Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Seattle ADU cost →Building an ADU in Seattle costs between $325,000-$550,000 for new construction, with per-square-foot costs averaging $400-$650. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $100,000-$225,000, while prefab/modular ADUs fall in between at $300,000-$450,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | $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) |
| Impact Fees | No City of Seattle impact fee is charged on an ADU permit, ADU floor area is excluded from Mandatory Housing Affordability payments (SMC 23.58C.040) and ADUs are exempt from public street improvement requirements (SMC 23.53.003). Budget instead for the King County sewage treatment capacity charge: 0.59 residential customer equivalent for an ADU, or $46.01 per month at the 2026 rate of $77.99 per RCE, billed quarterly for 15 years (about $8,280 total, payable early at a discount). Since Jan. 1, 2026 Seattle Public Utilities also charges water, wastewater and drainage System Development Charges when a project needs a new or larger water service or adds hard surface; the amounts are on SPU's SDC page and were not retrievable for this update |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. We pass your request to up to 5 local builders serving Seattle, where we have them — free, no obligation.
Get Seattle builder quotes →Permit fees in Seattle are $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). The review timeline is 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, with 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. Impact fees are No City of Seattle impact fee is charged on an ADU permit, ADU floor area is excluded from Mandatory Housing Affordability payments (SMC 23.58C.040) and ADUs are exempt from public street improvement requirements (SMC 23.53.003). Budget instead for the King County sewage treatment capacity charge: 0.59 residential customer equivalent for an ADU, or $46.01 per month at the 2026 rate of $77.99 per RCE, billed quarterly for 15 years (about $8,280 total, payable early at a discount). Since Jan. 1, 2026 Seattle Public Utilities also charges water, wastewater and drainage System Development Charges when a project needs a new or larger water service or adds hard surface; the amounts are on SPU's SDC page and were not retrievable for this update. Pre-approved plans: 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.
| 1-Bedroom Rent | $1,900-$2,600/month |
| 2-Bedroom Rent | $2,300-$3,000/month |
| Annual Rental Income (est.) | $27,000/year |
| Payback Period (est.) | 16.2 years |
| Short-Term Rental | Yes, with limits. |
| Overall ROI Estimate | 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. |
A typical ADU in Seattle generates approximately $27,000 per year in rental income based on average 1-bedroom rents. At an average construction cost of $437,500, the estimated payback period is 16.2 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Seattle and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Seattle: $100,000-$225,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $300,000-$450,000.
3. Pre-approved plans — available in Seattle, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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