Seattle DADU Property Tax: What a Backyard Cottage Does to Your Bill, Rent Roll and Resale

When you build a detached accessory dwelling unit in Seattle, the city is not the only one who shows up. King County shows up the following year with a new assessed value, and the Seattle DADU property tax question is the first one every owner asks me. The answer is more predictable than people fear, and it is one of three numbers a backyard cottage changes, along with the rent roll and the resale price.

I built cottages in Seattle for years. The owners who came out ahead were the ones who ran all three numbers on paper before the permit. The ones who got pinched priced only the construction.

How Seattle DADU property tax actually gets calculated

Washington does not cap your assessed value at the price you paid, the way California's Proposition 13 does. State law requires county assessors to value every parcel at 100 percent of its true and fair market value, and King County revalues every parcel on an annual cycle, not only when a property changes hands. There is no acquisition-value cap, and there is no one-time supplemental bill the way California sends after new construction. The new value simply appears on the next roll.

That value is taxed at a consolidated levy rate. For a standard Seattle parcel, levy code 0010 carries a 2026 rate of $9.90845 per $1,000 of assessed value — about 0.99 percent. The math is flat and linear:

If you are not sure whether your address sits in levy code 0010, the Assessor's eReal Property search shows the levy rate for any parcel. The customized feasibility report applies your parcel's own levy code rather than the citywide default.

The tax scales in a straight line with added value. A cottage the Assessor books at $194,000 — the measured example from a Licton Springs parcel — adds about $1,922 a year, or $160 a month.

Source: computed from the King County consolidated levy rate for Seattle code 0010 ($9.90845 per $1,000, 2026). The $194,000 figure is the new-construction dollars recorded for one DADU under construction, from the King County Assessor eReal Property record.

When the Assessor picks up your new DADU

In the site's own address-level pulls, a 2026 DADU permit still under construction on a Licton Springs parcel carried roughly $194,000 of "new dollars" in the King County Assessor's eReal Property record — the value the Assessor has already attached to the cottage before the framing is even dried in. At the levy code 0010 rate, that is about $1,922 a year that parcel will carry from the first full roll after completion.

There is no separate Seattle city tax on top; the county collects for the state, the city, and the school and fire districts in one consolidated rate, then sends it back out. Washington's Department of Revenue spells out the split: the assessor values, the treasurer collects.

The practical takeaway is timing. The value lands on the roll after construction is completed and inspected, so if you are holding the cottage empty, that first full tax year is out-of-pocket with no rent offsetting it. Run the cash timing, not just the annual figure.

What a DADU does to your rent roll

Seattle rents, as of Zillow's September 2026 market snapshot, average $1,800 a month for a one-bedroom and $2,695 for a two-bedroom, against a citywide all-types average of $2,200. A detached cottage — the unit most owners actually build for rent — sits inside that band, and the two-bedroom DADU is the workhorse because the extra bedroom pays for itself in a market where a second bedroom rents for hundreds more a month.

Two rules shape how you can collect that rent. First, there is no owner-occupancy requirement for a long-term rental: state law stops cities from requiring the owner to live on the lot, so you can rent both the house and the cottage. Second, the short-term path is more restricted. Seattle's short-term rental licensing charges $75 per unit per year and limits an operator to two units — the primary residence plus one secondary unit, which can be a DADU. You cannot run a cottage on Airbnb unless your primary residence is part of the same licensing picture.

Neighborhood matters to the rent number. A cottage near the light rail or the commercial core in Ballard, Green Lake, or Wallingford rents differently than the same floor plan farther out in Wedgwood. The neighborhood profiles measure the lot and hazard conditions that decide whether you can build a rentable unit there at all.

Selling the cottage separately: the condo right

Washington wrote the sale right into statute. RCW 36.70A.681(1)(k) says a city may not prohibit the sale or conveyance of a condominium unit independently of the principal unit solely on the grounds that the unit was originally built as an ADU. Seattle's SMC 23.42.022.K carries that right into local code. So a DADU is not automatically locked to the house; it can become its own condominium unit and be sold to a separate owner.

There are two paths, and they are not interchangeable. A condominium conveyance carves the cottage out as a condominium unit within a small common-interest project. A unit-lot subdivision is a different, heavier process that splits the parcel into two legal lots through the subdivision code, and it can trigger its own review and conditions. Neither is automatic, and both are work for a title professional and, usually, a surveyor. But the underlying right is real and was fought for in the 2023 state ADU bill.

That right matters even if you never split. A cottage that can legally be sold separately is worth more to a buyer than one that can only be rented, because it gives them an exit that does not require selling the whole property.

What appraisers do with a DADU at resale

Appraisers do not value a DADU the way you value it. For years most treated a backyard cottage as an amenity, because comparable sales with finished, permitted cottages were thin. That has shifted as DADUs became ordinary in Seattle, and as lenders pushed appraisers to use ADU comparables and, where a rent roll exists, the income approach alongside the sales comparison.

The honest pattern I have seen on resales across the city: a finished, permitted, rent-ready cottage supports value roughly in line with what a buyer will pay for that income stream — but usually below what it cost to build in a hot construction market. The cottage that appraises best is the legal one: permitted, finaled, with its own entry, its own address registered, and a documented rent history. The garage conversion done without permits supports almost nothing, and can actually drag the file.

For the resale value specifically, the single best thing you can do is keep the file clean: the permit, the final inspection, the separate address, and the rent history in one folder. That is what an appraiser can hang a number on.

The 15-year cash picture

Put the recurring numbers on one page, because that is what the bank will want to see before it lends. On top of the property tax there are two other recurring charges a DADU carries in Seattle.

• King County sewer capacity charge. A DADU is 0.59 RCE, billed at $77.99 per RCE per month for 15 years — about $46 a month, $552 a year, and roughly $8,283 over the full term.
• SPU drainage fee. Seattle's residential drainage fee is tiered by parcel size — $469.52 a year for a 2,000–3,499 sq ft lot and $976.13 for a 6,500–9,999 sq ft lot. Adding a DADU does not move your tier unless the parcel crosses 10,000 sq ft, so for most lots this line is already on your bill.

Against those, a two-bedroom cottage at the $2,695 average rents for about $32,340 a year gross. The chart below stacks the three recurring charges next to that gross rent, with the obvious caveat that rent is gross — it excludes vacancy, maintenance, insurance, and financing.

Even after property tax, the capacity charge, and drainage, a rent-ready two-bedroom cottage leaves a wide margin over its recurring carrying costs — which is why the rent roll, not the resale price, is usually the first way a DADU pays you back.

Sources: King County sewer capacity charge (2026, $77.99/RCE/month × 0.59 RCE); SPU 2026 residential drainage tiers; property tax computed at Seattle levy code 0010 on the measured $194,000 new-construction example; rent from Zillow Rentals Seattle, Sept 2026.

If you are still deciding whether your lot can produce a rentable cottage in the first place, start with the free homeowner checklist, then compare a pre-approved standard plan against a custom design — the permit path changes both your cost and your timeline. The Seattle DADU Pro Guide walks the whole sequence, from the first site check to what you do after the final inspection.

Frequently asked questions

Keep reading. If the number on your valuation notice looks wrong, see why a Seattle tax bill rises when values fall and the King County appeal deadline and the comps that win. For the income side, what a Seattle DADU rents for in 2026.

This article is educational and preliminary. It is not legal advice, tax advice, appraisal advice, or an official determination by the City of Seattle or King County. Levy rates, drainage tiers, capacity charges, and rental figures are dated to 2026 and can change. Verify all figures against current King County and City of Seattle records at the time of your decision, and consult a tax professional, a title professional, and a licensed appraiser before acting.