Ask a Seattle homeowner what a backyard cottage will bring in and you usually hear a top-line rent number: two thousand, twenty-five hundred, maybe more. What you almost never hear is the stack of bills that come out of that check before you spend a dollar of it. Seattle DADU rental income is not the rent. It is the rent minus the King County capacity charge, the SPU drainage fee, and a property tax bill that grows with the value you just added. Get those three wrong and a cottage that looks like it pays for itself quietly doesn't.
I have priced these out for homeowners for years, and the pattern never changes: the rent number gets the attention, and the recurring charges get discovered later. Here is the net math, start to finish, on a real two-bedroom cottage.
The rent is the easy number. The King County, SPU, and the Assessor are the three tenants you never see — and they bill on schedule whether your cottage is full or empty.
— Bob Hall, retired Seattle builderHow much can you rent a Seattle DADU for in 2026?
This site models a two-bedroom Seattle DADU at $2,700 a month — the citywide two-bedroom asking rent for September 2026 — which leaves about $17,759 in the first year if you built with cash. A cottage that is smaller or plainer than the apartment stock it is measured against asks less than that; the realistic asking range, and why, is below.
The best public data we have says Seattle rent has gone soft but stayed high. Zillow puts the average Seattle rent at $2,200 a month as of mid-September 2026 — down a grand total of $15 from a year earlier, with about 4,593 rentals on the market. Zumper's Seattle figure is $1,950, down 5% year over year. Nationally, Apartment List has the median at $1,390, up 0.1% month over month but still down 0.8% from a year earlier. Seattle stays well above the country, but it is not the bidding-war rental market of 2022.
The bedroom split is what matters for a cottage. Zillow's September numbers break out like this:
What Seattle rents, by bedroom count
Average monthly asking rent, Zillow Rentals data, September 2026.
Source: Zillow Seattle rental market trends, as read into the engine’s rent series on September 18, 2026. Bars scaled to the $3,661 three-bedroom figure.
Where does a detached cottage sit in that stack? Between the one-bedroom and the two-bedroom, and usually a little under the apartment two-bedroom. A DADU has no leasing office, no gym, and no elevator, and it is smaller than a two-bedroom apartment building unit. In the neighborhoods where cottages get built — Ballard, Wedgwood, Greenwood — a clean two-bedroom cottage realistically asks $2,300 to $2,500 a month. Zumper's neighborhood data runs the same direction: it lists Greenwood at $1,620 for its more modest stock. Transit access and a private yard push the number up; an unfinished basement or a shared laundry room pulls it down.
The rental income and ROI projection puts your parcel through the same engine as the numbers above — rent, vacancy, operating costs, the King County capacity charge, drainage, property tax and financing, over fifteen years.
Gross rent is not the number you keep
Three recurring charges come out of the rent check before you spend any of it, and none of them are optional.
King County capacity charge. This one surprises people because it shows up a year after move-in and runs for fifteen years. The county charges $77.99 per residential customer equivalent per month, and a DADU is 0.59 RCE. That works out to $46.01 a month, $552 a year, and $8,283 over the full term.
SPU drainage fee. Billed on the property tax statement, not the utility bill. A 2,000–3,499 sq ft parcel pays $469.52 a year in 2026; a 6,500–9,999 sq ft parcel pays $976.13. Adding a DADU under 10,000 square feet does not change your tier — the house already pays it — so the added drainage cost of the cottage is $0, and that is what the table above books.
Property tax on the added value. King County revalues every parcel annually, and the cottage's value lands on the next roll. At Seattle levy code 0010 — $9.90845 per $1,000, about 0.99% — a cottage that adds $350,000 of assessed value adds roughly $3,468 a year to the tax bill.
A worked 15-year example: 2-bed DADU, $2,700/mo
Planning estimates from this site's own engine — where each year's rent goes before debt service.
Light = vacancy (6%) and, in year 1, a month of lease-up. Bronze = fees, tax & upkeep. Green = net before debt.
| Milestone | Gross rent | Vacancy & lease-up | Fees, tax & upkeep | Net before debt |
|---|---|---|---|---|
| Year 1 | $32,400 | $4,644 | $9,997 | $17,759 |
| Year 5 | $35,415 | $2,125 | $10,975 | $22,315 |
| Year 10 | $41,055 | $2,463 | $11,980 | $26,612 |
| Year 15 | $47,594 | $2,856 | $13,148 | $31,590 |
| 15-year cumulative net | — | $375,076 | ||
Planning estimates, not a projection — computed by the same engine as the home-page calculator and the $59 projection (version roi-v2). Financing cash purchase; rent growth 2%/yr for 3 years then 3%/yr; vacancy 6%; 1-month lease-up in year 1; maintenance 7% and property management 8% of effective rent; insurance $1000/yr and RRIO $175/yr, both growing 3%/yr; property tax 9.90845/$1,000 on the added value, held flat; King County capacity charge $552/yr for 15 years; SPU drainage added cost $0. All estimates (◇). Build cost is the midpoint of the $420–$616/sq ft design-and-build band for an 800 sq ft two-bedroom (best case $336,000, recent Seattle builds $493,000), plus $18,150 of permits, studies and site prep. Your lot's tier and levy code move these numbers.
On rent alone, a $414,500 cottage netting around $17,759 its first year takes until about year 17 to pay for itself. That is the honest number, and most quick-take ROI articles won't show it to you. The reason it still pencils out is the part after the rent: the cottage adds to the value of the property, and under RCW 36.70A.681 and SMC 23.42.022 it can later be sold separately through a condo or unit-lot subdivision. Rent carries the first decade and a half; the resale right carries the rest.
What financing does to the cash flow
The same cottage changes character depending on how you pay for it.
- Built with cash. No debt service, so the $17,759 first-year net is yours from month one — 4.1% cash-on-cash on the $432,650 you put in (build plus fees), rising to 4.7% once the lease-up month is behind you. This is the cleanest way to read a DADU as income.
- 80% financed at 7.25%. Borrowing $331,600 over 20 years runs about $2,621 a month in principal and interest. Against $17,759 of first-year net, that is −$13,693 in year one — the cottage costs you money until rent growth catches the payment (payback about year 25).
- Financed against existing equity. Most Seattle DADU owners build on a HELOC or cash-out refi of a house they already own, often at a lower rate than new purchase money. That sits between the two cases above.
The pattern I saw on job sites: owners who treated a cottage as a get-rich-quick play got discouraged in year one. Owners who treated it as a slow, tax-advantaged second income stream were still collecting fifteen years later, with a paid-down asset and a rent roll that had grown every year.
The number that only your lot knows
Everything above is the citywide frame. Your actual Seattle DADU rental income depends on four things you cannot get from a rent index: whether your lot even fits a cottage, what it costs to build one there, what your parcel's drainage tier and levy code are, and whether the neighborhood will support the rent you need.
Lot geometry decides the first one. A Ballard lot with an alley gets a 0-foot rear setback and a short sewer run; a deep Wedgwood interior lot stretches the trench and the cost. The neighborhood feasibility profiles lay out those measured differences block by block, and the free homeowner checklist walks the go/no-go questions before you spend anything.
For the exact numbers on your address — buildable rectangle, fees, capacity charge, drainage tier, tax levy code, and a fifteen-year cash flow built from them — that is what the projection below is for.
Your address, your rent, your 15-year cash flow — all in.
A PDF built from your parcel's measured buildable zone, fees, drainage tier, and levy code, with a year-by-year cash-flow table.
- Projected rent range by bedroom count, adjusted for your neighborhood and transit
- All-in build cost and up-front fees from the site's 2026 rate engine
- King County capacity charge, SPU drainage, and property tax, line by line
- A 15-year cash-flow table with break-even and yield
$59 — delivered by email, automatically
The customized feasibility report is the escalation path if the numbers look good but you want the whole lot measured — setbacks, trees, critical areas, utilities, and the permit path — before you commit. The line-by-line cost guide and the property-tax and resale guide cover the two bills that move this math the most, and the September market notes explain why rent, not appreciation, is carrying the decision right now. If you would rather work it through yourself first, the free Pro Guide walks the same rules on paper.
Frequently asked questions
How much can I rent a Seattle DADU for?
Zillow puts Seattle's average rent at $2,200 a month in September 2026 — $1,800 for a one-bedroom and $2,695 for a two-bedroom. A detached backyard cottage typically rents a little below the apartment two-bedroom figure, roughly $2,300 to $2,500 a month, because it is smaller and lacks common amenities. Exact rent depends on neighborhood, transit access, and condition.
What recurring fees does a Seattle DADU rental carry?
Three bills keep arriving after the tenant does. The King County sewer capacity charge is $77.99 per RCE per month for 15 years, and a DADU is 0.59 RCE, so about $46 a month ($552 a year). The SPU drainage fee is $469.52 a year on a 2,000–3,499 sq ft lot and $976.13 on a 6,500–9,999 sq ft lot, but the parcel already pays it and a DADU under 10,000 square feet does not change the tier, so the added cost is $0. Property tax rises on the added value at about 0.99% for Seattle levy code 0010.
Does a Seattle DADU cash-flow positive from year one?
Built with cash, a two-bedroom DADU renting around $2,700 a month nets about $17,759 a year before financing, which is positive from the start. Financed at 80% at 7.25%, the debt service exceeds the first-year net income (−$13,693 in year one), and the cottage turns cash-flow positive a few years in as rent grows. It depends on build cost, rent, and how you finance it.
How long does a Seattle DADU take to pay for itself?
On rent alone, a $414,500 cottage renting for $2,700 a month pays back in about year 17, counting 2% annual rent growth for 3 years then 3%, 6% vacancy, 7% maintenance and 8% management. The real payback lands sooner because the DADU also adds to the property's value, and under RCW 36.70A.681 and SMC 23.42.022 it can later be sold separately through a condo or unit-lot subdivision.
This article is educational and preliminary. It is not financial, investment, tax, or legal advice, and it is not a projection of future rent or returns. Rent, expense, and appreciation figures are planning estimates built from the public sources linked above and may differ materially from your result. Fees, rates, and code change. Verify current figures against Zillow, SPU, King County, and SDCI schedules at the time of your application, and consult a qualified professional before making financial decisions.