Seattle is one of America's most progressive ADU markets, with by-right approval for both detached and attached ADUs. The city allows up to three residential units per lot in many zones, making it ideal for ADU development and multi-unit housing.
Zoning & Size Requirements [1][2]
| Allowed Zones | All residential zones |
| Max Size (Detached) | 1,200 sq ft |
| Max Size (Attached) | 1,000 sq ft |
| Max Size (JADU) | 800 sq ft |
| Max Height | 40 feet |
| Max Units | 3 units per lot possible (duplex + ADU) |
Setback Requirements [1][2]
| Rear Setback | 5 feet |
| Side Setback | 0 feet (easement) |
| Front Setback | 0 feet (alley lot) |
| Parking Required | Optional |
| Owner Occupancy | Not required |
| Min Lot Size | 3,000 sq ft |
Permit Process & Fees [3]
| Approval Process | Master use permit + building permit |
| Review Timeline | 30-45 days |
| Permit Fees | $1,800-$2,800 |
| Impact Fees | Minimal |
| Plan Check | 1-2 weeks |
| Pre-Approved Plans | Extensive library available |
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Construction Costs [4][5]
| Average Build Cost | $160,000-$320,000 |
| Cost per Sq Ft | $130-$270 |
| Garage Conversion | $75,000-$170,000 |
| Prefab ADU (Installed) | $110,000-$260,000 |
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Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Rental Income Potential [6][7]
| Avg Rent (1 Bedroom) | $1,600-$2,100/mo |
| Avg Rent (2 Bedroom) | $2,000-$2,800/mo |
| Short-Term Rental | Restricted (30 days minimum) |
| Est. Annual ROI | 6-8% annual return |
Explore Prefab ADU Options
Prefab ADUs can save 20-40% vs. traditional construction and cut build time in half.
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Recent Law Changes
What's Changed Recently
- HB 1337 full implementation (2026): All urban growth areas must allow 2 ADUs per lot
- Seattle eliminated single-family zoning: Up to 4 homes per lot in NR zones
- No owner-occupancy requirement statewide (HB 1337 provision)
- Washington ADU permit fee caps now in effect for units under 1,000 sq ft
Tips for Building an ADU in Seattle
- Seattle's 8-year property tax exemption significantly improves ROI
- Tech industry creates strong, stable rental demand
- Master use permit still required even with by-right status; plan timeline accordingly
- Pre-approved plans available; significant time savings possible
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Accessory Dwelling Unit — SDCI | seattle.gov Planning Department — Accessed 2026-02-24
- ADU Code Updates | seattle.gov Municipal Code — Accessed 2026-02-24
- SDCI Permits — ADU Projects Planning Department — Accessed 2026-02-24
- HomeAdvisor — ADU Construction Cost Data Market Data — Accessed 2026-02-24
- Angi — Local ADU Contractor Costs Market Data — Accessed 2026-02-24
- Zillow Rental Manager — Market Rent Data Market Data — Accessed 2026-02-24
- HUD Fair Market Rents Government Data — Accessed 2026-02-24
- Encouraging Backyard Cottages — OPCD Planning Department — Accessed 2026-02-24
- Washington ADU Legislation (HB 1337) Government Data — Accessed 2026-02-24
Frequently Asked Questions
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