Seattle vs Bellevue
vs Tacoma ADU cost.
Fifteen line items compared head-to-head — $/sqft, permit and capacity fees, achievable rent, and 5-year ROI. Numbers are pulled from 41 closed Golden State permits in the last 24 months and cross-checked against city fee schedules in effect for 2026.
All-in cost & fee comparison.
| Line item | Seattle | Bellevue | Tacoma |
|---|---|---|---|
Hard cost — $/sqft (detached, mid-finish) | $330–$380 | $335–$380 | $330–$360 |
Hard cost — 600 sqft DADU all-in | $240K–$280K | $245K–$280K | $200K–$240K |
Hard cost — 800 sqft DADU all-in | $290K–$330K | $295K–$335K | $265K–$300K |
Soft cost — Design + engineering | $18K–$30K | $22K–$35K | $14K–$22K |
Building permit fee Valuation-based; range covers 600–900 sqft. | $4,800–$7,200 | $5,500–$8,400 | $2,900–$4,400 |
Plan review fee | $2,400–$3,600 | $2,800–$4,200 | $1,450–$2,200 |
Sewer / water capacity charge (GFC) Bellevue is highest in the region; Tacoma the lowest. | $4,200–$7,500 | $8,600–$11,200 | $2,800–$4,500 |
School impact fee ADUs are exempt under HB 1337 in all three. | $0 | $0 | $0 |
Median permit timeline (submit → issue) | 14–22 wk | 12–18 wk | 8–14 wk |
Construction timeline (issue → CO) | 16–22 wk | 16–22 wk | 14–20 wk |
Typical achievable rent — 600 sqft 1BR | $2,250–$2,700 | $2,400–$2,900 | $1,650–$1,950 |
Typical achievable rent — 800 sqft 2BR | $2,800–$3,400 | $3,000–$3,650 | $2,000–$2,400 |
5-year gross rent (avg, 600 sqft) | $148K | $159K | $108K |
5-year ROI on avg all-in cost | 55% | 57% | 54% |
Median property-tax bump (yr 1) | $1,800–$3,200 | $2,200–$3,800 | $1,100–$2,000 |
Source: Golden State ADU Builders, 2024–2026 closed permits · cross-checked against SDCI, Bellevue DSD, Tacoma P&PD fee schedules (effective 2026)
What the table actually means.
Tacoma builds cheapest by 25–30%.
Lower labor rates, fewer geotechnical surprises in much of the city, and the smallest permit/capacity-fee stack in the Puget Sound. Rents are also lower — but the cap rate stays competitive because the build cost drops faster than rent does.
Bellevue is the most expensive city — and the highest-grossing.
PSE electric hookup costs more, Bellevue Sewer GFC is the region's highest ($8.6K+), and finish-level expectations from the rental market push spec materials up. But achievable rent is also the highest, so the math still works for owners holding 5+ years.
Seattle is the fastest path to permit IF you use a Pre-Approved DADU plan.
SDCI's pre-approved program drops typical review from 14–22 weeks to 6–10 weeks. Outside that program, Seattle's queue is the longest of the three cities. Bellevue and Tacoma do not yet offer a pre-approved equivalent.
School impact fee is $0 everywhere — for now.
HB 1337 (2023) exempts ADUs from school impact fees statewide. Pre-2023, Bellevue and parts of Tacoma assessed $4K–$8K per ADU. If you read older guides quoting these fees, they are out of date.
Property tax bump is roughly 0.95–1.10% of post-build assessed value.
King County levies in the mid-1% range; Pierce County is slightly lower. The full bump appears the year AFTER the ADU receives its Certificate of Occupancy. Budget the first-year bump as a cash-flow item; it is recoverable through rent.
Official resources & sources
- Source ↗WSEC 2021 Residential Energy CodeEnvelope, HVAC, hot-water and air-sealing requirements
- Source ↗WA HB 1110 — Middle housing lawStatewide middle-housing zoning preemption
- Source ↗WA HB 1337 — ADU statewide rulesTwo-ADU baseline, no owner occupancy, parking limits
- Source ↗WA Dept. of Commerce — ADU resources
- Source ↗King County Assessor
- Source ↗Pierce County Assessor-Treasurer
- Source ↗Snohomish County Assessor
- Source ↗Thurston County Assessor
- Source ↗WA L&I contractor license lookupVerify Golden State ADU Builders Inc · GOLDNAB882L2
Frequently asked
Why is Bellevue more expensive than Seattle for the same square footage?
Three reasons: (1) Bellevue's GFC + connection-fee stack is roughly $4K–$7K higher than Seattle's, (2) PSE residential electric hookups average ~$2K more than Seattle City Light's drop, and (3) Eastside finish-level expectations push HVAC, windows, and millwork specs up by 8–12% relative to Seattle-equivalent rentals.
Does Tacoma's lower cost mean lower quality?
No. Code requirements are identical — all three cities enforce the same 2021 IRC + WSEC. The cost delta is labor rates, fewer downtown logistical constraints, and a permit office that runs lighter loads. Material quality and structural standards are the same.
Which city has the best 5-year ROI for an ADU?
Bellevue is highest at ~57%, but Seattle and Tacoma are within 2–3 points of each other and within the noise of unit-level variation (corner lot vs interior, walk-out daylight vs slab, etc.). Pick the city based on where you already own — the build cost delta is bigger than the ROI delta.
Are these numbers fixed-price or estimates?
Estimates, drawn from 41 Golden State permits closed between 2024–2026. Your specific project depends on lot grade, soil class, tree retention, existing utility capacity, design complexity, and finish level. We publish ranges, not point estimates, because honest contractors do not promise the bottom of the range.
Drill into a single city
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Send the address and intended unit size. We'll return a fixed-fee estimate for the city, plus a side-by-side against an equivalent build in the other two — within two business days.