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ADU Rules in Seattle: 2026 Guide

Seattle liberalized its ADU rules in 2019 (MHA ordinance) and again in 2023 (Connected Communities zoning), making it one of the most permissive ADU markets on the West Coast. The city eliminated its owner-occupancy requirement in 2019 — meaning a homeowner can build both an Attached ADU (AADU) and a Detached ADU (DADU), rent both, and live in neither — and removed off-street parking requirements for ADUs near frequent transit in 2018. This guide walks through Seattle's 2026 DADU and AADU rules under Seattle Municipal Code (SMC) 23.44, the tree-ordinance overlay that blindsides many homeowners, and the SDCI timelines.

Authored by Netanel Presman — CSLB RMO #1105249 · Updated 2026-04-24

Regulatory framework in Seattle

Seattle Municipal Code (SMC) Title 23 (Land Use) governs ADUs. Key 2026 rules under SMC 23.44: (a) one AADU and one DADU are allowed on every single-family lot, up to two ADUs total; (b) DADU maximum size is 1,000 sq ft plus a 300 sq ft garage, or 1,200 sq ft without a garage; (c) AADU maximum size is 1,000 sq ft; (d) owner-occupancy is NOT required — this was eliminated in 2019; (e) short-term rental (under 30 days) is restricted, but long-term rental is permitted; (f) off-street parking is NOT required for ADUs within 1/4 mile of frequent transit (most of the city); (g) DADU height is limited to 18–24 feet depending on zone; (h) minimum 5-foot rear and side setbacks, 15-foot separation from primary structure or a 6-foot fire-rated wall.

Seattle's tree ordinance (SMC 25.11, substantially amended in 2023) adds a layered review for any ADU construction that would remove or encroach within the critical root zone of an Exceptional Tree (defined by species and size thresholds) or a Tier 1 or Tier 2 tree. For lots with mature trees, a licensed arborist assessment ($1,500–$4,500) and tree-protection plan become part of the permit package. SDCI reviews ADU applications under the Type I review process (ministerial, no discretionary review), with a 2026 target of 4–10 weeks for complete applications. The 'Connected Communities' ordinance also added density bonuses for ADUs on certain arterials, and updated the cottage cluster rules — relevant for larger lots.

Costs and timelines (2026)

A detached 800 sq ft DADU in Seattle in 2026 costs approximately $340,000–$485,000 all-in — $215K–$320K hard construction, $25K–$55K soft costs (architecture, arborist, engineering), $8K–$18K SDCI fees, $18K–$35K utility connections (Seattle City Light electrical upgrades, SPU water and sewer), $35K–$75K site prep including any required tree protection. AADU conversions from existing basement or attic run $120K–$220K. Pre-approved DADU plan-set programs (Seattle offers a pre-approved plan library since 2019) can save $15K–$35K in architecture + 3–5 weeks in plan review.

Timelines in 2026: SDCI plan review runs 4–10 weeks for a pre-approved plan DADU, 8–14 weeks for a custom DADU, 6–10 weeks for AADU conversion. Tree-ordinance review adds 3–6 weeks where it applies. Construction runs 5–8 months on-site for a DADU (Seattle weather compresses to April–October for most envelope work), 3–5 months for an AADU conversion. Total window from architect to occupancy: 10–18 months. Seattle's permit timeline is meaningfully faster than SF or LA, but the weather constraint (framing and roofing from November–March is difficult) is a real scheduling factor.

Four pitfalls specific to Seattle

  1. 1. Exceptional Tree trigger on backyard DADU. A mature Douglas Fir, Western Red Cedar, or any tree meeting Exceptional Tree criteria (species + size thresholds under SMC 25.11) in or adjacent to the proposed DADU footprint triggers tree-protection requirements that can force a redesign or rule out DADU placement entirely. Arborist assessment and tree-protection planning are mandatory. Homeowners routinely proceed to architecture without tree assessment, waste $8K–$18K on design, then redesign around an Exceptional Tree constraint.
  2. 2. Electrical service-upgrade blindside. Seattle's older neighborhoods (Ballard, Capitol Hill, West Seattle pre-1940) often have 100-amp or 200-amp service that can't support an added DADU's HVAC, appliances, EV charger, and lighting. Upgrade to 400-amp combined or a separate 200-amp DADU service runs $12K–$28K and Seattle City Light coordination adds 6–14 weeks. Require a formal electrical load calculation by the contractor's electrician before final contract.
  3. 3. Short-term rental restrictions. Seattle restricts short-term rental (under 30 days) to primary residences and one other unit per operator (City Regulation). If the ADU investment thesis is 'I'll Airbnb the DADU,' it likely doesn't work legally unless the DADU is the owner's primary residence. Investor math should assume long-term rental rates ($2,200–$3,400/mo for an 800 sq ft DADU in most Seattle neighborhoods), not short-term rental premium.
  4. 4. Stormwater overlay in Green Factor zones. Some Seattle zones require Green Factor landscape-score compliance or additional stormwater infiltration/detention for any impervious-surface addition. DADU construction converts permeable yard into impervious footprint, triggering stormwater requirements that can add $6K–$22K in infiltration systems. Check the Green Factor + stormwater overlay at the SDCI GIS viewer before design starts.

Five-item checklist before you sign

Frequently asked

Can I build both an AADU and a DADU on one lot in Seattle?

Yes. Seattle allows up to one AADU (attached to or within the primary dwelling) and one DADU (detached backyard cottage) on every single-family lot, for a maximum of three dwelling units including the primary residence. This was permitted under the 2019 MHA ordinance and reaffirmed in 2023. No owner-occupancy requirement applies, so all three can be rented long-term to tenants.

How does Seattle's tree ordinance affect DADU placement?

Seattle's 2023 tree ordinance (SMC 25.11) categorizes trees by species and size. Exceptional Trees (largest, rarest specimens) cannot be removed or have their critical root zone encroached without hardship waiver; Tier 1 and Tier 2 trees have graduated protection levels. Any DADU footprint within the critical root zone (typically 1 foot of radius per inch of trunk diameter) requires arborist evaluation and tree-protection measures, and in some cases requires redesign. If you have mature trees on the lot, arborist consultation should happen before architectural fees.

What's the fastest Seattle DADU path?

Choose a pre-approved plan from SDCI's DADU plan library (10+ designs as of 2026), use a contractor familiar with Seattle permit scheduling, site the DADU on a footprint that avoids any Exceptional or Tier 1 trees, and avoid Green Factor / stormwater-overlay triggers by minimizing net new impervious surface. This combination can get you from concept to occupancy in 10–12 months versus 14–18 for a custom design on a constrained lot.

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