Property Tax Guide

What Are Comparable Sales, and Why Do They Win Property Tax Appeals?

Published September 28, 2026

If you've read anything about fighting a high assessment, you've run into the word "comps." Comparable sales are the core of almost every successful residential property tax appeal. They're recent, closed sales of homes like yours, and they show what buyers actually paid, which is the thing your assessment is supposed to reflect.

The short version: your assessment is an estimate of market value, and comparable sales are the most direct evidence of market value there is. When the value the county implied for your home sits above what truly similar homes sold for, a few well-chosen comps make that case better than anything else you could bring.

A good comp is…	A weak comp is…
A closed sale	A listing, an asking price, or an online estimate
Recent, ideally near your assessment's valuation date	Two or three years old in a market that has moved
Nearby, in the same or a truly similar neighborhood	Across town, or across a school-attendance line
The same property type, size, age, and condition	A different style, a much bigger house, or a gut rehab
Arm's-length: unrelated parties, open market	A foreclosure, a family sale, or an estate liquidation

Why comparable sales carry so much weight

Assessors don't appraise homes one at a time. They use mass appraisal, which values thousands of parcels at once from models built on local sales data. The professional standard for that work, published by the International Association of Assessing Officers (IAAO), ranks the sales comparison approach first for single-family homes and calls it "the best approach" for them.

So when you bring comparable sales to a hearing, you're arguing on the assessor's own terms. You aren't asking the board to accept a new theory of value. You're showing that the method they already rely on gives a lower number when it's applied carefully to your house. A model tuned to thousands of sales can still miss on any one home: a dated interior it never saw, a busy road, a floor plan buyers don't want. A handful of close comps is how that miss becomes visible.

It also explains why the other kinds of evidence homeowners bring tend to fall flat. Your tax bill going up, a neighbor's lower assessment on a different house, or a feeling that the number is high aren't evidence of market value. Closed sales are.

What makes a sale "comparable"

A sale earns the name by resembling your home on the features that drive price. In rough order of importance:

1. Location. Same neighborhood first. A sale a few streets away usually beats a closer match on paper in a different part of town.
2. Timing. Assessments are valued as of a specific date, which may be months before your notice arrived. Sales close to that date are strongest. Fannie Mae's appraisal guidance expects comparables that closed within the last 12 months, while noting that the best comp isn't always the most recent one.
3. Type and size. Same style of house (ranch against ranch, townhouse against townhouse) and similar finished square footage.
4. Age, condition, and features. Bedrooms, bathrooms, garage, basement finish, lot size, and how recently the house was updated.

The sale also has to be a real market transaction. The IAAO defines an arm's-length sale as one between unrelated parties who each know the market and neither of whom is under undue pressure to buy or sell. Its sales-verification standard lists the kinds of transfers often thrown out as invalid, including sales between relatives, sales involving banks as buyer or seller, estate settlements, and court-ordered sales. If the assessor wouldn't use a sale, the board probably won't credit it either, even when it helps you.

Why listings and online estimates don't count

A listing is what a seller hopes to get. An automated home-value estimate is a model's guess. Neither one shows what a buyer actually paid. Even in mortgage appraisal, where lenders care a great deal about getting value right, Fannie Mae allows current listings only as supporting data next to closed sales. Treat them the same way: fine as context, never the core of your case.

How to adjust a comp, and why you adjust against yourself

No two houses are identical, so a comp's raw sale price is only a starting point. You adjust it for each meaningful difference to estimate what that house would have sold for if it matched yours:

• The comp has a finished basement and yours doesn't → adjust the comp's price down.
• Your house has an extra bathroom the comp lacks → adjust the comp's price up.
• The comp sold eight months before the valuation date in a rising market → adjust for time. The IAAO standard calls for time adjustments based on market analysis, not guesswork.

The adjustments that go against you matter most. Boards see a steady stream of homeowners who pick only the cheapest sales and ignore every difference that would push the number up. If your comps plainly include the ones that hurt your case, with honest adjustments, the board has much more reason to trust the rest of your numbers.

Putting comps to work in your appeal

1. Find the market value your county implied. On many notices that means dividing the assessed value by your state's assessment ratio. Our guide to whether your home is over-assessed walks through it.
2. Pull three or four closed sales that match on location, timing, type, and size. That's also the floor in residential appraisal: Fannie Mae requires at least three closed comparables in an appraisal report.
3. Compare on price per square foot, then make honest adjustments.
4. Check that the gap is worth arguing. A few percent is normal noise in any mass appraisal. A gap in the low double digits is where comps tend to carry the day. Then work out what a win is actually worth.
5. Check your state's rules before you file: the deadline, any assessment cap, and whether the board can raise your value. Your state's property tax appeal guide covers all three.

PROppeal does this comparable-sales analysis for you. It pulls licensed sale records for your address, applies your state's ratio and cap rules, and tells you plainly whether the comps support an appeal, including when they don't.

Property tax rules and deadlines vary by jurisdiction and can change — verify with your county before relying on this.