Zillow Zestimate vs. CMA: What Your King County Home Is Really Worth

Every King County homeowner has checked their Zestimate at least once. Most check it a lot more than that. It is free, instant, and sitting right there at the top of every Zillow listing, so it is easy to treat that number as a real answer instead of what it actually is: an educated guess built entirely from a spreadsheet, with no one having ever walked through your front door.

Zillow is more upfront about this than most people realize. The company’s own numbers show the Zestimate misses by a wide margin once a home is off the market, and King County’s mix of older housing stock, view premiums, and wildly different micro-markets from one zip code to the next makes that gap even more likely to matter here than in a lot of the country.

What a Zestimate Actually Is

Zillow is upfront about this, even if the giant number on every listing does not make it obvious: a Zestimate is an automated valuation model, not an appraisal. Zillow’s own help documentation describes it as an estimate built from public and user-submitted data, and states plainly that it is “not equivalent to an appraisal.”

Under the hood, it is a neural network, what Zillow calls the Neural Zestimate, mapping hundreds of millions of data points nationwide. For any single home, it pulls from county and tax assessor records, prior sale history, direct data feeds from MLS systems and brokerages (NWMLS included here in King County), basic home characteristics like square footage and bed and bath count, and for on-market homes, the current listing price and days on market.

That is a genuinely large amount of data. It is also entirely data the algorithm can read from a spreadsheet, nothing it can see, smell, or stand inside.

The Real Numbers Behind “Accurate”

Zillow reports two different accuracy figures, and the gap between them tells you most of what you need to know.

The gap is not a coincidence. Once a home is listed, the Zestimate leans heavily on the actual list price a human being set, an agent’s professional judgment gets folded back into the “estimate,” which makes it look more accurate without the algorithm actually getting smarter. Off-market, there is no such anchor. The Zestimate is working from stale data and statistical inference alone, and the error rate roughly quadruples.

A 7% error rate sounds small until you attach a real King County number to it. At the county’s current median sold price of $838,842, that is a swing of roughly $58,700, enough to change whether a seller is pricing competitively or leaving real money on the table, and enough to make a buyer’s “the Zestimate says it’s worth less” negotiating line something worth pushing back on with actual comps.

Why the Gap Runs Wider in King County

Automated valuation models struggle everywhere, but a few things about King County widen the miss specifically.

Huge price dispersion over short distances. A county that spans Sammamish and Auburn, Mercer Island and South Park, does not have one housing market, it has dozens of overlapping micro-markets, sometimes a few blocks apart, with very different price ceilings.

An older, heavily renovated housing stock. A large share of King County homes were built in the 1970s through 1990s and have since been updated, sometimes with permits that update county records, sometimes without. A fully remodeled kitchen and a 1988 original kitchen can sit in homes with identical tax records and wildly different Zestimates.

View and lot premiums the algorithm cannot price. Lake Washington, Puget Sound, and Mount Rainier views add real value that shows up in comparable sales but not in any structured data field. Two homes with identical square footage a block apart can differ by six figures because one has a filtered territorial view and the other looks at a fence.

Legal versus livable square footage. Basement conversions, ADUs, and finished spaces that were never permitted often are not reflected, or are reflected incorrectly, in county records, which throws off a model that leans on those records for its baseline.

Zestimate vs. County Assessed Value vs. an Agent’s CMA

These three numbers get confused constantly, and they’re answering three different questions.

Three numbers, three different jobs. Only one of them is built to set a list price.

Zillow’s own guidance agrees with this ranking: for serious pricing decisions, a CMA from an agent is the more reliable number, precisely because it incorporates local MLS detail and professional judgment that public data alone cannot capture. For the full mechanics of how a CMA is actually built and how to read one, see how to read a CMA in King County.

What This Means for You

If you’re selling: Use your Zestimate as a rough starting point at most. Before you set a list price, get a CMA, most agents, including me, will run one at no cost and no obligation. Off-market accuracy is roughly a coin flip on being within a meaningful margin; don’t price a six- or seven-figure asset off it.

If you’re buying: Don’t anchor an offer to a seller’s Zestimate, and don’t let a low Zestimate talk you out of a fair offer on a home you actually want. Ask your agent to run comps on the specific home, not the algorithm’s version of it.

If you’re just curious: A Zestimate is fine for casually watching your equity trend over months or years. It’s not the number to make a five- or six-figure decision on.

Frequently Asked Questions

A Zestimate is a fine place to start a conversation about your home’s value. It is a poor place to end one. If you are within a year or two of buying or selling in King County, the fastest way to close the gap between the algorithm’s guess and reality is still the same one it has always been: get someone who actually knows the neighborhood to look at the actual house.

Reach me at greg@livingoutsideseattle.com or 253-350-0045 whenever you want a real number instead of an estimated one.