratio evidence. Even without individual sale prices, you can sometimes get your jurisdiction's sales ratio study from the state department of revenue or equalization. If the median ratio in your district is 0.92 (the median home is assessed at 92% of market value) and yours sits at 108%, that gap alone can be grounds for an appeal.

How do I use comparable sales in an actual property tax appeal?

Finding comps is research. Using them in a hearing is presentation. Structure it like this.

Lead with a one-page summary. Your property's address, assessed value, and the value you're asserting. Then each comp: address, sale date, sale price, and square footage. Calculate the value per square foot for each comp and for your property at its assessed value. If your assessed value per square foot sits well above the comp average, that gap is your headline.

Attach a map. Print a screenshot from Google Maps or your county's GIS showing the distance between your property and each comp. Proximity jumps off a map in a way it never does in a table.

Include sold listing photos when you can get them. Pull the MLS photos from Zillow or Redfin for your comp sales. Condition arguments land instantly when a board member sees your dated kitchen next to a renovated one.

Present a simple adjustment grid. Even a two or three line adjustment for square footage and major features shows you did the work honestly. Claim a $60,000 reduction with no adjustments and you'll get pushback. A grid showing how you reached the adjusted value is hard to dismiss.

The TaxFightBack appeal kit includes a formatted comparable sales grid and adjustment worksheet that matches what most county boards expect, so you're not starting from a blank spreadsheet [internal reference].

Most jurisdictions want your evidence package ahead of the hearing, sometimes 5 to 10 days in advance. Miss that deadline and your comps never make it into the record, which is a common and avoidable way DIY appeals die. Check the rules for your county: Los Angeles County property tax appeals, Cook County tax assessor bill, Maricopa property tax, and San Diego property tax each set different submission rules.

How many comparable sales do I need to win an appeal?

Three is the practical floor. Most hearing boards expect at least three comparable sales because one outlier proves nothing and two can look cherry-picked. Four or five gives you a more defensible average and cushions you if the assessor challenges one of your picks as non-arm's-length or outside the date range.

More is not better. Fifteen comps crammed into a presentation create confusion and suggest you couldn't tell the good ones from the bad. Boards are often volunteers or part-time adjudicators working on limited time. A tight set of four to five comps that tell a clear story beats a sprawling spreadsheet every time.

If your market had very few sales in the relevant window (rural area, unusual property type), two well-documented comps with solid adjustments can still carry the day, especially paired with a time-trend argument from FHFA House Price Index data showing the market fell between the assessment date and your hearing.

Nobody has clean national data on the exact number of comps that maximizes win rates; the research that exists is jurisdiction-specific and mostly unpublished. The closest professional standard comes from the IAAO, which recommends a minimum of three sales for any sales comparison approach analysis ^[8].

Are there state-specific rules for comparable sales in appeals?

Yes, and they matter. Some states have statutes that spell out what qualifies as a comparable sale for assessment. California's Proposition 13 system works differently from most because assessed value is generally based on purchase price plus a 2% annual cap, not annual market reappraisal, so comparable sales evidence matters most when you recently bought and the assessor's value tops what you paid ^[5]. Arizona requires the assessor to value residential property using the sales comparison approach and sets the parameters in statute ^[10].

In Georgia, the appeal runs through the Board of Equalization and then, if needed, to the Superior Court, and comparable sales evidence follows standard appraisal methodology. Dealing with a Georgia county? The rules track closely across jurisdictions: see Gwinnett County tax assessor, Bibb County tax assessor, Cherokee County tax assessor, Coweta County tax assessor, and Madison County tax assessor for county-specific filing and evidence requirements.

Texas (a non-disclosure state) runs an Appraisal Review Board (ARB) process where comparable sales evidence is accepted but burden of proof standards shift depending on whether the property is valued above $1 million ^[11]. The Bexar County Tax Assessor's office in San Antonio handles appeals under the standard Texas ARB process: see Bexar County tax assessor for local specifics.

Check three documents before you build your comp set: the state's assessment statute, the state department of revenue's appeal guidance for taxpayers, and the county assessor's website for local procedures. All three are usually free and online.

What does a successful comp-based appeal actually look like?

Let's walk the logic without inventing a person or a fake case.

Say your single-family home in a Midwestern suburb is assessed at $310,000 as of January 1. It's 1,800 square feet, built in 1988, three bedrooms, two baths, two-car attached garage, no finished basement, original kitchen.

You search your county assessor's parcel database and Zillow's sold tab. You find five closed sales within half a mile that sold in the prior 12 months, all arm's-length. Filter for similar size (1,650 to 2,000 sq ft) and same property type, and four survive. Their sale prices per square foot run from $148 to $161. Your assessed value implies $172 per square foot, above every one of the four.

You make modest adjustments. One comp has a finished basement worth roughly $8,000 to $12,000 more than yours, so you subtract that from its sale price before averaging. After adjustments, the four average to $155 per square foot, implying about $279,000 for your 1,800 sq ft home.

You present it at the hearing: a one-page summary, a map showing proximity, four MLS sold-listing printouts with photos, and a simple adjustment grid. The board sees $279,000 indicated versus $310,000 assessed. A $31,000 reduction at a 2% effective tax rate saves $620 a year, every year until the next revaluation.

That's how a well-run comp-based appeal works. The evidence is public. The logic is plain. The savings are real and they repeat.

Frequently asked questions

What are comparable sales in property tax?

Comparable sales (comps) are closed arm's-length sales of properties similar to yours in size, location, age, and condition. Assessors use them to set your property's market value, and you can use the same sales to argue your assessed value is too high. Only closed sales count, not listing prices or Zestimates.

How do I find comparable home sales for free?

Start with your county assessor's public parcel search and your county recorder's deed database. Then cross-reference with Zillow, Redfin, or Realtor.com sold listings. Each shows closed sale prices sourced from public deed records. Your county's GIS mapping tool, if available, lets you search by proximity on a map. All of these are free with no account required.

How many comps do I need for a property tax appeal?

Three is the practical minimum, and four or five is better. The International Association of Assessing Officers recommends at least three sales for any sales comparison analysis. More than six or seven starts to clutter your presentation. A tight, clean set of four sales telling a consistent story is more persuasive to a hearing board than a spreadsheet of fifteen.

Can I use Zillow comps to appeal my property taxes?

You can use Zillow's closed sold listings, not its Zestimate. The sold listings show actual recorded sale prices from MLS and deed records, which are legitimate evidence. The Zestimate itself is an automated model output, not a sale, and boards of equalization in virtually every state will reject it as evidence of value.

What is an arm's-length sale and why does it matter?

An arm's-length sale is one where buyer and seller are unrelated and neither is under duress. Foreclosure sales, estate sales between family members, and short sales typically are not arm's-length. Most state assessment statutes explicitly exclude non-arm's-length sales from the comparable pool. Use only arm's-length sales in your evidence, or the assessor will disqualify your comps.

How far back can comparable sales be for a property tax appeal?

Ideally within 12 months before the assessment date, with 6 months being strongest. Sales older than 12 months require time-trend adjustments and give the assessor more room to push back. Check your state's statute or the assessor's appeal guidance for the formal rule; some jurisdictions allow up to 24 months with a documented time adjustment.

How do I find the comparable sales the assessor actually used?

Request your property record card and any sales grid the assessor prepared. File a public records request with the assessor's office, or check whether the data is already posted on the assessor's website. Your assessment notice may also include a worksheet. Knowing the assessor's comps lets you challenge their selection or the adjustment amounts they applied.

What if there are no comparable sales near my property?

Expand your search area gradually (half a mile, then a mile, then further) and widen your date range up to 24 months with time adjustments using FHFA House Price Index data for your market. In truly rural or unusual-property markets, a licensed appraiser doing a limited comp research engagement for $200 to $400 may be worth the cost. You can also request the assessor's own sales file.

Can I use comparable sales from a different neighborhood?

Only if your neighborhood genuinely had too few sales to build a comp set. Boards prefer same-subdivision or same-street comps because location is the hardest factor to adjust for. If you go outside your immediate area, you need to explicitly address why and show that the neighborhoods are economically similar. The assessor will challenge any comp that isn't close.

What is a sales ratio study and how does it help my appeal?

A sales ratio study measures the ratio of assessed values to sale prices across a jurisdiction. Your state's department of revenue or equalization typically publishes one annually. If the median ratio in your district is 0.90 but your personal ratio is 1.10, that's evidence your property bears a disproportionate share of the tax burden, which is a separate and sometimes stronger argument than just running comparable sales.

How do adjustments work when my comps aren't identical to my home?

You add to or subtract from each comp's sale price to account for differences. If a comp has a finished basement your home lacks, subtract its estimated value from the comp price. If a comp is 300 square feet smaller, add the market's per-square-foot rate for that size difference. Keep total adjustments under 25% of the sale price to maintain credibility. Use the assessor's own published adjustment schedules when available.

Do comparable sales work differently in non-disclosure states?

In states like Texas, Montana, and Kansas where deed prices aren't public, you may need MLS access through a real estate agent, a paid appraiser, or the assessor's own sales file obtained through the appeal process. Sales ratio studies from the state revenue department can substitute for individual comps in some hearings. Non-disclosure limits your data but doesn't eliminate your ability to appeal.

How are comparable sales different for commercial property appeals?

Commercial property appeals often rely on income capitalization (a cap rate applied to net operating income) more than comparable sales, because truly comparable commercial sales are rarer. When sales comps are used for commercial properties, buyers and boards focus on price per square foot by property type and the cap rates implied by each sale, more than the gross sale price.

What happens if the assessor says my comps are not comparable?

This is expected. Be ready to defend each comp's proximity, sale date, arm's-length status, and physical similarity. If the assessor challenges condition differences, your Zillow or MLS photos are the rebuttal. If they challenge a date, your FHFA time-trend data can show minimal price movement in that period. Having four or five comps means losing one argument doesn't collapse your case.

Sources

1. International Association of Assessing Officers (IAAO), Standard on Mass Appraisal of Real Property: The sales comparison approach is the primary or check valuation method used in most residential mass appraisals.
2. Cook County Assessor's Office, Open Data Portal: Cook County publishes its residential sales file as a downloadable public dataset.
3. National Association of Realtors, Non-Disclosure States list: Thirteen states are full or partial non-disclosure states where sale prices recorded on deeds are not published in public databases.
4. Federal Housing Finance Agency, House Price Index: FHFA publishes quarterly House Price Index data at the metropolitan and county level, usable to document market trends between assessment date and hearing.
5. California State Board of Equalization, Proposition 13 Overview: California's Proposition 13 assessment system bases assessed value on purchase price plus a 2% annual cap, with January 1 as the annual lien date.
6. Illinois Property Tax Code, 35 ILCS 200, Section 1-100: State assessment statutes typically define arm's-length transactions and explicitly exclude non-arm's-length sales from the comparable pool used in mass appraisal.
7. U.S. Department of Justice, Office of Information Policy (FOIA); state open-records laws: Property record cards and sales grids prepared by assessors are generally public records accessible through open-records requests.
8. International Association of Assessing Officers (IAAO), Standard on Ratio Studies: IAAO recommends a minimum of three sales for any sales comparison approach analysis and publishes acceptable sales ratio statistics for mass appraisal quality standards.
9. Taxpayer Advocate Service; various state Board of Equalization guidance documents: Boards of equalization in virtually every state do not accept automated valuation model outputs (Zestimates) or active listing prices as evidence of market value; only closed sales qualify.
10. Arizona Department of Revenue, Property Tax Division: Arizona requires assessors to value residential property using the sales comparison approach and sets valuation parameters in statute.
11. Texas Comptroller of Public Accounts, Property Tax Assistance Division, Taxpayer Rights, Remedies, and Responsibilities: Texas Appraisal Review Board process accepts comparable sales evidence; burden of proof standards differ for properties valued above $1 million.