Your county appraisal district has a hard job. In Harris County alone, the appraisal district (HCAD) has to place a value on roughly 1.8 million parcels every single year. They don't send an appraiser to walk through your home. They don't check whether your kitchen was remodeled in 1985 or last summer. They don't know that your foundation has a crack, or that a busy commercial parking lot opened next door.
Instead, they use something called mass appraisal. A computer model takes a handful of data points about your home — square footage, year built, neighborhood, recent sales nearby — and spits out a number. Then that number becomes the basis for your property tax bill.
Mass appraisal works reasonably well on average. The problem is, your home isn't average. And when the model is wrong about your home, you end up paying taxes on a value that doesn't reflect reality.
Here are the most common errors we see on appraisal records, and how you can spot them before they cost you thousands of dollars.
Error #1: Wrong Square Footage
This is the single most common error, and it's also one of the easiest to prove. Appraisal district records are often built from old permits, original builder plans, or even rough sketches from decades ago. If the record says your home is 2,400 square feet but it's actually 2,150, you're being taxed on 250 square feet that doesn't exist.
How to verify: Measure your home yourself, or pull your original closing documents and compare the listed square footage. You can also pull up your parcel on your county's online portal (for Harris County, that's hcad.org) and compare. Any discrepancy larger than 50 square feet is worth fighting.
Error #2: Missed Condition Issues
The mass appraisal model assumes your home is in "average" condition for its age. It has no way of knowing about:
- Foundation cracks or slab movement
- An outdated kitchen with 1970s cabinets
- A roof that's near the end of its life
- Old HVAC systems or water heaters
- Plumbing or electrical issues
If your home has any of these, it is worth less than the model thinks. Photos, repair estimates, and contractor bids are powerful evidence at a hearing. A $15,000 foundation repair estimate can directly reduce your assessed value by that amount or more.
Error #3: Incorrect Property Features
Sometimes the record just has the wrong facts. We've seen appraisal records that list:
- 4 bedrooms when the home has 3
- A two-car garage when it's actually a carport
- A swimming pool that was filled in years ago
- Lot size that's larger than the actual deed describes
- A finished basement that was never finished (or doesn't exist — common in Texas)
Every wrong feature inflates your value. Pull your property card from the county portal and check every single line. If you bought the home recently, compare it against your inspection report.
Error #4: Using Bad Comps
The appraisal district justifies your value by pointing to "comparable sales" — recently sold homes that supposedly match yours. But the computer picks these comps automatically, and it often picks poorly.
Bad comps include:
- Homes in a different, nicer neighborhood
- Homes of a different type (a two-story comp for a one-story home)
- Homes with major upgrades yours doesn't have
- Sales from 18+ months ago in a shifting market
- Homes on quiet streets compared to yours on a busy road
This is often the strongest line of attack in an appeal. If you can show that three recently sold homes similar to yours went for less than your assessed value, you have a case. We cover this in detail in our guide on how comparable sales lower your property tax.
Error #5: Failure to Adjust for Market Shifts
Appraisal districts send out notices in the spring based on sales data that may be six to twelve months old. In a cooling market, that lag matters a lot.
If home prices in your neighborhood peaked in 2022 and have since come down 5-10%, your 2026 assessment might still reflect the peak. The district is slow to recognize downturns, and it's on you to point this out. Pull recent sales (last 3-6 months) from Zillow, Redfin, or Realtor.com and compare them to what was selling a year ago.
Error #6: Not Accounting for Location Negatives
The computer model knows your ZIP code. It doesn't know that your lot backs up to a high-voltage power line, or that a noisy industrial yard is across the fence. Location negatives that the model typically misses include:
- Backing up to commercial property (stores, gas stations, warehouses)
- High-voltage transmission lines overhead
- Flood plain (especially Zones AE and VE)
- Major road or highway noise
- Drainage ditches or retention ponds
- Train tracks within earshot
These all reduce market value and should reduce your assessment. Bring photos and a map to your hearing. A typical overassessment analysis often reveals 8-12% overvaluation from location factors alone.
Error #7: Ignoring Deferred Maintenance
"Deferred maintenance" is the industry term for repairs you haven't gotten to yet — peeling paint, a failing deck, cracked driveway, old windows, worn carpet. None of it appears in the appraisal record. All of it reduces what a buyer would actually pay.
Walk around your home with a notebook. Write down every item a buyer would ask to be fixed or priced into the deal. Get rough repair estimates — most contractors will provide them free. Add it up. That number is a direct argument for reducing your assessed value.
How to Verify Your Records
Before you can fight an error, you need to know what the county thinks is true. Here's your checklist:
- Pull your property card. For Harris County, go to hcad.org and search by address. Every county has a similar portal.
- Print it. You'll want to mark it up.
- Check every field: square footage, bedroom count, bath count, year built, lot size, garage type, pool, features.
- Compare against your closing documents or a recent home inspection.
- Walk your home with the property card in hand. Note every discrepancy.
- Document condition issues with dated photos and repair estimates.
- Pull 3-5 recent comparable sales from your neighborhood using a free real estate site.
If you find one or more errors — and almost everyone does — you have the basis for a protest. Our complete guide to the Harris County appeal process walks through the filing and hearing steps.
The Bottom Line
Mass appraisal is a volume business. It is built to be roughly right across millions of homes, not precisely right about yours. The appraisal district isn't being malicious — they're being efficient. But efficient systems make mistakes, and those mistakes cost homeowners real money every year.
The good news: once you know what to look for, errors jump off the page. And Texas law gives you a formal process to correct them. You don't have to accept whatever number arrives in your mailbox.
If you're in Harris County and you'd like a free analysis of whether your home is overassessed — with real comparable sales data pulled for your specific address — visit our Harris County property tax appeal page. We'll tell you in about 60 seconds whether you have a case worth fighting.
Related Guides
- Complete Guide to Harris County Property Tax Appeals in 2026
- Is My Home Overassessed? How to Tell in 10 Minutes
- Comparable Sales in a Property Tax Appeal: What Counts
- How Comparable Sales Lower Your Property Tax
- Harris County Property Tax Appeal Guide
Ready to find out if your home is overassessed? Start your free analysis for Harris County — no account required, results in under a minute.