ROI in seconds.
Move four sliders. See your monthly cash flow, payback timeline, and 10-year property uplift — based on real Puget Sound rent and build-cost data.
Ownership covers property tax, insurance, vacancy, and maintenance. Default 22% reflects King County 2024 norms.
Conservative model — does not include appreciation, tax shield, or rent growth. Real returns historically outperform on Puget Sound lots.
DISCLAIMER · Modeled returns, not financial advice. Confirm rents and costs in a feasibility study.
Want our model with your numbers?
We'll run a free, lot-specific pro-forma using comparable rents within a half mile of your address.
ROI calculator — frequently asked
How accurate is the ROI calculator for a Puget Sound ADU?
It uses 2025 market rent data from RentCafe, Zillow ZORI, and CoStar quarterly reports for King/Pierce/Snohomish/Thurston counties, blended with our own closed-contract cost averages. Output is within ±10% of pro-forma reality for a code-built turn-key ADU on a typical lot. Edge cases (steep slope, off-grid utilities, critical areas) need a custom pro-forma from our team.
What rental yield should I expect from a new ADU?
Median 2025 gross yields by city: Seattle 8.4–10.2%, Bellevue 7.6–9.1%, Kirkland 7.9–9.4%, Redmond 8.1–9.7%, Tacoma 9.8–11.6%, Olympia 9.1–10.8%. Net of vacancy (5%), maintenance reserve (8%), management (8% if outsourced), property tax increase, and insurance, net cap rates typically land 5.4–6.8%. Higher than most multifamily syndications in 2025.
Does the calculator account for property tax reassessment?
Yes. We pull the current year's levy rate from each county (King ~$8.10/$1,000, Pierce ~$10.40, Snohomish ~$9.80, Thurston ~$11.60 in 2025) and apply the city's cost-to-assessed-value ratio. The output line "+$ property tax annual" is the increase you should expect under RCW 84.40.0301, not the absolute tax bill.
Go deeper: Read: What an ADU does to your property tax in King County
How does the calculator handle depreciation?
Federal residential rental depreciation under IRS Pub 527 uses 27.5-year straight-line on the building basis (land is not depreciable). We allocate cost between land and building using county assessor ratios as the IRS recommends. Output shows annual depreciation deduction and approximate federal tax benefit at your specified marginal rate. Washington has no state income tax to model.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
Can I model short-term rental income?
Yes, with the caveat that Seattle (SMC 6.600), Bellevue (LUC 20.20.900), and other AHJs restrict short-term rentals on ADUs to specific zoning overlays or require operator licensure. Our STR mode uses AirDNA market data but caps usage at jurisdiction-permitted nights/year. For most owner-occupant DADUs, long-term rental almost always nets more after platform fees, cleaning, and vacancy.
Go deeper: Read: Long-term vs. short-term rental ADU in Seattle
What financing assumptions does it use?
Default is a 30-year fixed renovation loan at current Fannie HomeStyle conventional rate (updated weekly from Mortgage News Daily). You can toggle to HELOC at prime+0.5, construction-to-perm at +1.25% over conventional, or cash. Each scenario shows monthly debt service, total interest over 30 years, and break-even month on net rent.
Does it model appreciation?
Optional. We provide three appreciation scenarios using Case-Shiller Seattle MSA historical bands: conservative 2.5%/yr, base 4.5%/yr, aggressive 6.5%/yr. We default to base because Case-Shiller Seattle's long-run average is ~5.1% (1990–2024). Aggressive should not be your planning scenario.
Can I save and share my pro-forma?
Yes — generate a PDF or a unique URL with all inputs baked in. The PDF includes a one-page summary suitable for a lender pre-qual conversation. We do not require an email to generate the PDF, and we do not retain the inputs on our server unless you create a free account.
What's a realistic cap rate for ADU rentals in 2025?
Net cap rates of 5.4–6.8% are common in the Puget Sound for a code-built, professionally managed ADU. That beats most stabilized small multifamily syndications (3.8–5.2%) and most STR markets after platform/cleaning costs. The wildcard is your cost basis — at the high end of build cost, returns compress to 4.2–5.0%.
What if I plan to live in it instead of rent?
We model that too. Owner-occupied use loses rental income but typically saves the owner $1,800–$3,400/mo versus renting an equivalent unit in the same neighborhood. Lifetime equity capture and avoided rent often beats rental yield over a 10-year hold for multi-generational households.
Go deeper: Glossary: RRIO (Rental Registration & Inspection Ordinance)
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