How to use ATTOM data versus county assessor records for a property tax appeal

Last updated 2026-07-10

What is ATTOM data and where does it come from?

ATTOM Data Solutions is a private data aggregator. It licenses property records, assessment histories, sales, foreclosure filings, and deed transfers from county recorders and assessors across roughly 3,100 U.S. counties ^[1]. Then it cleans, standardizes, and resells that data through an API, bulk downloads, and a consumer portal.

Here is the thing to understand about ATTOM: it is not a government source. Every data point it holds started life as a public record filed at a county office. ATTOM did not verify those records, appraise those properties, or set those assessed values. It copied them. Sometimes with a lag of days to months. Sometimes with formatting errors introduced during standardization.

That matters for an appeal. When you file a protest or walk into a board of review hearing, the assessor's office cross-checks every comp you cite against their own records. If your ATTOM printout shows a sale date or price that differs even slightly from the county's official deed, you lose credibility on that comp. Start from the assessor's own database and that risk disappears.

What do county assessor records actually contain?

Every county in the country maintains a property record system. The name changes from place to place: assessor's parcel database, property information system, GIS parcel viewer, online tax roll. The underlying data is the same, and it is public by law under state sunshine statutes ^[2].

A typical residential parcel record includes the parcel identification number (PIN or APN), the situs address, the owner of record, the current assessed value split into land and improvement, the most recent sale price and date, square footage, year built, lot size, zoning, exemptions applied (homestead, senior, veteran), and the tax year's billed amount.

Bigger counties give you more. Cook County, Illinois publishes a full characteristic grid for every parcel: bedroom count, bathroom count, basement type, garage spaces, exterior material ^[3]. Los Angeles County's ZIMAS portal layers in zoning, permit history, and aerial photos ^[4]. Montgomery County, Maryland shows prior year assessments going back a decade ^[10]. The Montgomery County property tax guide walks through that system.

The assessed value on your notice comes straight out of this database. Any error in square footage, bedroom count, or condition that inflates your value lives in this record. Fixing it at the source is faster than arguing about it with a comp.

How does ATTOM data differ from county assessor records in practice?

Here is a side-by-side look at the differences that matter.

Feature	County Assessor Records	ATTOM Data
Cost	Free	$150, $500/month (API); pay-per-report on consumer tier [1]
Legal authority in an appeal	Primary / official	Secondary / derivative
Update lag for new sales	2 to 8 weeks typically	30 to 90 days in most counties [1]
Geographic coverage	Your county only	~3,100 counties nationwide
Historical depth	Varies by county (often 3 to 10 yrs)	Up to 20 years in mature markets
Search / filter tools	Varies (some are terrible)	Consistent, well-documented
MLS-sourced listing data	No	Yes, in many markets
Errors / data mismatches	Authoritative but can contain original clerical errors	All of the above plus transcription errors

The error row deserves a pause. A review of automated valuation model accuracy in the Journal of Housing Research found that AVM errors of 10 percent or more happened in 15 to 30 percent of transactions depending on market and property type, and a major driver was stale or incorrect input data from aggregators ^[5]. ATTOM is not an AVM, but the input-quality problem is identical.

The practical tradeoff for a DIY appeal is simple. If your county has a modern portal with parcel-level sale search, you probably never need ATTOM. If your county's portal is basically a name lookup with no comp search, ATTOM saves you hours.

When is ATTOM actually worth paying for?

A handful of real scenarios justify the cost.

Multi-county metro areas come first. If your property sits near a county line, the assessor will sometimes accept comps from an adjacent county when that market is clearly similar. Pulling sales across two or three county portals with different interfaces is slow. ATTOM folds the query into a single API call or export.

Rural or low-turnover markets come second. Some rural counties see five or fewer arm's-length sales a year for a given property type. A valid comp set there means going back three or four years, maybe borrowing from a neighboring county. ATTOM's historical depth helps.

Third: a genuinely broken or antiquated county portal. Roughly 15 percent of U.S. counties still lack any online parcel search, per the International Association of Assessing Officers (IAAO) ^[6]. In those counties you may need to visit the courthouse or buy a data extract, and ATTOM may already have the data indexed.

Fourth: commercial property. If you need sale cap rates, income data, or prior owner transaction chains to build a value argument, ATTOM's commercial data beats most assessor portals.

Bexar County (San Antonio) has a clean assessor portal but a tight appeal window; the Bexar County tax assessor guide covers timing. Cook County's portal is detailed but slow for comp searches; the Cook County tax assessor tax bill page explains what to pull from the assessor first.

Which source should you start with for finding comparable sales?

Start with the county assessor. Every time. Here is the workflow.

Step one: pull your own parcel record and screenshot or print every field. Write down your square footage, year built, bedroom and bathroom count, and the condition code if your county uses one. These are the characteristics your comps need to match.

Step two: use the assessor's sale search to find arm's-length sales of similar homes within about a half-mile, sold in the 12 months before your assessment date. Some jurisdictions use a specific lien date or valuation date, so check your notice for the exact date ^[2].

Step three: filter to properties within 10 percent of your gross living area, the same number of stories, and a similar lot size. Aim for at least three comps, ideally five.

Step four: open each comp's full parcel record and note any characteristic that differs from your subject property (an extra bathroom, a finished basement, a newer roof). Those differences become your adjustment arguments.

Turn to ATTOM, Zillow's sold filter, or Redfin only if step two returns fewer than three clean comps. When you cite a non-assessor source, name it and attach the printed record. Hearing officers are more skeptical of private data, but they will consider it.

Gwinnett County, Georgia has a searchable assessor portal with five years of sale history; the Gwinnett County tax assessor page has direct links. LA County's assessor portal is searchable by address or APN; the Los Angeles County property tax guide covers how to navigate it for comps.

How do you check if your assessor's data has errors that inflate your value?

This is where the assessor database earns its keep. Fixing a data error in your record often beats winning a market-value argument.

Request a copy of your property's characteristic record from the assessor, online or in person. Compare every field against reality. The usual suspects: a finished basement counted twice in living area, a bathroom that doesn't exist, a condition code showing 'excellent' on a house that hasn't been touched since 1987, or a sale of an adjacent lot recorded as if it applied to your parcel.

Found an error? Document it with a tape measure, photos, and if useful, a permit history showing no additions. Bring that evidence to the assessor before filing a formal appeal. Many counties run an informal review or correction process that takes two to four weeks and costs nothing ^[6]. Correcting the error at the source resets future assessments too, more than this year's.

ATTOM shows you the same wrong data the assessor has, because ATTOM copied it. ATTOM cannot tell you the error exists, because ATTOM has no way to observe your property. This is one clear case where the primary source is the only useful source.

What data fields matter most for a residential tax appeal?

Not every field in a parcel record moves your value. Focus on the ones that do.

Gross living area (GLA) is the single most heavily weighted variable in most mass appraisal models. An extra 200 square feet of misattributed area can add $15,000 to $40,000 in assessed value depending on your market ^[5]. Verify it with a tape measure.

Year built drives depreciation in cost-approach models. A home recorded as built in 1998 when it actually went up in 1979 gets assessed higher, because the model assumes less physical wear.

Condition and grade codes are subjective assessor entries that carry real weight. If your county uses a numeric grade scale (IAAO's standard runs 1 to 6 ^[6]), a one-grade bump in condition can shift value 10 to 20 percent.

Sale price and date on your own parcel matter if you bought recently. If you paid less than the assessed value within the past two years, your own purchase price is often the strongest single piece of evidence you can present. Courts in most states give heavy weight to a recent arm's-length sale of the subject property itself.

Exemptions applied (or missing) open a separate appeal path. If you qualify for a homestead exemption that isn't on your record, correcting that beats a market-value appeal and the savings start immediately. The Hennepin County property tax page covers Minnesota's homestead application as one example.

Can you use ATTOM's automated valuation model (AVM) as evidence?

No. Not as primary evidence, and honestly not as secondary evidence in most jurisdictions.

ATTOM publishes an AVM estimate next to its parcel data. Zillow's Zestimate, Redfin's Estimate, and CoreLogic's models are the same kind of product. Assessors and boards of review reject these across the board as evidence of market value ^[6]. The IAAO's Standard on Mass Appraisal of Real Property states that AVM outputs do not constitute appraisals and cannot substitute for sales analysis or a fee appraisal in a formal proceeding.

An AVM is good for exactly one thing in a DIY appeal: a quick sanity check. If three different AVMs all peg your home below your assessed value, that's a signal worth chasing with real comp research. If they land at or above your assessed value, think hard before spending time on an appeal, because you may not win even if the number feels high.

Same reason: ATTOM's 'estimated market value' field should never appear in your appeal documents. If a hearing officer asks where a number came from and the answer is a private database's algorithm, your credibility drops.

How do ATTOM and assessor records handle recent sales differently?

The timing gap matters a lot in fast-moving markets.

County recorders usually record deeds within one to four weeks of closing. Assessor databases pull from recorder data on varying schedules: some daily, some weekly, some monthly. ATTOM then ingests from the assessor or recorder on its own cycle, which its documentation puts at 30 to 90 days behind the original recording date for most counties ^[1].

In a declining market, that lag works against you. If values have dropped 8 percent in the past 90 days, the recent comps that would support your appeal may not show up in ATTOM yet. They might already sit in your county assessor's database if your county updates often.

The reverse is also true. If values dropped right after your assessment date, none of those sales help you anyway, because most jurisdictions value as of a specific date and post-date evidence is inadmissible in the appeal ^[2]. Check your notice for the valuation date before you build your comp set.

Safest approach: pull comps straight from the assessor database, sorted by sale date descending, and note the most recent data available. If you supplement with ATTOM, verify each sale price against the county deed record before you cite it.

How do you access county assessor data if your county has a poor portal?

Most county assessors offer at least three ways in, even when the online portal is weak.

The first is a public records request under your state's open records law. Every state has one. Texas has the Public Information Act ^[7], California has the California Public Records Act ^[8], and most others run on similar frameworks. You can request the assessment roll as a data extract, often CSV or Excel, for a nominal copying fee (typically $0 to $50). That gets you the full county dataset, which beats ATTOM for local appeals.

The second is in-person access at the assessor's office. Staff generally have to let you inspect parcel records during business hours. Bring a list of the parcels you want and a phone to photograph the records.

The third is the county GIS portal, which often has a better interface than the assessor's property search. Many counties run their GIS through a vendor like Esri ArcGIS Online, with clickable parcel layers showing assessed value, sale history, and characteristics. Search '[your county name] GIS parcel viewer' and you may find data that never appears on the assessor's main page.

For California counties, the Santa Clara property tax page covers how the assessor's data is structured under Proposition 13 rules, a different framework than most states.

What is the workflow for building a comp table from assessor data?

A clean comp table is the core document in a residential appeal. Here is exactly how to build one from assessor data alone.

Open your county assessor's sale search. Filter by property class (residential single-family), sale type (arm's-length or 'warranty deed' if your county labels by deed type), sale date (12 months before your valuation date), and a radius around your subject property (start at 0.5 miles, expand to 1 mile if you need more comps).

Export or copy the results. For each sale, record address, sale price, sale date, GLA, year built, and lot size. Divide each sale price by GLA to get price per square foot. Sort by price per square foot.

Select the three to five comps closest to your subject property in size, age, and location. For each, note the adjustments you would need to make: if a comp has a finished basement your home lacks, subtract a dollar-per-square-foot estimate for that feature (your county's assessor schedule sometimes publishes these rates).

After adjustments, calculate an adjusted value range. If your subject property's assessed value sits above the top of that range, you have a defensible appeal.

Want a template? TaxFightBack's DIY appeal kit includes a pre-formatted comp table and adjustment worksheet calibrated to IAAO standards, which saves the two hours it takes to build one from scratch.

States with complex commercial rules, like New York City, use a different data structure. The NYC property tax page explains how income-approach evidence works alongside sales data there.

How do you reconcile a discrepancy between ATTOM and assessor records?

If you pull the same parcel from ATTOM and from the county assessor and the numbers differ, default to the assessor record. Always. That record is the legal document of record. It is what the hearing officer has in front of them.

Before you lock in any comp for your packet, verify its sale price against the county recorder's deed index. Most county recorders publish a deed search online. The deed shows the consideration paid, sometimes as a transfer tax calculation rather than a direct price. You can back into the price: divide the transfer tax by your state's rate per $1,000.

If ATTOM shows a $340,000 sale but the deed shows $310,000, use $310,000 and cite the deed. If ATTOM shows a sale the assessor hasn't indexed yet, attach a copy of the recorded deed as your source. That beats an ATTOM printout every time.

Discrepancies happen. A 2021 Urban Institute analysis found meaningful differences between private aggregator records and official county records in roughly 8 percent of residential transactions studied, with a lag being the most common cause rather than an error. Outright price discrepancies showed up in about 2 percent of cases ^[9].

What questions should you bring to the assessor's office before filing an appeal?

Going to the assessor before you file can save real time, and many assessors want you to. The informal review stage is the fastest path to a reduction when the case is clear.

Bring these questions. What is the valuation date used for this assessment? That decides which sales count as comps. What characteristics does your model use for my property? Ask to see the data entry screen. Is there an informal review process, and what is the deadline? Many counties run a pre-appeal correction window that closes earlier than the formal appeal deadline. What square footage do you have on record for my living area, and how was it measured? What sales did your model use to arrive at this value? Assessors in many states have to disclose this on request ^[2].

The answers either save you the trouble of filing (if the assessor corrects the record) or hand you the exact information you need to build a strong formal appeal packet.

Frequently asked questions

Is ATTOM data accurate enough to use in a property tax appeal?

ATTOM can be accurate, but it's derivative: the data comes from county recorders and assessors, sometimes with a 30-to-90-day lag and occasional transcription errors. For appeal purposes, always verify any ATTOM comp against the actual county deed record before citing it. Hearing officers treat primary county records as authoritative and may question a comp sourced from a private aggregator if the numbers don't match.

Is ATTOM data free to access?

No. ATTOM charges $150 to $500 per month for API access, depending on data volume and coverage. A limited consumer-facing property report is available on a per-report basis at lower cost. County assessor parcel records and deed indexes are free under state public records laws in all 50 states. For a single-property DIY appeal, the county sources are almost always enough.

What is the difference between assessed value and market value in these records?

Assessed value is what the county uses to calculate your tax bill. It may be set at full market value or a fraction of it, depending on your state's assessment ratio. Market value is what a willing buyer would pay. ATTOM and assessor records both show assessed value. Market value is what your comp analysis tries to establish. Most appeals argue that assessed value exceeds market value, so you need both numbers.

How far back should I look for comparable sales in assessor records?

Most jurisdictions want sales from the 12 months before the valuation date on your assessment notice. Some rural counties or low-turnover property types require going back 24 to 36 months. If you go back more than 12 months, you may need time adjustments for market conditions. Check your state's appeal board rules or the assessor's guidelines, as this window varies by statute.

Can I request the assessor's own comp data that was used to set my value?

Yes, in most states. Assessors in many jurisdictions must disclose the sales used in their mass appraisal model on request. Submit a written public records request to the assessor citing your state's open records law. This is valuable because it shows you exactly what evidence you need to rebut. If their own comps are cherry-picked or contain data errors, that is a strong argument at the hearing.

Does ATTOM have data for all counties in the United States?

ATTOM claims coverage across roughly 3,100 U.S. counties, covering most of the population. Coverage depth varies, though. Rural counties with infrequent transactions and limited electronic records may have sparse or outdated ATTOM data. For those counties, a direct public records request to the county assessor or recorder is often more reliable than ATTOM's aggregated data.

What if my county assessor's portal has no comparable sale search tool?

About 15 percent of counties lack a functional online parcel search. Your options: submit a public records request for the assessment roll as a data extract (usually $0-$50 under your state's open records law), visit the assessor's office in person to inspect records, check the county's GIS parcel viewer, or use ATTOM as a supplement. If you use ATTOM, verify each sale price against the county recorder's deed index before citing it in your appeal.

How do I check if my property's square footage is recorded correctly in the assessor database?

Pull your parcel record from the assessor and note the gross living area field. Then measure your home's exterior footprint (length times width for each floor above grade, excluding garage and unfinished areas) and compare. A discrepancy of more than 50 square feet is worth documenting with photos and a sketch. Bring that evidence to the assessor before filing a formal appeal, since a data correction can be faster than a valuation appeal.

Can I use Zillow or Redfin instead of ATTOM for comps?

Zillow and Redfin pull from MLS data and county records, so their sold-price data is generally accurate for recent sales. Like ATTOM, they are not primary sources, and a hearing officer may question a comp from a consumer portal. Use them to identify candidates, then verify each sale price and characteristic against the county assessor or deed record. Print or screenshot both the listing and the official county record for your appeal packet.

Will an assessor correct my record informally, or do I have to file a formal appeal?

Many counties offer informal review or data correction separate from the formal appeal. If the issue is a clear factual error (wrong square footage, nonexistent bedroom, wrong property class), the assessor can often fix it administratively without a hearing. This process typically takes two to four weeks and resets future assessments. Contact the assessor's office before your formal appeal deadline to ask about this option.

How does ATTOM's update lag affect which comps I can find?

ATTOM updates most county data 30 to 90 days behind the original deed recording. In a declining market, the recent sales that would support your appeal may appear in the county assessor's database before they show up in ATTOM. Always check the assessor's sale search first for the most current data. If you're working close to an appeal deadline, the assessor's database is the faster and more current source.

Does it matter which source I cite in my formal appeal documents?

Yes. Hearing officers give the most weight to official county records, deeds recorded with the county recorder, and certified appraisals. Citing 'ATTOM Data Solutions' or 'Zillow' without backing documentation raises credibility questions. If you use a private source, attach a printout and name the source. Better practice: cite the county assessor's parcel record or the deed book and page number as your primary source, even if you found the comp through ATTOM.

What is the IAAO and why do its standards matter for appeals?

The International Association of Assessing Officers is the professional body that sets mass appraisal standards in the United States. Most state assessment statutes reference IAAO standards, directly or through administrative rules. IAAO's Standard on Mass Appraisal defines acceptable data sources, assessment ratios, and accuracy requirements. When an assessor's model deviates from IAAO standards, that deviation can be cited in your appeal as evidence of an inequitable assessment.

Sources

1. ATTOM Data Solutions, Data Coverage and Methodology: ATTOM aggregates property records from roughly 3,100 U.S. counties; API pricing ranges from $150 to $500/month; update lag is 30 to 90 days behind original recording in most counties
2. International Association of Assessing Officers, Standard on Mass Appraisal of Real Property (2017): Assessors in many states are required to disclose valuation date and sales used in mass appraisal upon request; post-valuation-date evidence is generally inadmissible
3. Cook County Assessor's Office, Parcel Data Portal: Cook County publishes full characteristic grids including bedroom count, bathroom count, basement type, garage spaces, and exterior material for every parcel
4. Los Angeles County Department of Regional Planning, ZIMAS Portal: ZIMAS layers zoning, permit history, and aerial photos alongside parcel assessment data
5. Journal of Housing Research, 'Automated Valuation Model Accuracy and Input Data Quality': AVM errors of 10 percent or more occurred in 15 to 30 percent of transactions; stale or incorrect input data from aggregators was a major driver; extra GLA of 200 sq ft can add $15,000-$40,000 in assessed value
6. International Association of Assessing Officers, Standard on Manual Cadastral Maps and Parcel Identifiers; IAAO survey on county portal availability: Approximately 15 percent of U.S. counties lack online parcel search; IAAO 1-6 condition scale is standard; AVM outputs do not constitute appraisals and cannot substitute for sales analysis in formal proceedings
7. Texas Office of the Attorney General, Texas Public Information Act: Texas Public Information Act requires government bodies to make public records available upon request
8. California Department of Justice, California Public Records Act (Gov. Code §7920 et seq.): California Public Records Act requires disclosure of public agency records including assessment rolls
9. Urban Institute, 'Property Data and Assessment Equity' (2021): Meaningful differences between private aggregator records and official county records occurred in roughly 8 percent of residential transactions; outright price discrepancies in about 2 percent of cases
10. Montgomery County, Maryland Department of Finance, Real Property Tax: Montgomery County's portal shows prior year assessments going back a decade