Seattle is one of America's most progressive ADU markets, with by-right (building-permit) approval for both detached (DADU) and attached (AADU) ADUs and no owner-occupancy requirement. Up to two ADUs are allowed per lot, and many lots can reach three units total under middle-housing rules.
Zoning & Size Requirements [1][2]
Yes, you can build an ADU in Seattle. A detached ADU can be up to 1,000 sq ft under current Washington and Seattle zoning rules.
| Allowed Zones | All residential zones (Neighborhood Residential and other residential zones) |
| Max Size (Detached) | 1,000 sq ft of floor area above grade (size also constrained by FAR and lot coverage) |
| Max Size (Attached) | 1,000 sq ft |
| Max Size (JADU) | Seattle does not use a separate JADU category; second unit treated as an AADU/DADU |
| Max Height | Up to 32 ft in Neighborhood Residential and most residential zones (raised by 2025 reform; height also governed by zone limits) |
| Max Units | 2 ADUs per lot (up to ~3 units total on many lots under middle-housing rules) |
Setback Requirements [1][2]
Seattle requires a rear setback of about 5 ft for a detached ADU, and no additional parking is required. Owner-occupancy is not required — you can rent it out without living on the property.
| Rear Setback | 5 feet |
| Side Setback | 5 feet (reduced setbacks allowed along an alley/easement) |
| Front Setback | Per zone; DADU generally behind primary structure or 20 ft from front lot line |
| Parking Required | Not required (no parking minimum for ADUs) |
| Owner Occupancy | Not required |
| Min Lot Size | No minimum lot size for an ADU (varies by zone) |
Permit Process & Fees [3]
An ADU permit in Seattle typically costs $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) and takes Several weeks to a few months depending on review queue, through a standard permit review.
| Approval Process | Building permit (no Master Use Permit required for most ADUs); SDCI review |
| Review Timeline | Several weeks to a few months depending on review queue |
| Permit Fees | $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) |
| Impact Fees | ADUs exempt from MHA affordability contributions and most impact fees |
| Plan Check | 1-3 weeks for pre-approved (ADUniverse) plans; longer for custom |
| Pre-Approved Plans | Yes — ADUniverse pre-approved DADU plan library |
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Construction Costs [4][5]
Building an ADU in Seattle typically costs $160,000-$320,000 ($130-$270 per sq ft); a garage conversion runs $75,000-$170,000.
| 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 |
Finance Your ADU Project
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]
A 2-bedroom ADU in Seattle rents for around $2,000-$2,800/mo, an estimated 6-8% annual return.
| Avg Rent (1 Bedroom) | $1,600-$2,100/mo |
| Avg Rent (2 Bedroom) | $2,000-$2,800/mo |
| Short-Term Rental | Allowed with a Seattle STR operator license ($75/yr); a 'short-term rental' is under 30 consecutive nights, and an ADU/DADU can be a licensed STR subject to the operator/primary-residence rules |
| Est. Annual ROI | 6-8% annual return |
Explore Prefab ADU Options
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Recent Law Changes
What's Changed Recently
- Data accuracy correction (2026-06-28): updated per official city/state sources following a regulatory audit.
- WA HB 1337 (2023) middle-housing/ADU law in full effect: cities must allow up to 2 ADUs per lot, no owner-occupancy mandate, no off-street parking requirement within 0.5 mi of major transit
- Seattle 2025 ADU reform: detached/attached ADU height raised to 32 ft in Neighborhood Residential and most residential zones; 1,000 sq ft above-grade floor-area cap retained
- ADUniverse pre-approved DADU plans continue — reduced review time (1-3 weeks) and lower fees
- Two ADUs allowed per lot; size primarily constrained by FAR and lot coverage in addition to the 1,000 sq ft cap
Tips for Building an ADU in Seattle
- Seattle's 8-year property tax exemption significantly improves ROI
- Tech industry creates strong, stable rental demand
- Most ADUs need only a building permit — no Master Use Permit (discretionary review) for a by-right ADU; ADUniverse pre-approved plans clear review fastest
- Pre-approved plans available; significant time savings possible
ADU Builders in Seattle
Looking for an ADU builder in Seattle? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match Seattle homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
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 2026-06-28
- ADU Code Updates | seattle.gov Municipal Code — Accessed 2026-06-28
- SDCI Permits — ADU Projects Planning Department — Accessed 2026-02-24
- Angi — 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
- Seattle SMC 23.42.022 — Accessory Dwelling Units (Municipal Code) Municipal Code — Accessed 2026-06-28
Frequently Asked Questions
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