Most homeowners start their ADU research on the cost side, and for good reason. But the number that actually decides whether a project pencils out is the one on the other side of the ledger: what the unit earns once it's built. A backyard rental that covers its own loan payment is a very different financial decision than one that doesn't. Before you commit, it's worth running the return math the same way a small investor would. For the cost half of the equation, start with our breakdown of ADU costs in Washington and our guide on how to budget for an ADU project.
What Does an ADU Rent For in the Puget Sound Region?
Rent depends heavily on size, layout, and neighborhood, but 2026 asking rents for a well-built detached ADU (DADU) in the Seattle-Tacoma area generally land in these ranges:
- Studio / 1-bath (roughly 350–500 sq ft): $1,400 to $1,900 per month
- 1-bedroom (roughly 500–750 sq ft): $1,700 to $2,400 per month
- 2-bedroom (roughly 750–1,000 sq ft): $2,200 to $3,200 per month
Close-in Seattle neighborhoods and the Eastside sit at the top of those ranges; Tacoma, Auburn, and outer suburbs run lower but also cost less to build. A separate private entrance, in-unit laundry, and off-street parking are the features that move a unit toward the upper end of its band.
Long-Term vs. Short-Term Rental
The same unit can be run two very different ways:
- Long-term (12-month lease): Predictable monthly cash flow, lower turnover cost, and the simplest path with lenders and insurers. This is the baseline most homeowners underwrite against.
- Short-term (Airbnb / midterm): Higher gross revenue in strong-demand areas, but with real offsets — furnishing, cleaning, platform fees, higher vacancy, and city regulations. Seattle requires a short-term rental license and limits operators, so confirm the current rules for your specific address before you count on nightly income.
For a conservative plan, underwrite the ADU on long-term rent and treat any short-term upside as a bonus rather than the base case.
A Simple ROI Formula
You don't need a spreadsheet full of assumptions to get a usable estimate. Start here:
- Annual gross rent = monthly rent × 12
- Net operating income (NOI) = annual gross rent − operating costs (property tax increase, insurance, maintenance, and a vacancy allowance of roughly 5–8%)
- Cash-on-cash return = annual cash flow after loan payment ÷ cash you put in
- Simple payback = total project cost ÷ annual NOI
Worked Example
Say you build a 1-bedroom DADU in Tacoma for $260,000 and rent it long-term for $2,000/month.
- Annual gross rent: $24,000
- Operating costs (taxes, insurance, maintenance, ~7% vacancy): roughly $6,000
- NOI: ~$18,000/year
- Simple payback: $260,000 ÷ $18,000 = ~14 years on NOI alone
That's before two things that meaningfully improve the picture: the appreciation of the added square footage, and the equity you build if the unit is financed rather than paid in cash. Run the same math on a Seattle build at higher rent and you'll often see a stronger payback despite the higher cost. Use a real builder estimate rather than a guess for the cost input — you can request quotes from verified builders to anchor the number.
Financing Changes the Return
If you borrow to build, the relevant question isn't just payback — it's whether the rent covers the loan. When monthly rent exceeds the loan payment plus operating costs, the tenant is effectively buying you an asset. That's why the financing structure matters as much as the rent itself. Compare HELOCs, construction loans, and renovation loans in our guide on how to finance an ADU in Washington, and remember that HB 1337 removed statewide owner-occupancy rules, so you can build purely as a rental and count the projected income.
What Lifts (or Sinks) Your Return
- Build cost discipline. Every dollar saved on construction shortens payback directly. A tight scope and an experienced builder matter more than premium finishes a renter won't pay extra for.
- Layout efficiency. A well-designed 1-bedroom often out-earns a cramped 2-bedroom of the same footprint. Rentability per square foot beats raw size.
- Location. Proximity to transit, employers, and walkable amenities is the single biggest rent driver in the Puget Sound market.
- Quality of the build. Turnover, repairs, and vacancy quietly erode returns. A durable, code-compliant build from a verified local builder protects your NOI for years.
The Bottom Line
A detached ADU in the Seattle-Tacoma region typically rents for $1,700 to $3,200/month depending on size and location, and a conservatively underwritten unit often reaches simple payback in the low-to-mid teens of years — faster once appreciation and financed equity are included. The projects that pencil out best pair disciplined build costs with an efficient, well-located design. Start with a real number from a builder, then run the formula above against it.
Ready to see what a project would actually cost on your lot? Get free quotes from top-rated ADU builders in the Seattle and Tacoma area.