MLS data vs. Zillow for property tax appeals: what actually works

Last updated 2026-07-09

Why does the source of your comps matter so much in a tax appeal?

An appeal is one argument: what was your home worth on a single date, usually January 1 of the tax year. The board is not weighing your opinion. It is weighing your evidence. That gap is where most homeowners lose.

Most boards of equalization and assessment review boards run on rules that spell out what counts as credible proof of market value. In nearly every state the standard is a verified arm's-length sale ^[1]. That means a willing buyer and a willing seller, neither one forced, agreeing on a price. The assessor built your assessment from sales like that. You beat it with sales like that.

A Zillow Zestimate does not show a sale. It shows a guess. Zillow's own disclaimer reads: "The Zestimate is not an appraisal and can't be used in place of an appraisal" ^[2]. Boards know this. Most will not even log a Zestimate into the record. Bring one as your only support and you will lose.

MLS data is different. It records closed transactions with verified prices, dates, addresses, square footage, and condition notes. That is the same raw material your assessor used. Show up with MLS comps that match your property better than the ones the assessor picked, and you are fighting on the board's terms. That is when you actually win.

What exactly is a Zestimate and why do boards reject it?

A Zestimate is a machine-learning model that eats public records, user-submitted data, and listing history and spits out an estimated value. Zillow publishes its own error rates. The national median error for on-market homes runs about 2.4%, but for off-market homes it jumps to roughly 6.9% ^[2]. On a $400,000 house, 6.9% is a swing of $27,600 in either direction.

That spread is one reason boards say no. The bigger problem is where the number comes from. A Zestimate is not a sale. It is not an appraisal signed by a licensed professional following the Uniform Standards of Professional Appraisal Practice. It has no signer, no license number, and no liability attached to it.

Some states put this in writing. The Illinois Property Tax Code requires evidence of value in the form of comparable sales or a certified appraisal ^[3]. An automated valuation model from a consumer website is neither. Other states use similar language.

Here is the practical test. Can you put a person on the stand who will swear to the number and explain how it was built? If not, the board cannot cross-examine it. Zestimates fail that test every time. A closed MLS sale passes, because the sale itself is a public record.

What makes MLS data credible evidence for an appeal?

MLS stands for Multiple Listing Service, the regional databases where licensed brokers share listings and log closed sales. The data is not spotless. It is still the closest thing residential real estate has to a verified transaction record.

When an agent lists a home and it sells, the MLS captures the closing price, date, days on market, square footage, bed and bath count, lot size, and condition notes. After closing, that sale usually gets recorded with the county recorder and flows into the assessor's public database. So MLS records and assessor sales records draw from the same pool.

What makes MLS data stronger than a bare county printout is the extra detail: condition at the time of sale, whether the deal was arm's-length, whether the seller paid concessions. Those details change the math. A sale where the seller kicked in $15,000 toward the buyer's closing costs is not a clean $300,000 sale. It is closer to $285,000.

For an appeal, you want comps that:

• Sold within 12 months before your assessment date (some states allow 24 months in a thin market) ^[4]
• Sit within about one mile in a city, or the same school district or neighborhood in a rural area
• Land within roughly 20% of your home's gross living area
• Match on lot size, age, and condition
• Were arm's-length deals, not foreclosures, estate sales, or family transfers, unless the market is very thin

Five to seven solid comps is the target. Three is the floor. Past ten, you start watering down your own case.

How do you actually get MLS data without a real estate license?

This is the wall most DIY appellants hit, and there are four real ways around it.

First, ask a real estate agent for a Comparative Market Analysis. Most agents run one free, hoping you list with them someday. A CMA pulls straight from the MLS and formats the comps for you. Be blunt about what you need: sold comps from the 12 months before your assessment date, not current listings. Current listings are asking prices, and asking prices are not market value.

Second, your county assessor's own sales database. Nearly every assessor in the country publishes recent sales, often searchable by neighborhood, subdivision, or zip code. This data comes from deed recordings, the same source the assessor used against you. The catch is that it usually lacks the condition detail the MLS carries.

Third, Redfin's sold listings. Redfin is an MLS-affiliated brokerage, and its sold data is far more reliable than Zillow's because it pulls directly from MLS feeds. Filter for "sold" and set your date range. Those are real closed prices, not estimates.

Fourth, pull deed transfers straight from your county recorder's website. These are public records. You will not get condition notes, but you get the sale price, date, and parcel number. Cross-reference with the assessor's property record card for size and age.

If you want a step-by-step process for collecting, formatting, and presenting all of it, the TaxFightBack DIY appeal kit walks you through building a package boards accept, without handing a contingency firm 30 to 50 percent of your savings.

Local assessor sites often have their own sales search tools. Start with our guides on the cook county tax assessor tax bill, gwinnett county tax assessor, and bexar county tax assessor.

MLS comps vs. Zillow: a direct comparison for appeal purposes

Here is how the two stack up on the things that decide appeals.

Factor	MLS / Assessor Sales Data	Zillow Zestimate
Source	Actual closed transaction, recorded deed	Algorithmic model, public + user data
Verifiable?	Yes, via county recorder	No independent verification
Accepted as evidence?	Yes, in virtually all jurisdictions	Rarely, often explicitly excluded
Condition detail	Often included (MLS remarks)	None
Date of sale	Exact	N/A (no sale)
Error rate	Sale price = actual price paid	Median off-market error ~6.9% [2]
Cost to obtain	Free (assessor) to free-with-agent (MLS CMA)	Free
Examiner can cross-examine?	Yes (public record)	No

The cost row surprises people. Everyone assumes MLS data means paying for expensive tools. It does not. Your assessor posts free sales data. A friendly agent runs a free CMA. Redfin shows sold prices for free. The one time you pay is when you hire a licensed appraiser for a formal USPAP appraisal, which runs roughly $300 to $600 for a residential property ^[5]. That carries the most weight of any evidence, but you rarely need it for a smaller dispute.

Can you use Zillow for anything useful in an appeal?

Yes, within tight limits. Here is where Zillow earns its keep.

Use the sold listings page. Zillow pulls sold data from public records and MLS feeds, the same as Redfin. The sold prices for specific addresses are generally accurate because they trace back to deed recordings. That is usable. The Zestimate is not the sold price. Do not mix them up.

Use it for market trend context. If Zillow shows values in your zip code fell 8% between January 2023 and January 2024, that trend supports your story. You still need actual comps to anchor the number.

Use it to find properties to investigate. Zillow can surface houses near yours that sold recently. Once you have the addresses, verify the prices through your county recorder or assessor database. Zillow is the search box, not the source.

The line is simple. Zillow as a directory that points you to real sales is fine. Zillow as your evidence of value gets your appeal denied.

How should you format your comp evidence for the hearing?

Presentation matters more than DIY appellants expect. A board reviewer may see 40 to 80 cases in a single day. A clean, organized package gets read. A stack of screenshots gets skimmed and set aside.

The standard format is a comp grid, also called a sales adjustment grid. Each comparable sale gets a row, with columns for address, sale date, sale price, square footage, price per square foot, lot size, age, bed and bath count, and condition. Your subject property sits at the top. Then you show how each comp measures against it.

A comp bigger than your house? Adjust its price per square foot down. It has a finished basement and yours does not? Adjust down. This is what appraisers do, and you do not need a license to run a simplified version. The goal is a range of adjusted values that cluster below your assessed value.

Add a cover sheet with your parcel number, assessment date, the assessor's value, and your claimed value. Attach a map showing where each comp sold relative to your home. Print it all in a stapled packet with numbered exhibits.

In high-volume urban counties like Los Angeles or Cook County, a cleaner packet means better odds. Our guides on la county property tax and cook county tax assessor tax bill cover local board expectations.

What do assessors and boards actually use to value your home?

Know the assessor's method and you know where to hit it.

Most residential assessors use mass appraisal, a statistical process that groups similar properties and applies valuation models to the whole batch. The core input is the same sales data you can pull: recent arm's-length sales in each neighborhood. The assessor then runs a ratio study, comparing assessed values to sale prices to check accuracy ^[6].

The International Association of Assessing Officers sets the professional standards for mass appraisal. Its Standard on Ratio Studies says a well-performing assessment should have a median sales ratio between 90% and 110% of market value, with a coefficient of dispersion (a measure of uniformity) below 15% in urban areas ^[6]. When your property falls outside those bounds, that is an argument for assessment error.

Show that comparable sales in your neighborhood imply your home is over-assessed relative to the IAAO ratio standard, and you have a technical argument boards respect. It beats a personal opinion every time.

That is the whole reason MLS data wins and Zestimates lose. A ratio study needs actual sale prices. A Zestimate is a prediction of a possible sale price, which is circular and untestable.

Does a formal appraisal beat MLS comps in an appeal?

Usually yes, in a formal contested hearing. A USPAP-compliant appraisal from a licensed appraiser is the strongest single piece of evidence you can hand a board. It carries a signature and license number, the appraiser can testify and be cross-examined, and it follows a method boards are trained to read ^[5].

The catch is cost. A residential appraisal runs $300 to $600 in most markets, more in high-cost areas ^[5]. That only pencils out if your potential tax savings clear the fee. On a $10,000 annual bill with a 10% over-assessment, you might save $1,000 a year, and a $400 appraisal pays for itself in five months. On a $2,000 bill, the math gets tight.

For informal hearings and lower-dollar disputes, clean MLS comps often do the same job for free. Many assessors settle at the informal level before a case ever reaches a formal board, especially when a homeowner arrives with five solid, well-formatted comps that point to a lower value. Save the full appraisal for cases where the assessor stonewalls at the informal stage or the dollars justify it.

For commercial property in high-value markets like Santa Clara County or New York City, a formal appraisal is almost always worth it. See our guides on santa clara property tax and nyc property tax.

Are there states where online AVM data is specifically excluded from appeals?

Several states limit admissible evidence to verified sales and certified appraisals, which functionally shuts out any automated valuation model output, Zestimates included.

Illinois statute 35 ILCS 200/16-180 sets the evidence bar at the Property Tax Appeal Board: evidence must establish "the fair cash value of the subject property," and in practice the PTAB wants comparable sales or an appraisal ^[3]. Boards of review in Cook County and elsewhere apply similar rules locally.

In Texas, Appraisal Review Board procedures under Texas Tax Code Section 41.43 put the burden of proof on the owner, who must establish value "by a preponderance of the evidence" ^[7]. That standard needs verifiable data. Texas ARBs routinely toss Zestimate printouts as insufficient.

In New York, Small Claims Assessment Review proceedings under Real Property Tax Law Article 7 require the petitioner to provide evidence of market value, which in practice means comparable sales or an appraisal ^[8].

No state statute I know of names Zillow or the Zestimate outright. The exclusion is practical, not literal. Boards apply evidence standards that automated estimates simply cannot meet.

What mistakes do homeowners make when using online valuation tools for appeals?

The top mistake is submitting a Zestimate printout as the main evidence. It is an easy trap, because Zillow is the first place most people check to see what their home is worth. But a Zestimate is an estimate of an opinion, not evidence of a sale.

Second most common: using current active listings as comps instead of sold listings. Asking prices are not market value. A seller can list at any number they dream up. The only price that tells you what the market really thinks is the one a buyer agreed to pay at closing.

Third: cherry-picking comps that are too different from your home and hoping nobody notices. If your house is 1,800 square feet and your comp is 1,200, an experienced board member flags it in seconds. The size adjustment matters, and if you skipped it, you look unprepared.

Fourth: leaning on sales that are too old. In an active market, a sale from 18 months ago may not reflect current conditions. Check your state's rules. Most want sales within 12 months of the assessment date ^[4].

Fifth: ignoring condition. A house that sold cheap because it needed a new roof is not a good comp for your well-kept home, and a board will adjust it back up. Use comps in similar shape, or make the adjustment yourself and show your work.

Where can you find your county's own sales data for free?

Every county assessor in the United States has to keep property records, and the vast majority post recent sales online. The layout changes county to county. The data is almost always free.

Start at your county assessor's website. Look for "sales search," "recent sales," "property lookup," or "comparable sales." Many assessors let you search by neighborhood, zip code, subdivision, or a radius you draw on a map.

Some state-level tools are even better. Ohio county auditor sites often carry full sales history with transfer details. California counties pair a recorder and an assessor, and you can pull recorded sale prices by parcel. Florida's county property appraiser sites (the state calls the assessor an appraiser) are among the most data-rich anywhere.

The IAAO keeps a directory of assessing offices if you cannot find your county's site ^[6]. Your state's department of revenue or taxation often runs a statewide sales ratio database too.

For specific county tools, check our resources on montgomery county property tax, hennepin county property tax, and bibb county tax assessor.

Frequently asked questions

Will a board of equalization accept a Zillow Zestimate as evidence?

Almost never. Boards require proof of actual market value, meaning verified arm's-length sales or a certified appraisal. A Zestimate is an algorithmic prediction with no signer, no verification, and no evidentiary chain. Zillow's own site states the Zestimate is not an appraisal and should not replace one. Submit a Zestimate printout alone and your appeal will almost certainly be denied.

How many comparable sales do I need for a property tax appeal?

Three is the floor most boards accept, but five to seven gives you a credible range that is harder to wave off. More than ten rarely helps and can dilute your case if some are weak matches. Each comp needs a sold date, verified sale price, square footage, and address. All comps should fall within the 12 months before your assessment date.

Can I get MLS data without being a real estate agent?

Yes. Ask a local agent for a free Comparative Market Analysis, which pulls straight from the MLS. You can also use Redfin's sold listings (MLS-sourced), your county assessor's or recorder's free sales database, or Zillow's sold prices (the recorded sale price by address, not the Zestimate). The assessor's own sales data is the most credible, because it is the same source they used.

What is the difference between a Zestimate and a sold price on Zillow?

They are entirely different. The Zestimate is a computer model's prediction of what a home might sell for today. The sold price is the recorded price from a closed transaction, pulled from public deed records. For an appeal, only the sold price is usable. Look for properties marked 'sold' with a specific date, not the Zestimate on a current listing.

How old can comparable sales be for a property tax appeal?

Most states prefer sales within 12 months before the assessment date. Some boards allow up to 24 months if the local market has few sales. Check your state's rules: Illinois PTAB prefers sales within 12 months of the assessment year, and Texas ARBs follow similar guidance. Sales older than two years get little weight almost anywhere.

Is a formal appraisal always better than MLS comps in an appeal?

At formal contested hearings, a USPAP-compliant appraisal from a licensed appraiser outweighs raw MLS comps. But for informal reviews and lower-dollar disputes, clean, well-formatted MLS comps often produce the same result at zero cost. A residential appraisal runs roughly $300 to $600. Only pay for one when the potential savings clearly clear that fee over a reasonable time.

What radius should I use when searching for comparable sales?

In urban and suburban areas, start with one mile from your property. In rural areas where sales are sparse, expand to the same school district, township, or neighborhood your assessor uses. Aim for geographic proximity, not a fixed rule. If 20 homes sold within half a mile last year, stay tight. If three did, widen the search until you have enough comps.

Can I use Redfin data instead of Zillow for an appeal?

Redfin's sold data beats Zillow's for appeals because Redfin is an MLS-affiliated brokerage and pulls sold prices straight from MLS feeds. The sold prices for specific addresses are real closed transaction prices. Still, present the underlying sale (address, date, recorded price, square footage) rather than a Redfin screenshot. The county recorder record or assessor database is the most defensible source.

What adjustments do I need to make to comparable sales?

Adjust for differences in square footage, lot size, bed and bath count, age, condition, garage, and major amenities. A simple approach is to calculate price per square foot for each comp, then note whether your home is better or worse on each feature. Comp has a pool and yours does not? Cut its price per square foot. Show your math. Boards respect transparency over false precision.

Does it help to show that my neighbor's identical home has a lower assessment?

Yes. This is an equity argument, valid in most states. Uniform and equal treatment is a constitutional requirement in property taxation. If a nearly identical home one street over is assessed 15% lower, that is evidence of unequal treatment. Pull the neighbor's assessment from the public database, document the physical similarities, and present both parcel records side by side. It works alongside or instead of a sales comp argument.

What if my county's assessor website does not have a sales search tool?

Go to the county recorder's or clerk's website, which records deed transfers separately from the assessor. Most recorder offices post recent real estate transfers as public records. You can also visit in person and request a sales report. If both fail, a local agent's MLS CMA is your next best source. The IAAO directory at iaao.org lists assessing offices that can point you to the right county resource.

Can commercial property owners use the same MLS approach?

Commercial appeals are more complex. The income approach (based on rent and capitalization rates) often outweighs the sales comparison approach for income-producing properties. MLS data covers some small commercial sales, but dedicated databases like CoStar are more complete. For high-value commercial property in markets like New York City or Los Angeles, a licensed commercial appraiser is almost always worth the cost given the stakes.

How do I know if my assessment is worth appealing in the first place?

Pull three to five recent sold comps from your assessor's database or Redfin right now, using the criteria above. Average the price per square foot, multiply by your home's square footage, and compare to your assessed value. If your assessed value runs more than 5 to 10 percent above what the comps imply, you likely have a case. Most jurisdictions have an informal review stage where even a small savings is easy to capture.

Sources

1. International Association of Assessing Officers (IAAO), Standard on Ratio Studies: Verified arm's-length sales are the standard input for mass appraisal and assessment ratio studies.
2. Zillow, Zestimate Accuracy page: Zillow states the Zestimate is not an appraisal and cannot be used in place of one; median off-market error rate is approximately 6.9%.
3. Illinois General Assembly, 35 ILCS 200/16-180, Property Tax Appeal Board evidence standards: Illinois PTAB requires evidence of fair cash value through comparable sales or a certified appraisal.
4. Illinois Property Tax Appeal Board, PTAB Rules and Procedures: Comparable sales used as evidence should generally be from within 12 months of the assessment date.
5. Appraisal Institute, Residential Appraisal Cost Guide: A USPAP-compliant residential appraisal typically costs $300 to $600 in most U.S. markets.
6. International Association of Assessing Officers, Standard on Ratio Studies (2013): IAAO standard requires median assessment-to-sale ratio between 90% and 110% and coefficient of dispersion below 15% in urban areas.
7. Texas Comptroller of Public Accounts, Texas Tax Code Section 41.43, Protest Procedures: Texas Tax Code Section 41.43 requires property owner to establish value by a preponderance of the evidence at an ARB hearing.
8. New York State Department of Taxation and Finance, Small Claims Assessment Review (SCAR) procedures: New York SCAR proceedings require petitioner to provide evidence of market value, meaning comparable sales or an appraisal.
9. Uniform Standards of Professional Appraisal Practice (USPAP), The Appraisal Foundation: USPAP defines the standards a licensed appraiser must follow; appraisals meeting USPAP are generally accepted as evidence in tax appeal proceedings.
10. National Association of Realtors, MLS Explained: MLS databases record closed sale prices, dates, and property details for transactions facilitated by licensed brokers.