Zoning & Size Limits
| Requirement | Seattle, WA | Portland, OR |
|---|
| Allowed Zones | All residential zones (Neighborhood Residential and other re | All residential zones |
| Max Size (Detached) | 1,000 sq ft of floor area above grade (size also constrained by FAR and lot coverage) | 800 sq ft or 75% of primary (whichever is less) |
| Max Size (Attached) | 1,000 sq ft | 800 sq ft or 75% of primary living area (same cap as detached; no separate 1,000 sq ft limit) |
| Max Height | Up to 32 ft in Neighborhood Residential and most r | 20 ft (outside setbacks); 15 ft (within setbacks) |
| Parking Required | Not required (no parking minimum for ADUs) | Optional |
| Owner Occupancy | Not required | Not required |
Setback Requirements
| Setback | Seattle, WA | Portland, OR |
|---|
| Rear Setback | 5 feet | 5 feet |
| Side Setback | 5 feet (reduced setbacks allowed along an alley/easement) | 0 feet with easement |
| Front Setback | Per zone; DADU generally behind primary structure | 0 feet (alley lot) |
Permits & Timeline
| Detail | Seattle, WA | Portland, OR |
|---|
| Permit Timeline | Several weeks to a few months depending on review queue | 30-45 days |
| Permit Fees | $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) | ~$1,900 permit and plan review; plus System Development Charges (SDCs) of about $19,000 - see impact fees |
| Impact Fees | ADUs exempt from MHA affordability contributions and most im | SDCs total roughly $18,900 for a detached ADU (Parks ~$7,100 |
| Pre-Approved Plans | Yes — ADUniverse pre-approved DADU plan library | Extensive pre-approved library available |
Construction Costs
| Cost | Seattle, WA | Portland, OR |
|---|
| Avg Build Cost | $160,000-$320,000 | $150,000-$300,000 |
| Cost Per Sq Ft | $130-$270 | $125-$250 |
| Garage Conversion | $75,000-$170,000 | $70,000-$160,000 |
| Prefab ADU | $110,000-$260,000 | $100,000-$240,000 |
Rental Income Potential
| Rental | Seattle, WA | Portland, OR |
|---|
| 1BR Avg Rent | $1,600-$2,100/mo | $1,400-$1,900/mo |
| 2BR Avg Rent | $2,000-$2,800/mo | $1,800-$2,600/mo |
| Short-Term Rentals | Allowed with a Seattle STR operator license ($75/yr); a 'sho | Yes, with registration |
| Est. ROI | 6-8% annual return | 6-9% annual return |
Which City Is Better for ADUs?
Seattle scores 9/10 while Portland scores 9/10 on our ADU-friendliness index.
Both cities offer viable ADU development opportunities — the best choice depends on your specific property, budget, and goals.
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