Zillow says your house is worth $342,000. Your neighbor sold an identical floor plan for $361,000 two months ago. Both numbers are right. They are also both wrong. Here is the real math, what the Zestimate gets right, what it misses, and what it means if you are pricing your home in Central PA.
What the Zestimate actually does
Zillow runs your property through a national algorithm using:
- Public records (square footage, lot size, beds/baths, last sale price, year built)
- Recent comparable sales within a half-mile radius
- Tax assessments
- Aerial imagery for lot characteristics
- User-submitted updates if homeowners have claimed and updated their listing
It is good. It is also generic.
How wrong is it, on average?
Zillow itself publishes the data. Per Zillow's official Zestimate accuracy page:
| Home Status | Median Error Rate |
|---|---|
| On-market homes (currently listed for sale) | 1.74% |
| Off-market homes (not currently listed) | 7.20% |
That sounds small until you do the dollar math:
| Home Value | Off-Market 7.20% Error | On-Market 1.74% Error |
|---|---|---|
| $200,000 | $14,400 | $3,480 |
| $300,000 | $21,600 | $5,220 |
| $400,000 | $28,800 | $6,960 |
| $500,000 | $36,000 | $8,700 |
| $750,000 | $54,000 | $13,050 |
And that is the median error. Half of homes have larger errors. Some Zestimates are off by 15 to 20% — common for older homes with significant renovations not yet reflected in public records.
What it does not know
Zillow does not know:
- You put a new roof on last spring
- Your kitchen was renovated in 2024
- Your basement is finished (or that the listing data says "unfinished" when it is not)
- The comp three blocks over had foundation issues that knocked $25,000 off its price
- The comp on your street sold to a cash buyer who waived inspection (so the sale price does not reflect a normal financed transaction)
- Your HVAC is 25 years old and needs replacement
- You added a $20,000 fence and a $15,000 deck since the last public record
- The new development going in two streets over is going to drop comparable values
And critically, it does not know neighborhood-level nuance. In Chambersburg, a home four blocks from the high school sells for measurably more than the same home four blocks the other direction. Zillow does not see that. The algorithm uses a half-mile radius for comps and weights distance, not the nuance of which side of which street.
What a real CMA looks like
A Comparative Market Analysis from us starts with three to five recent sales of comparable homes within a half-mile, all closed in the last 90 days, all similar in size, age, condition, and finish level. Then we adjust line by line:
| Adjustment Factor | Typical Value Impact (Central PA) |
|---|---|
| Finished basement | +$10,000 to $25,000 |
| 2-car attached garage vs. detached | +$5,000 to $10,000 |
| Updated kitchen (last 3 years) | +$8,000 to $20,000 |
| Updated primary bath (last 3 years) | +$5,000 to $12,000 |
| Brand new roof (last 2 years) | +$5,000 to $10,000 |
| HVAC 20+ years old (vs. new) | -$5,000 to $8,000 |
| Corner lot vs. mid-block | +$3,000 to $7,000 |
| Backs to busy road | -$5,000 to $15,000 |
| Pool (in PA) | +$0 to $10,000 (often a wash) |
The result is usually within 1 to 2% of the eventual sale price, because we account for what the algorithm cannot see.
When Zillow is most accurate
The Zestimate works best for:
- Tract-built homes in large subdivisions where every house is similar
- Newer suburban developments (post-2000) with consistent floor plans
- Homes recently sold (the algorithm has fresh comp data)
- Homes in active markets with high transaction volume
When Zillow is least accurate
- Older homes (most of downtown Chambersburg, Mercersburg, Carlisle's historic district)
- Custom-built homes
- Rural properties with acreage
- Anything with major condition variation from neighbors
- Anything with significant updates not yet in public records
- Unique architectural styles (Victorian, mid-century, contemporary)
- Properties with land use complications (in-law suites, ADUs, mixed-use, etc.)
For most Central PA homes — particularly older ones in town centers or rural acreage properties — the Zestimate is a starting point only.
The Central PA market context
Here is why this matters right now: Chambersburg prices are up 13.9% year over year and 17201 specifically is up over 20%. Zestimates lag the market because they pull from completed sales (which closed 30 to 60 days after the offer was made). In a fast-appreciating market like ours, the Zestimate is often under what your home is actually worth — sometimes by tens of thousands of dollars.
If you priced off Zillow last year and pulled your home off the market because "Zillow says I am only worth $X," there is a real chance you left money on the table.
How to use the Zestimate appropriately
The Zestimate has its uses. Here is how to leverage it:
- Initial price-range research: Browsing homes for sale? The Zestimate gives you a quick sanity check on whether a listing price seems reasonable.
- Equity tracking over time: Watching your own home's Zestimate trend gives a rough sense of appreciation direction. Use it for trend, not absolute number.
- Tax appeal supporting evidence: If your county tax assessment exceeds your Zestimate by 15%+, you may have grounds for a tax appeal.
- Negotiating awareness: If you are buying, knowing the Zestimate gives you a data point to discuss with your agent. If you are selling, knowing yours helps you anticipate buyer questions.
What it should NOT do:
- Determine your asking price
- Decide whether to buy or sell
- Form the basis for an insurance valuation
- Drive an estate planning conversation
- Anchor a divorce settlement
For all of those, get a real CMA or a licensed appraisal.
Real Chambersburg case study
A recent client: 1940s brick colonial in 17201 ZIP. Zestimate said $268,000. Redfin Estimate said $281,000. Our CMA, accounting for a 2023 kitchen remodel, recent roof, and finished basement, priced it at $312,000 to $322,000.
It listed at $315,000. Sold for $328,000 in 9 days, 7 offers.
If the seller had priced off Zillow alone, they would have lost approximately $60,000.
Want a real number on your home?
We do free home valuations with no obligation. Takes about 30 minutes in person. We do a walk-through, pull current comps from the MLS (not the public record Zillow uses), make line-item adjustments for your home's specific features, and write up a one-page report.
The number is yours to use however you want — for tax appeals, insurance reviews, refinance conversations, divorce settlements, estate planning, or just curiosity. No obligation to list with us. Reach out when you are ready.