Seattle rewrote its ADU code in mid-2025: Ordinance 127211 (HB 1337 compliance) and Ordinance 127219 (HB 1110 interim middle-housing zoning, since replaced by permanent Ordinance 127376 effective Jan 21, 2026) both took effect June 30, 2025, consolidating all ADU standards into SMC 23.42.022 and applying them uniformly across every zone that allows residential development. Every lot may now add 2 ADUs on top of the primary home — including two detached backyard cottages — with the size cap at 1,000 sq ft for ADUs up to 2 bedrooms (1,200 sq ft for 3+ bedrooms under the 2026 permanent code) and DADU height allowed up to 32 ft in most Neighborhood Residential zones (higher in Lowrise). Setbacks and height standards can be no more restrictive than those for the primary house, no parking or owner-occupancy is required, and ADUs are exempt from street-improvement requirements. Under HB 1337 (statewide from July 1, 2025), ADUs can also be sold separately as condominiums — confirm the condominiumization process with SDCI before designing around it. The same reforms allow 4 units per residential lot by right (6 with affordable units or near major transit), so many owners now weigh 'house + 2 ADUs' against a middle-housing project on the same lot.
Zoning & Size Requirements [1][2][3][4]
Yes — Seattle allows ADUs. A detached ADU can typically be up to 1,000 sq ft under current Washington and Seattle zoning rules.
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| Allowed Zones | All residential zones (Neighborhood Residential and other residential zones) |
| Max Size (Detached) | 1,000 sq ft (up to 2 bedrooms); 1,200 sq ft for 3+ bedrooms |
| Max Size (Attached) | 1,000 sq ft (up to 2 bedrooms); 1,200 sq ft for 3+ bedrooms |
| Max Size (JADU) | Seattle does not use a separate JADU category; second unit treated as an AADU/DADU |
| Max Height | Base 32 ft in Neighborhood Residential zones (up to 42 ft with tree-protection/setback/green-factor conditions) |
| Max Units | 2 ADUs per lot (up to ~3 units total on many lots under middle-housing rules) |
Setback Requirements [1][2][3][4]
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 (0 feet where the lot abuts an alley) |
| 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 [5]
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 [6][7]
Building an ADU in Seattle typically costs $160,000-$320,000 ($130-$270 per sq ft); a garage conversion runs $75,000-$170,000.
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| 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 [8][9]
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
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
- Ordinance 127376 (CB 120993, passed Dec 16, 2025; effective Jan 21, 2026) replaced the interim HB 1110 rules: permanent Neighborhood Residential standards, ADU size now 1,000 sq ft (up to 2 BR) / 1,200 sq ft (3+ BR), still max 2 ADUs per lot with no parking required; ADUs now count toward density, FAR, and lot coverage
- June 30, 2025 — Ordinance 127211 (HB 1337 ADU compliance, adopted May 2025) and Ordinance 127219 (HB 1110 interim middle-housing zoning, adopted June 2025) took effect; all ADU standards consolidated into SMC 23.42.022 and made consistent across all residential-eligible zones (NR, LR, MR, HR, NC, SM, downtown).
- June 30, 2025 — ADU size limit raised to 1,000 sq ft in every zone where ADUs are allowed; DADU height increased to 32 ft in most Neighborhood Residential zones and higher in Lowrise zones, with ADUs permitted in the rear yard up to 32 ft.
- June 30, 2025 — Two ADUs allowed per lot in addition to the primary unit on all lots, including the option of two DADUs; attached, detached, and stacked-flat configurations all permitted, and ADU setback/height standards may be no more restrictive than the primary home's.
- June 30, 2025 — Ordinance 127219 allows 4 units on every residential lot (6 with income-restricted units or near major transit stops), adding townhouses, rowhouses, and stacked flats to former single-family zones — an alternative development path to the 2-ADU route.
- July 1, 2025 — HB 1337 statewide provisions fully in force: owner-occupancy requirements eliminated, off-street parking cannot be required for ADUs, and sale of ADUs as separate condominium units is legalized (Seattle implementation via Ordinance 127211).
- 2025 — ADUs remain exempt from street-improvement requirements regardless of zone, and SDCI continues its pre-approved DADU plan program (ADUniverse) with permits issued in as little as 2-6 weeks.
Tips for Building an ADU in Seattle
- Use a pre-approved DADU plan from Seattle's ADUniverse library — SDCI issues permits for pre-approved designs in roughly 2-6 weeks versus months for fully custom plans.
- You can now build two detached ADUs (e.g., two backyard cottages) behind one house — but total floor area is still governed by FAR and lot coverage, so model those limits before assuming 2 x 1,000 sq ft.
- The 32 ft height allowance in most Neighborhood Residential zones makes a two-story DADU (or a unit over a garage) feasible on lots where it wasn't before June 2025 — re-run old feasibility studies that assumed lower limits.
- No owner-occupancy and no parking requirements mean a pure investment build is legal; if you want short-term rental income instead, budget for Seattle's STR operator license and its primary-residence rules.
- HB 1337 legalized selling an ADU separately as a condominium — a potential exit strategy that changes project economics — but confirm the condominiumization and unit-lot process with SDCI before you design for it.
- If your lot is large, compare the 2-ADU path against a 4-6-unit middle-housing project under Ordinance 127219 — different development standards apply, and one may pencil significantly better on your parcel.
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.
- Seattle CB 120993 / Ordinance 127376 (permanent HB 1110 + One Seattle Plan implementation) Government Data — Accessed 2026-07-27
- ADUs and Middle Housing — SDCI Building Connections (official Seattle.gov blog, July 7, 2025) municipal — Accessed 2026-07-20
- Accessory Dwelling Units Code Updates — What & Why (Seattle SDCI) municipal — Accessed 2026-07-20
- ADUs and Middle Housing Update (Broadview Community Council summary of SDCI notice) News Source — Accessed 2026-07-20
- Accessory Dwelling Units — Common Projects (Seattle SDCI permits page) municipal — Accessed 2026-07-20
- 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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