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
| 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
| 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
| 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
| 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 (installed) |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders in minutes.
Compare ADU Financing Options →Rental Income Potential
| 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. Compare top prefab manufacturers.
Compare Prefab ADUs →Recent Law Changes
What's Changed Recently
- 2024: Expanded middle housing allowance in many zones
- 2023: Streamlined ADU approval to 30-45 days
- 2024: Increased max detached ADU size to 1,200 sq ft
- 2023: Created ADU property tax exemption for 8 years
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
Frequently Asked Questions
Can I build an ADU in Seattle?
Yes. ADUs are allowed in All residential zones in Seattle. The maximum size for a detached ADU is 1,200 sq ft. No owner-occupancy is required.
How much does an ADU cost in Seattle?
ADU construction in Seattle typically costs $160,000-$320,000. Garage conversions run $75,000-$170,000, and prefab ADUs cost $110,000-$260,000 (installed).
How much can I rent an ADU for in Seattle?
Average ADU rents in Seattle are $1,600-$2,100/mo for a 1-bedroom and $2,000-$2,800/mo for a 2-bedroom unit.
Do I need parking for an ADU in Seattle?
Optional. Check with Seattle planning department for the latest Washington ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Seattle?
The permit review timeline in Seattle is 30-45 days. Fees range from $1,800-$2,800.
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