How is a Zestimate calculated?
Zillow's model compares your house with others using public data (bedrooms, square footage, year built, past sale prices) and homes currently or recently listed. Zillow itself publishes that the error for homes not listed for sale is much larger than for listed homes.
Why it misses so much on older homes
- It cannot see inside: the model does not know the kitchen is from 1970, the roof leaks, or the pipes are corroded. Two neighbors, one renovated and one original, can have nearly the same Zestimate but sell for very different prices.
- Square footage mismatches: a garage conversion or unpermitted addition is often not in county records. Zillow may undercount, or an owner may edit the data to overcount. For buyers using a loan, appraisers usually do not count unpermitted space.
- Stale data: older homes sell less often, so the model leans on farther or different homes.
Your property tax value is not market value either
Many owners look at the assessed value on the tax bill and assume it is the price. Under California's Proposition 13, assessed value rises at most 2% per year from purchase. For a house bought in the 1990s, that figure is usually far below market.
Estimate more accurately in 30 minutes
- Find homes that have sold (not just listed) in the last 3–6 months, within about half a mile
- Pick similar ones: same bedrooms and baths, size within about 20%, same era
- Look at the photos: which are renovated, which are original like yours
- Take the price per square foot of the ones closest to your condition
- Multiply by your permitted square footage
For more confidence: ask an agent for a comparative market analysis (CMA), usually free, and get a few written cash offers to compare.
How we do it
When we make an offer, we include the list of sold comparables we used and the repair costs by item, so you can check our work. Try the comparison calculator →
This article is general information only, not legal or tax advice. Laws change; ask an attorney or CPA about your specific situation.