Two numbers describe every house, and they disagree. The county prints one on the tax bill. The market sets the other at closing, and only for the houses that sell. Most explanations of assessed value vs market value stop at "the assessed one is lower." That is true on average and wrong often enough to cost you money.
I run FlaggedLeads, which joins code enforcement records to the county assessor's roll for every parcel in seven Puget Sound markets, and the roll carries the assessed value of each one. King County also records every sale. Join the two and you get something no explainer publishes: the actual gap, measured, across 20,172 arm's-length Seattle sales.
Assessed value is the dollar figure the county assessor assigns to a property for the purpose of calculating its property tax, set once a year as of a fixed date using mass appraisal. Market value is the price a willing buyer pays a willing seller, and it is only known when a sale closes. In Washington the assessor is required by law to aim at market value, so the gap between the two is timing, method, and condition rather than a built-in discount.
The assessor's roll is one of the five public records that describe a property, and the assessed value is the one field on it that investors misuse most.
For a repair project, after repair value (ARV) adds another condition assumption: what the house would be worth with a specified scope completed. Support that estimate with finished-home comps.
The five numbers on one house
| Number | Who sets it | When | What it is for | Where it goes wrong |
|---|---|---|---|---|
| Assessed value | The county assessor | Once a year, as of a fixed valuation date | The property tax bill | Lags the market by a year or more and never sees the inside |
| Appraised value | A licensed appraiser hired by a lender | At a purchase or refinance | Sizing the loan | One person's opinion on one day, anchored to the contract price |
| Market value | Buyers and sellers | Only when a sale closes | What the house is worth | Only known after the fact, and only for houses that sold |
| Automated estimate | A data vendor's model | Continuously | A starting guess | No inspection, thin on unusual houses |
| Asking price | The seller | At listing | Opening a negotiation | Aspirational by design |
Assessed and appraised get confused constantly. An appraisal is a licensed inspection of one house for one transaction. An assessment is a model run over every house in the county on the same morning.
Why the two numbers drift apart
Washington is a 100% state. RCW 84.40.030 says "all property must be valued at one hundred percent of its true and fair value in money," using comparable sales from the past five years, cost, or capitalized income. Many states assess at a fraction of market value on purpose, which is where the "assessed is always lower" folk wisdom comes from. Here the assessor is aiming at the sale price. The drift comes from three things.
The valuation date. King County's own explanation: "For taxes paid this year, we value most properties as of January 1 of last year." The bill you pay in 2026 rests on what the house was worth on 1 January 2025. In a rising market that alone puts the assessment under the sale price by a year of appreciation. In a falling one it does the opposite.
Mass appraisal. The county values residential property with a sales comparison approach and a cost approach, run as a model across the whole neighborhood. Nobody walks through your kitchen. RCW 84.41.030 requires that all real property "be revalued annually" and "physically inspected at least once every six years," so the model can carry a remodel, or a collapse, for years before an inspector sees it.
Condition the model cannot see. A house with a failed roof and a house with a new one, on the same street with the same square footage, get nearly the same assessment. The market treats them very differently. This is the drift that matters for finding deals, and it shows up in the numbers below.
The state checks the assessors' work every year. The Department of Revenue's 2025 ratio study compared assessed values with sale prices and found a statewide median assessment ratio of 0.94, meaning half of sold properties were assessed above 94% of their sale price and half below. The professional standard the report uses, from the International Association of Assessing Officers, calls anything between 0.90 and 1.10 acceptable, and King and Pierce counties both met every standard in the study. The ratio program uses sales from May 1 to April 30 each year to set the number.
So in this state the assessed value is not a discount. It is a year-old estimate of market value for an average house on that block.
What 20,172 Seattle sales show
Here is the measurement. I took every arm's-length King County sale in Seattle between 3 June 2024 and 1 September 2026, kept the ones recorded on a warranty or statutory warranty deed with a price above zero, and divided the sale price by the assessor's appraised total (land plus improvements) for that parcel as held in our property table. That left 20,172 sales.
| Sales | Median sale ÷ assessed | 10th percentile | 90th percentile | |
|---|---|---|---|---|
| All sales | 20,172 | 1.075 | ||
| Under $500,000 | 671 | 0.853 | 0.500 | 1.097 |
| $500,000 to $1M | 9,134 | 1.047 | 0.878 | 1.222 |
| $1M to $2M | 7,625 | 1.116 | 0.930 | 1.354 |
| $2M and up | 2,742 | 1.118 | 0.938 | 1.505 |
Source: FlaggedLeads live database, King County recorded sales joined to the assessor's roll, 20,172 arm's-length Seattle sales, June 2024 to September 2026. The middle half of all sales landed between 0.986 and 1.178.
Three readings.
The median Seattle house sold for 7.5% more than its assessed value, which is about what a year of appreciation and a conservative model should produce. Still, 28.6% of sales closed below the assessed value. The assessment is a center, and the spread around it is wide.
The gap grows with price. Houses over $1 million cleared their assessments by 11 to 12% at the median, and a tenth of the $2 million sales went 50% over. The model is most conservative exactly where the sales are least comparable.
The bottom of the market runs the other way. Houses under $500,000 sold for a median 85% of assessed value, and a tenth of them went for half. That band is where the condition the model cannot see lives: teardowns, estates, houses that have not been maintained in a decade. The assessor sees square footage and a lot. The buyer sees the roof.
One caveat on method. The assessed value is the roll as of our last refresh, so a 2024 sale is being compared with a slightly newer assessment than the one on its tax bill that year. If anything that pushes the 2024 ratios down, and the year-by-year medians (1.062 for 2024, 1.082 for 2025, 1.080 for 2026) suggest the drift is small.
Where the gap becomes a signal
This is why I care about assessed value at all. A house selling under its assessment is a house the market priced below the county's idea of an average house on that block, and the reasons for that are the reasons I look for.
Of the 20,172 sales, 1,701 were on properties with a code violation on file before the sale date. Those sold at a median 1.066 of assessed, barely below the 1.076 for everything else. The median is not the story. The tail is. Sales at 20% or more under the assessed value were 8.4% of the prior-violation group and 4.4% of the rest, nearly double.
Narrow it to the 84 arm's-length sales of Seattle properties with a vacant building case and the tail becomes the whole distribution. Median sale: 91.6% of assessed. Below assessed: 64.3%. Twenty percent or more below assessed: 31.0%. Median price $668,500. And 48 of those 84 buyers, 57%, were an LLC, trust, or other entity, against 14.3% of all buyers.
The assessed value did not find those deals. The code record did. What the assessed value does is give you a denominator, the same one for every parcel in the county, so you can rank a list of distressed properties by how far under the county's number a deal would have to land. That is the arithmetic behind the equity estimate in every tool that sells one, including the vendors who buy the assessor roll in bulk.
How to use assessed value, and how not to
Use it to rank, never to offer. The assessed value tells you what the county thought an average house of that size on that block was worth in January of last year. It is the right number for sorting a thousand leads. It is the wrong number for pricing one. The offer comes from comparable sales and a repair budget.
Read it off the parcel record. Every county assessor publishes it free, next to the owner of record and the tax-billing mailing address, when you look up a property by address. The county GIS parcel map shows the same field on a click. For a whole market at once it comes in the bulk parcel roll as one row per parcel.
Compare a debt to it. When you are working a tax delinquent list, the balance owed against the assessed value tells you how much pressure the owner is under.
Distrust it under $500,000, on lots worth more than the house, and right after a remodel. The sub-$500k spread above is the widest band in the table. In neighborhoods where land carries most of the value, the improvement figure is close to noise. And a house gutted and rebuilt last year keeps last year's assessment until the model catches up.
If the assessed value on your own house is wrong
The same lag cuts the other way for owners. If the county's number is high and your tax bill with it, the appeal goes to the county board of equalization, and the clock starts on the date of the valuation notice. The evidence that works is the evidence the assessor uses: recent sales of comparable houses, adjusted to the valuation date, plus anything about condition the model could not see. The DOR ratio study is public and shows how your county's assessments compare with sales as a whole.
Choose one metro and start a 14-day trial, with no card required. The paid view attaches the assessed value and the owner's mailing address to each parcel, which is the ranking arithmetic above, already done.
Frequently asked questions
Is assessed value higher or lower than market value?
Usually lower, by a little, but not reliably. Across 20,172 arm's-length Seattle sales the median house sold for 7.5% more than its assessed value, and 28.6% sold for less. Washington assesses at 100% of market value as of a valuation date a year or more before the sale, so the gap is mostly timing and condition.
Why is the assessed value lower than the market value?
Three reasons. The valuation date is old: King County values property as of 1 January of the prior year for this year's bill. The method is mass appraisal, a model run across every house at once with a physical inspection only every six years. And in a rising market a year of appreciation lands between the assessment and the sale. In states that assess at a fraction of market value the gap is also deliberate.
What is the difference between assessed value and appraised value?
An appraised value is one licensed appraiser's opinion of one house on one day, ordered for a specific loan or sale. An assessed value is the county's model estimate for every parcel, set annually for tax purposes. Lenders use the appraisal. The treasurer uses the assessment. Neither is the sale price.
Can you sell a house for less than its assessed value?
Yes, and 28.6% of Seattle sales did. Under $500,000 the median sale closed at 85% of assessed value. Nothing about the assessment binds a sale price. It is an estimate for taxation, and the market is free to disagree in either direction.
How is assessed value calculated?
By mass appraisal. King County runs a sales comparison approach, adjusting recent sales of similar houses to the valuation date, and a cost approach that estimates replacement cost less depreciation plus land. Washington law requires the result to be 100% of true and fair value, revalued every year, with a physical inspection at least once every six years. The Department of Revenue audits the results annually against actual sales.