Comparable sales approach to value: how it works in a property tax appeal

Last updated 2026-07-09

What is the comparable sales approach to value?

The comparable sales approach estimates what your property would sell for by looking at what genuinely similar homes actually sold for recently. Appraisers call it the sales comparison approach. The Appraisal Institute defines it as "a method of estimating the value of a property by analyzing recent sales of comparable properties." ^[1] Assessors lean on it for houses because houses trade often enough to leave a real trail of market evidence.

The logic is simple. Three houses nearly identical to yours sold for $320,000, $330,000, and $315,000 in the past year. The market is telling you your house is worth around $320,000. The assessor's job is to find those sales, adjust for the differences between each one and your property, and land on a supported number.

Adjustments are the hard part. No two houses match exactly. The assessor has to account for square footage, lot size, age, condition, garage, basement, bathrooms, where the house sits in the neighborhood, and sometimes a view or a busy road out front. Each difference gets a dollar or percentage adjustment. Get the adjustments wrong and the whole value is wrong. That's where most appeals are won or lost.

How do assessors actually choose comparable sales?

Most county assessors run a computer-assisted mass appraisal (CAMA) system that pulls sales automatically by property type, neighborhood code, year built, and square footage. ^[2] The software spits out sales that fit the model. An appraiser reviews the output, or in a lot of offices barely glances at it.

The International Association of Assessing Officers (IAAO) writes the standard most assessors are supposed to follow. The IAAO Standard on Mass Appraisal of Real Property says comparable sales should be recent (within 12 months of the assessment date, or 24 months when the market is thin), arm's-length (no family sales, foreclosures, or estate sales at distressed prices), and genuinely similar in physical characteristics. ^[3]

Assessors stretch those rules all the time. You'll see comps from 18 or 24 months back sitting next to fresh ones with no time adjustment. You'll see a 1,400-square-foot ranch used as a comp for your 2,100-square-foot two-story. You'll see a sale across a six-lane highway used when closer sales existed that just didn't fit the model as cleanly.

Here's what to check when you pull the assessor's comp selection:

• Sales date: anything older than 12 months should raise a flag, especially in a fast market.
• Proximity: comps should sit in your neighborhood or a truly similar one. A cross-ZIP comp needs a real justification.
• GLA (gross living area): most appraisers want comps within 20-25% of your square footage. A 2,000 sq ft house against a 1,200 sq ft house is a stretch.
• Property type: a two-story comp for a ranch works only with careful adjustments. A condo as a comp for a detached single-family home is usually flat wrong.
• Condition and age: a renovated 1960s house and an untouched 1960s house are different animals even on the same street at the same size.

What adjustments do assessors make and why do they matter?

Every difference between a comp and your property gets a dollar or percentage adjustment. Comp A has a two-car garage and your house has none, so the assessor subtracts a garage adjustment from comp A's sale price to make the two apples-to-apples. Your house has an extra bathroom, so the assessor adds a bathroom adjustment to comps that don't.

Adjustments are where assessors have the most discretion and the most room to be wrong. The IAAO says adjustments should come from market data, ideally through paired sales analysis (two nearly identical homes that differ in one feature) or regression. ^[3] In mass appraisal, adjustments often come from tables built years ago and rarely touched since.

Common adjustment categories:

Feature	Typical adjustment range	How assessors derive it
Gross living area (GLA)	$40-$150 per sq ft	Regression or paired sales
Garage (per stall)	$5,000-$25,000	Paired sales
Bathroom (full)	$3,000-$15,000	Paired sales
Lot size (per sq ft over/under)	$1-$10 per sq ft	Regression
Year built / condition	0.5%-2% per decade	Depreciation tables
Location (busy road, view, etc.)	3%-15%	Market surveys

The ranges are wide because markets are different. A GLA adjustment in rural Iowa isn't the same as one in suburban Chicago. ^[4]

For your appeal, the question is whether the assessor's adjustments match your local market. Say the assessor used $80/sq ft for GLA but recent paired sales in your neighborhood support $55/sq ft. That's an argument the adjustments are inflated and the value should drop. The argument needs numbers behind it, not a hunch.

How do you find your own comparable sales to challenge the assessment?

This is the core skill in a DIY appeal. You're building an alternative sales grid that, once adjusted, points to a lower value than the assessor's number.

Start with your county's recorded deed database or a free aggregator like Zillow or Redfin. ^[5] You want arm's-length sales from the 12 months before your assessment date. Note that: the assessment date, not the appeal deadline. It's usually January 1 of the tax year. If your assessment date is January 1, 2024, you want sales from roughly January 1, 2023 through December 31, 2023.

Criteria for a strong comp:

1. Same neighborhood or subdivision where you can. If you have to cross a major boundary, explain why in writing. 2. Within 20% of your GLA. Smaller is fine. A much larger house is a harder sell to a board. 3. Same property type. Single-family detached only, unless your property is a townhouse or condo. 4. Condition as close to yours as you can find. If your house needs a roof and the comp is fully updated, the hearing officer will notice. 5. Arm's-length. Pull the deed. If grantor and grantee share a last name, skip it. If it was a bank sale or a quit-claim, skip it.

Three comps is the floor. Five is better. More than seven and you'll bury a hearing officer who has six minutes for your case.

Build a simple grid. Your property in the first column, each comp in the columns after. List sale price, sale date, GLA, lot size, garage, bathrooms, and anything else that matters. Apply your adjustments. The adjusted sale prices should cluster below your assessed value. That gap is your argument.

Want a structured template and step-by-step instructions for building the grid? The TaxFightBack appeal kit walks you through this exact process so you keep 100% of whatever reduction you win.

What makes a comp "arm's length" and why does it matter?

An arm's-length sale is one where buyer and seller are unrelated, both are informed, neither is under duress, and the property got reasonable market exposure. ^[6] Assessors are supposed to use only arm's-length sales as comps because distressed and related-party sales don't reflect true market value.

Sales that usually are not arm's-length:

• Foreclosures or bank-owned (REO) sales, which often sell at a discount.
• Short sales, where the lender takes less than the balance owed.
• Family transfers, gifts, or divorce settlements.
• Estate or probate sales with limited market exposure.
• Sales between business entities under common ownership.

Use this two ways. First, if the assessor used a non-arm's-length sale as a comp, challenge it. A foreclosure at $240,000 in a market where clean sales run $310,000 has no business in the comp pool. Second, keep non-arm's-length sales out of your own grid. If you must use one, adjust for the distress discount and say so out loud.

Most county assessors post a list of sales they excluded from their ratio studies, which tells you which sales they already flagged as non-arm's-length. In Illinois, the Cook County Assessor's office publishes its sales validation procedures. ^[7] Look for a similar document from your own assessor.

How is the sales comparison approach different from the income or cost approach?

Assessors have three recognized approaches to value. The sales comparison approach uses market sales. The income approach capitalizes rental income into a value and mostly applies to apartment buildings, commercial property, and other income-producing real estate. The cost approach estimates the depreciated replacement cost of the improvements plus land value, and assessors use it for new construction, special-use properties, and places where there simply aren't enough sales. ^[1]

For single-family homes, the sales comparison approach almost always carries the most weight, and most hearing boards expect you to argue it. If your property is a rental or a small multifamily, the income approach might hand you a lower value, so it's worth running both.

The cost approach can work in your favor for older homes with real depreciation. If the assessor built the value from cost but skimped on physical depreciation for a 50-year-old house with original systems, you can argue the cost approach overstates the number.

For most homeowners reading this, stick to comparable sales. It's the language hearing officers speak, it's the most defensible, and the data is public.

What time adjustments are required if the market changed?

Markets move. A sale from 18 months ago in a rising market is worth less as a comp than a sale from six months ago. Appraisers call this a market conditions adjustment or a time adjustment. ^[1]

Say your assessment date is January 1, 2024 and you're using a comp that sold in June 2022. You need to show how much the market moved between June 2022 and January 2024 in your area, then adjust the comp's sale price up (or down, if prices fell) by that percentage.

You can pull the number from the Federal Housing Finance Agency's House Price Index (FHFA HPI), which publishes quarterly appreciation rates by metro area. ^[8] If the FHFA HPI shows your metro rose 6% from June 2022 to January 2024, you multiply that older comp's sale price by 1.06 before it goes in your grid.

This cuts both ways for an appellant. In a falling market (late 2022 through 2023 in many metros), older comps from the peak need a downward time adjustment, which drags them toward a lower value. That's a strong argument if the assessor used peak-market sales to set your current-year value.

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

Three is the floor. Five is the sweet spot. Boards and hearing officers want a pattern, not a single outlier. One low sale proves nothing. Three low sales that all bracket your property in size, age, and location tell a story.

Quality beats quantity. One perfectly matched comp from three doors down that sold 12% below your assessed value is stronger than six mediocre comps from across town. If you've got one great one, lead with it and back it up with two or three supporting sales.

Keep your grid under six comps in most cases. A long grid just hands the assessor's rep more targets to pick apart. If you've found eight strong comps, use the five most similar and drop the other three into a backup exhibit labeled "additional supporting sales."

Some states hand you an extra edge. In Texas, the Appraisal Review Board decides value on the evidence presented, and the appraisal district has to give you its evidence before the hearing. ^[9] You can see their comps ahead of time and sharpen your grid to match.

What if there are no recent sales near your property?

Rural properties, unusual homes, and stable neighborhoods where nobody sells hit this wall constantly. No recent sales doesn't kill your appeal. It means you work harder and stay honest about the limits of your evidence.

Your options:

• Widen the search radius, but stay in the same market area and document why the farther-out comps are representative.
• Extend the window to 24 months, apply a market conditions adjustment for the gap, and disclose what you did.
• Shift to the cost approach as your main argument if your home is old and the assessor underapplied depreciation.
• Request the assessor's own comp grid through a public records request. Many states require them to produce it. If their comps are weak, argue the weakness instead of building your own.
• Use listing prices as supporting data. Listings aren't sales, so they aren't true comps, but an active listing at $280,000 for a house larger and newer than yours supports an argument that your $310,000 assessment runs high.

In thin markets, the income approach sometimes produces a more defensible value for single-family rentals. If your property is rented, run the numbers.

How do hearing boards evaluate comparable sales evidence?

Hearing boards and appraisal review boards (the name varies by state) are usually three people who aren't professional appraisers. They see dozens of cases a day. They want clear, organized evidence that makes one simple point: the comps say X, the assessment says Y, the assessment should come down to X.

Put your grid on a single one-page summary if you can. Add a map showing where each comp sits relative to your property. Add a photo of your house and each comp. Show your math. The adjusted sale prices should obviously cluster below your assessed value.

Skip the jargon. Say "adjusted sale price," not "reconciled value indication." Say "the assessor's number is $345,000, but these five recent sales of similar homes average $298,000 after adjustments," and then stop talking.

Expect pushback. The assessor's rep will argue your comps are inferior locations, worse condition, or that your adjustments aren't supported by market data. Have a short answer ready for each comp: why this sale matches, what adjustments you made, and where the adjustment figures came from.

For how boards run in your area, your county assessor's appeal page is worth reading. See how the Los Angeles County property tax appeal process works, how Maricopa property tax hearings are structured, or if you're near Chicago, how the Cook County tax assessor tax bill process flows.

What are the most common mistakes homeowners make with comparable sales?

Using active listings instead of closed sales. A listing is an asking price, not a transaction. Boards generally won't take it as primary evidence.

Picking the cheapest sales without checking comparability. Cherry-pick a distressed sale or a much smaller house just because it's cheap, and the assessor's rep shreds it in 30 seconds.

Forgetting to adjust. If your comp has one fewer bathroom than your house, subtract a bathroom adjustment from your property's value rather than citing the comp's raw sale price. Raw prices without adjustments aren't analysis.

Using sales after the assessment date. If the assessment date is January 1, 2024, a sale from March 2024 stays out of your primary grid. Some states allow post-assessment sales as supporting evidence but not as primary comps.

Ignoring the assessment ratio. Some states assess at less than 100% of market value. There, the comparable sales approach gives you market value, and then you apply the state's ratio to get the assessed value that lands on your bill. Check your state's statutory ratio before you walk in. ^[10]

Overcomplicating it. A clean three-comp grid with clear adjustments beats a ten-comp spreadsheet nobody can follow in the six minutes you get at the hearing.

How does the comparable sales approach apply in your specific county or state?

The mechanics are the same everywhere. The procedural rules aren't. Deadlines, forms, how many hearings you get, whether you can appeal to a court after the board rules, and whether the assessor has to hand over its evidence in advance all change by jurisdiction.

A few examples of how local rules bite into comp evidence:

In Texas, Property Tax Code Section 41.43 puts the burden of proof on the appraisal district if it raised your assessed value from the prior year. That flips the room. You show up with your comps, and the district has to prove theirs are better. ^[9]

In Illinois, the Cook County Assessor reassesses on a three-year cycle and uses a sales ratio standard. If the median ratio for your property class sits well above the state's 10% residential level, that's a systemic argument stacked on top of your comp-by-comp case. ^[7]

In California, Proposition 13 caps increases at 2% a year until the property sells, so comparable sales aren't typically used to raise assessments mid-ownership. They do come into play when a change in ownership triggers reassessment and in supplemental assessments. ^[11]

In Georgia, the Gwinnett County tax assessor posts appeal forms and comp requirements online. The Coweta County tax assessor and Cherokee County tax assessor follow similar Georgia Board of Equalization procedures, where comparable sales are the primary evidence in a residential appeal.

In Texas, the Bexar County tax assessor and surrounding counties use Appraisal Review Boards that now have to give you the district's evidence packet at least 14 days before your hearing under HB 988 (2023). That changes how you prep, because you can react to their comps before you build your own. ^[9]

The TaxFightBack appeal kit includes state-specific checklists and deadline calendars so your comp evidence gets filed in the right format at the right time. The comp approach is universal. The rules around it are local.

Frequently asked questions

What is the comparable sales approach in simple terms?

It's figuring out what your home is worth by looking at what similar nearby homes actually sold for recently. Assessors use it to set your assessed value. You can use it to challenge that value by finding recent sales of similar homes that closed for less than your assessment implies, then adjusting them for any differences.

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

Three is the minimum most hearing boards expect. Five is the practical sweet spot because it shows a pattern rather than a single outlier. Keep it under seven unless the comps are exceptionally strong. A longer grid just gives the assessor's representative more to attack.

Can I use Zillow or Redfin sales data to find comps?

Yes, for identifying candidate comps. Both aggregate MLS and public deed data and are fine starting points. Verify each sale against your county's recorded deed database before you use it. Zillow and Redfin sometimes have errors in sale dates, prices, or square footage, and you need the recorded figure, not the estimate.

What is an arm's-length sale and which sales should I avoid?

An arm's-length sale is one between unrelated buyers and sellers with no duress and reasonable market exposure. Avoid foreclosures, bank-owned sales, short sales, family transfers, estate sales with limited exposure, and any sale where the parties look related. Non-arm's-length sales in your grid will sink your credibility with the board.

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

The IAAO standard recommends sales within 12 months of the assessment date as primary comps. Going back 24 months is acceptable when sales are thin, but apply a market conditions adjustment (using FHFA HPI data or local MLS statistics) to account for price changes over that period.

What adjustments do I need to make to comparable sales?

Adjust for every meaningful difference between each comp and your property: gross living area, lot size, garage stalls, bathrooms, year built, condition, and location factors. Each adjustment moves the comp's sale price toward what your home would have fetched under the same conditions. The adjusted prices, not the raw sale prices, support your value conclusion.

Can I use comparable sales from a different neighborhood?

Only if you can show the neighborhoods are genuinely similar in price level, property type, and demand. Assessors and boards are skeptical of cross-neighborhood comps. If you must go outside your neighborhood, document why those sales are representative and admit the limitation in your presentation rather than hiding it.

What if the assessor's comparable sales are different from mine?

That's the whole point of the hearing. You present your grid arguing your comps are more similar to your property. They defend theirs. Attack theirs on comparability (size, location, condition, sale date) and defend yours on the same grounds. The board decides whose evidence is more persuasive.

Does the comparable sales approach work differently for condos or townhouses?

The approach is the same but the comp pool is narrower. Use only condos or townhouses as comps, never detached single-family homes. Focus on the same complex or a similar one. HOA fees, floor level, view, and parking are important adjustment factors that don't come up in single-family comparisons.

What is a market conditions adjustment and when do I need one?

A market conditions adjustment accounts for price changes between a comp's sale date and your assessment date. If the market rose 8% in the 18 months between a comp's sale and your assessment date, that comp needs an 8% upward adjustment. In a falling market it goes the other way. Use FHFA HPI quarterly data for your metro to calculate it.

How do I find the assessor's comparable sales to see what they used?

Start with your assessment notice. Many counties attach a comp grid or point you to one online. If not, file a public records request for the property record card and the sales grid for your parcel. Most states require the assessor to produce this. In Texas, the appraisal district must send you their evidence at least 14 days before your ARB hearing.

Can I use a pending sale of my own home as evidence of value?

A signed purchase contract for your property is the strongest possible evidence of its market value, since it's a real buyer and seller agreeing on a price for that exact property. Present it. Boards and courts generally view it favorably. A listing without a contract is weaker but still worth submitting as supporting context.

What if the comparable sales approach gives a higher value than my assessment?

If your honest comp analysis lands at or above your current assessment, you don't have a viable appeal on value grounds, and filing one could invite an upward correction in some jurisdictions. Run your own analysis honestly before you file, and walk away if the numbers say your assessment is already low.

Is the comparable sales approach used for commercial property appeals too?

Yes, though commercial hearings often lean on the income approach for properties with reliable rental income. For commercial properties without strong income data, like owner-occupied small businesses or special-use buildings, comparable sales may be the only viable approach. The mechanics are the same. Finding genuinely similar sales is just harder.

Sources

1. Appraisal Institute, The Appraisal of Real Estate, 15th Edition (summary definitions): The sales comparison approach is defined as a method of estimating value by analyzing recent sales of comparable properties; the three approaches to value are sales comparison, income, and cost.
2. International Association of Assessing Officers (IAAO), Standard on Mass Appraisal of Real Property: Most county assessors use computer-assisted mass appraisal (CAMA) systems; the IAAO publishes standards for mass appraisal practice including comp selection criteria.
3. IAAO, Standard on Mass Appraisal of Real Property (Section 5): Comparable sales should be recent (within 12 months, or 24 months when the market is thin), arm's-length, and physically similar; adjustments should be derived from market data such as paired sales analysis or regression.
4. IAAO, Standard on Ratio Studies: Adjustment amounts vary significantly by market area; no single adjustment table applies nationally.
5. Zillow Research, Zillow Data: Zillow aggregates MLS and public deed sale data and is an acceptable starting point for identifying comparable sales; individual records should be verified against county deed databases.
6. IAAO, Glossary for Property Appraisal and Assessment: An arm's-length sale is defined as a transaction between unrelated parties with no duress and reasonable market exposure; non-arm's-length sales are excluded from valid comp pools.
7. Cook County Assessor's Office, Illinois: The Cook County Assessor publishes sales validation procedures and conducts reassessments on a triennial cycle; the Illinois statutory assessment level for residential property is 10% of market value.
8. Federal Housing Finance Agency (FHFA), House Price Index (HPI): FHFA publishes quarterly house price appreciation rates by metropolitan statistical area, which appraisers use to calculate market conditions adjustments for comparable sales.
9. Texas Comptroller of Public Accounts, Property Tax Code Chapter 41: Texas Property Tax Code Section 41.43 places the burden of proof on the appraisal district when it raised the assessed value from the prior year; HB 988 (2023) requires the appraisal district to provide their evidence at least 14 days before an ARB hearing.
10. Lincoln Institute of Land Policy, Significant Features of the Property Tax: Many states assess residential property at less than 100% of market value; the statutory assessment ratio varies by state and must be applied to convert a market value conclusion into the correct assessed value.
11. California State Board of Equalization, Proposition 13 Overview: California Proposition 13 limits annual assessment increases to 2% per year until the property is sold; sales comparison is used when a change in ownership triggers reassessment.
12. National Taxpayers Union Foundation, Property Tax Appeal Data: Studies suggest a significant share of residential properties in the U.S. are overassessed, though the exact percentage varies by jurisdiction and study methodology.