Everything homeowners ask about building an ADU in Seattle — answered with 2026 data.
Yes, ADUs are allowed in Every zone where housing uses are allowed (SMC 23.42.022.A): Neighborhood Residential NR1-NR3, Lowrise LR1-LR3, Midrise, Highrise, Neighborhood Commercial, Seattle Mixed and downtown zones. In the Shoreline District an ADU must also meet Chapter 23.60A. ADUs may only be accessory to a housing use, and both units must be on the same lot (23.42.022.B and F) in Seattle, WA. The maximum size for a detached ADU is 1,000 sq ft of gross floor area for an ADU with up to two bedrooms; 1,200 sq ft with three or more bedrooms (SMC 23.42.022.G.1, as amended by Ord. 127376). Not counted: up to 250 sq ft of attached garage, any story that is underground, and up to 35 sq ft of long-term bike parking. A 1,500 sq ft ADU is allowed only on a Lowrise lot inside a frequent transit service area that has not been sold for more than $1,000 in 20 years (community land trust case). ADU floor area does count toward the lot's FAR, lot coverage and density. Attached ADUs can be up to Same 1,000 / 1,200 sq ft cap as a detached unit. Seattle has an ADU-friendliness score of 9/10. Check our full Seattle ADU rules page for complete zoning details.
ADU construction in Seattle typically costs $325,000-$550,000. The cost per square foot averages $400-$650. Garage conversions are more affordable at $100,000-$225,000, while prefab ADUs cost $300,000-$450,000. Permit fees add $3,453 in plan-review and building-permit fees for a 500 sq ft DADU valued at $93,145, per SDCI's own 2026 fee example (up 18.7 percent from $2,908 in 2025). Fees are valuation-based (Fee Subtitle Table D-1); at SDCI's 2026 rate of $186.29 per sq ft for wood-frame R-3 construction, the 2026 Fee Estimator gives roughly $4,990 for a 1,000 sq ft DADU, 75 percent due at intake. Add a 5 percent technology fee, the $6.50 state surcharge, hourly reviews at the 2026 base rate of $292 per hour where triggered, plus separate electrical, plumbing and side sewer permits. A subject-to-field-inspection permit pays 40 percent of the plan-review fee, and a pre-approved standard-plan submittal gets a reduced plan-review fee (Tip 310) to your budget. See our detailed Seattle ADU cost guide.
Detached ADUs in Seattle can be up to 1,000 sq ft of gross floor area for an ADU with up to two bedrooms; 1,200 sq ft with three or more bedrooms. Attached ADUs are limited to Same 1,000 / 1,200 sq ft cap as a detached unit. Junior ADUs (JADUs) have a cap of Seattle has no junior ADU category. A second unit carved out of the house is simply a second attached ADU and counts toward the two-ADU limit. Maximum height is 32 ft in Neighborhood Residential zones, the same limit as the main house (SMC 23.44.070.A.1); a pitched roof of 4:12 or steeper may rise 5 ft more, a shed or butterfly roof 3 ft. The limit is 42 ft on lots that keep a Tier 1 or Tier 2 tree or meet the tree-point score, or for developments with three or more principal units and a 20-ft front setback (23.44.070.A.2). In Lowrise and other zones the ADU takes the height allowed for a principal unit in that zone (23.42.022.E). An accessory structure placed inside a required setback is limited to 12 ft, which is why a DADU must stay out of the setbacks. You are allowed up to Two ADUs per lot in any mix: two attached, two detached on a qualifying lot.
Rear setback: 5 ft for an ADU in Neighborhood Residential zones; 0 ft where the rear lot line abuts an alley (Table A for 23.44.090, footnote 3). The house itself needs 15 ft (10 ft with three or more principal units), so the cottage can sit closer to the back line than the house can. Structures containing floor area must be 5 ft apart, eave to eave (23.44.100.A). Side setback: 5 ft average with a 3 ft minimum in Neighborhood Residential zones; a flat 3 ft on lots under 5,000 sq ft inside a frequent transit service area (Table A for 23.44.090). Eaves and gutters may project 2 ft if they stay 3 ft from the line. Lowrise zones use the principal-unit minimum setback with no averaging (23.42.022.E). Front setback: 15 ft on a lot with one or two dwelling units, 10 ft on a lot with three or more (Table A for 23.44.090). The table counts dwelling units, which include ADUs, so a house plus two ADUs appears to qualify for the 10-ft figure; confirm with SDCI zoning review. Nothing in the code requires the cottage to sit behind the house any more; it simply has to meet the lot's setbacks. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None required for any ADU (SMC 23.42.022.I and Table B for 23.54.015, footnote 4). If you do build a space, it must be reached from an improved street, alley or easement, may not sit within 20 ft of the front lot line except on the driveway, and alley access is required where an improved alley exists (23.44.160). The state floor is the same: RCW 36.70A.681(2) bars any parking requirement for ADUs within a half mile of a major transit stop. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required. SMC 23.42.022 contains no owner-occupancy rule and RCW 36.70A.681(1)(b) forbids one. Owners of ADUs permitted before 2019 may still have an owner-occupancy covenant recorded on title; SDCI publishes a 'Release of Owner Occupancy Covenant for ADUs' form to remove it. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Seattle is typically No published SDCI target for ADU review. Pre-approved ADUniverse plans: 2-6 weeks to permit on most sites (SDCI and OPCD). Custom plans: SDCI's Tip 116B warns the intake appointment alone may be 2-4 months out, and 2026 industry guides report 4-8 weeks of review for a straightforward custom DADU and 12-18 months from design start to move-in overall (8-10 months with a pre-approved plan). SDCI's Construction Permit Timeline tool (April 2026) shows the actual review history of any permit since 2018. Plan check: Electronic plan review through the Seattle Services Portal. Plans must state the number of ADUs, whether the unit is in a new or existing structure, the ADU size, total lot coverage and any parking provided (Tip 116B). The ADU must meet the Seattle Residential, Building, Energy, Stormwater and side sewer codes; a converted structure must fully meet the Energy Code; sleeping rooms need egress windows (5.7 sq ft opening, sill no higher than 44 in.); the ADU needs its own heating controls plus smoke and CO alarms. Tips 116A and 116B were last updated Aug. 18, 2023 and still print the pre-2025 rules (3,200 sq ft lot, 40 percent rear-yard rule, 650 sq ft in Lowrise), which the 2025-26 ordinances superseded. Pre-approved plans availability: Yes. The City's ADUniverse gallery lists 7 pre-approved DADU designs (10 floor plans, 288 to 1,000 sq ft, studio to 3 bedrooms). Using one gets a permit in 2-6 weeks on most sites and a reduced plan-review fee, but only on a Neighborhood Residential lot with no environmentally critical area and under 750 sq ft of ground disturbance (SDCI help article, May 2026). ADUniverse 2.0 selected 15 new ADU and middle-housing designs from 110 submissions in 2026; the City expects them to be available from January 2027.
Average ADU rental income in Seattle: 1-bedroom units rent for $1,900-$2,600/month, 2-bedroom units for $2,300-$3,000/month. Short-term rentals: Yes, with limits. Expected ROI: 6-8% gross rental yield on a $325,000-$450,000 detached build renting at $2,000-$2,600/mo, before vacancy, taxes, the King County capacity charge and financing; conversions of existing space at $100,000-$225,000 yield ro.
The minimum lot size for an ADU in Seattle is No minimum lot size for an ADU. Ordinance 127211 repealed former SMC 23.44.041, whose Table A required a 3,200 sq ft lot at least 25 ft wide and 70 ft deep for a backyard cottage; SDCI Tip 116B (Aug. 2023) still prints those numbers. Lots under 5,000 sq ft without wetlands, riparian corridors or steep-slope non-disturbance areas may hold up to four units (23.44.060.C.1); lots with those critical areas get a reduced unit count (23.44.060.C.5) and lose the state ADU guarantees (RCW 36.70A.681(4)). This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Seattle, a garage conversion typically costs $100,000-$225,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
Tell us about your project and we'll pass it to ADU builders serving Seattle. Free, no obligation — you choose who to work with. We don't have builders in every area yet; if nobody covers yours, we'll email you to say so.
🔒 Your info is secure. We share it only with contractors quoting your project. No spam, no obligation.
Just browsing? Search ADU builders near you by ZIP code.