Seattle · DADU cost guide
How much does a DADU cost in Seattle? (2026)
Seattle is the most expensive Pacific Northwest ADU market, but its ADUniverse pre-approved DADU program meaningfully cuts both cost and permitting time. Here's what to expect.
Last reviewed: 2026-06-14
Quick answer
In 2026, building a detached ADU (DADU) in Seattle typically costs $250,000–$450,000 for mid-grade builds — pushing to $650,000 for premium designs in tight inner-city lots. Garage conversions run $90,000–$180,000. Soft costs add $18,000–$40,000 on top, but Seattle's ADUniverse pre-approved program can cut both cost and timeline meaningfully.
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Get matched with up to 3 Seattle DADU buildersCost breakdown by build type
Total project costs in 2026 Seattle — hard construction plus typical soft costs — for a mid-finish build. Capitol Hill, Queen Anne, and Ballard push to the top of each range due to lot constraints and labor demand.
| Build type | Typical size | Total cost | Per sqft |
|---|---|---|---|
| Detached ADU (DADU) | 600–1,200 sqft | $250K–$650K | $320–$550 |
| Garage conversion | 350–500 sqft | $90K–$180K | $230–$370 |
| Attached ADU | 500–1,000 sqft | $210K–$380K | $300–$420 |
| Basement conversion | 500–900 sqft | $120K–$240K | $220–$340 |
| Prefab / modular | 400–1,000 sqft | $170K–$320K | $260–$370 |
ADUniverse — Seattle's pre-approved DADU program
Seattle's ADUniverse program offers a curated catalog of DADU designs from local architects that have been pre-approved by SDCI for code compliance. Picking an ADUniverse plan dramatically accelerates permitting — typical projects move through plan check in 2–6 weeks instead of the usual 6–12 weeks.
Cost savings stack: $5K–$15K saved on design fees (you're skipping the custom architecture phase) plus carrying-cost reductions from faster permitting (less interest on construction loans, fewer months of soft rent). Total typical savings: $10,000–$20,000 + 4–6 weeks.
The trade-off: ADUniverse plans are pre-approved as designed and can't be modified. Tight Seattle lots with unusual access, slope, or constraints may not fit any catalog design.
Hard costs vs. soft costs
On a typical Seattle DADU project, soft costs are $18,000–$40,000on top of construction — one of the higher soft-cost ranges in the Pacific Northwest due to Seattle's SDCI permit fees.
Hard costs
- · Site prep, excavation, foundation
- · Framing, roof, exterior
- · Plumbing, electrical, HVAC
- · Insulation, drywall, paint
- · Cabinets, counters, appliances
- · Flooring, fixtures, finishes
Soft costs (Seattle)
- · Design / architectural plans: $10K–$22K
- · Structural & energy engineering: $2K–$5K
- · SDCI plan check + permit: $8K–$18K
- · Trade permits (electric / plumb / mech): $1.5K–$4K
- · Utility hookups (SCL / SPU): $0–$10K
- · Survey, soils, contingency: $2K–$6K
ROI — what a Seattle DADU returns
1-bedroom DADUs in Seattle typically rent for $1,800–$2,500/month in 2026 — Capitol Hill, Ballard, Queen Anne, Wallingford top the range. The premium over apartments is meaningful (privacy, yard, standalone) but high Seattle construction costs mean cash-on-cash returns are modest.
Sample ROI — $325K DADU 800 sqft
- · Construction cost: $325,000
- · Gross rent (1BR @ $2,200/mo): $26,400/yr
- · Net rental income after expenses: ~$20,500/yr
- · Cash-on-cash return: ~6.3%
- · Property value lift: $175K–$275K
- · Approx. rental payback: 13–16 years
How to lower the cost
1. Use an ADUniverse pre-approved plan ($10K–$20K + 4–6 weeks saved)
Highest-leverage Seattle move. Pick from the catalog and skip custom architecture.
2. Consider basement or garage conversion
$90K–$240K vs $250K+ for new DADU. Big caveat: older Seattle homes often need significant work to make basements code-compliant.
3. Get 3+ written bids
Seattle builder bids vary 20–40% for identical scopes.
4. Keep finishes mid-grade, not premium
Premium finishes add 15–20% to hard cost but rarely add proportional rent.
When Seattle DADU costs go up unexpectedly
- Tight lots + alley-only access. Common across central Seattle. Restricted equipment access slows builds 1–3 months and adds 5–15% to construction.
- Sloped / hillside lots. Common in Magnolia, West Seattle, parts of Queen Anne. Retaining walls + deeper foundations add $15K–$50K.
- Tree protection. Seattle's tree-protection ordinance can constrain footprint or require certified arborist plans. Add $3K–$10K + design rework.
- Old service panel. Pre-1985 Seattle City Light services often need upgrade for DADU panel. Add $4K–$12K.
- Long sewer runs. Deep Seattle lots may need new sewer laterals to reach the DADU. Add $8K–$20K.
Sources
- Seattle SDCI — ADUniverse Pre-Approved DADU Plans ↗Official ADUniverse catalog and process.
- Washington HB 1110 — Statewide ADU rules ↗Statewide ADU permitting rules enforced statewide.
- Apartment List — Seattle Rent Report ↗Current 1-bedroom rent data for ROI estimates.
- Washington L&I — Contractor verification ↗License verification for any Seattle ADU/DADU builder.
Last reviewed: 2026-06-14. Cost ranges are 2026 estimates and shift with materials prices, labor markets, and lot conditions. Always verify with the official source and get written bids before making decisions.
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Frequently asked
Seattle DADU/ADU cost — common questions
How much does it cost to build an ADU/DADU in Seattle in 2026?
In 2026, a detached ADU (DADU) in Seattle typically costs $250,000–$450,000 for mid-grade builds, with premium DADUs in tight inner neighborhoods like Capitol Hill or Queen Anne reaching $650,000. Garage conversions run $90,000–$180,000. Seattle is the most expensive Pacific Northwest market — driven by tech-economy labor costs and tight lot conditions.
What's Seattle's ADUniverse program?
ADUniverse is Seattle's pre-approved DADU plan program, offering a catalog of designs from local architects. Picking an ADUniverse plan cuts permitting from 6–12 weeks down to 2–6 weeks — and reduces design fees significantly (typically $5,000–$15,000 saved on architecture). Plans cover a range of styles and sizes within Seattle's DADU rules.
What's the difference between an ADU and a DADU in Seattle?
In Seattle's permit language, ADU usually means an attached or interior unit (a Mother-in-Law apartment, basement conversion, etc.), while DADU specifically means Detached Accessory Dwelling Unit — a standalone backyard cottage. Seattle's rules and pre-approved program (ADUniverse) are specifically built around the DADU type.
What does Washington's HB 1110 do for ADUs?
Washington's HB 1110 (2023) is the state's missing-middle housing law. For ADU owners specifically, it requires cities of 25,000+ to allow at least two ADUs (or DADUs) per residential lot and limits cities' ability to impose owner-occupancy requirements, design conditions, or off-street parking mandates on ADUs. Seattle's rules already met most HB 1110 requirements, but the law made statewide ADU permitting more predictable.
How much rent can a 1-bedroom DADU in Seattle get?
1-bedroom DADUs in Seattle typically rent for $1,800–$2,500/month in 2026. Capitol Hill, Ballard, Queen Anne, and Wallingford command the top of the range; South Seattle, North Seattle, and West Seattle run more moderate. The DADU premium over apartments is meaningful — standalone units with a yard pull higher rents.
What's the ROI on a DADU in Seattle?
On a $325,000 mid-build DADU renting at $2,200/month, annual net rental income runs roughly $20,500 after expenses — a cash-on-cash return of about 6.3%. Lower cash-on-cash than other Pacific Northwest cities because Seattle's higher build cost outpaces the rent premium. However, property value lift tends to be high ($175,000–$275,000) due to tight Seattle housing supply.
How long does DADU construction take in Seattle?
Plan on 14–18 months for a detached DADU: 2–4 months for design, 3–6 months for permitting through SDCI (faster with ADUniverse), and 8–10 months for construction. Garage conversions run 10–14 months total. Tight Seattle lots often add 1–2 months for access and staging logistics.
Do I need a contractor licensed in Washington specifically?
Yes — Washington requires builders to register with the Washington State Department of Labor & Industries (L&I) as a registered contractor. Verify the L&I number is active and check for complaints at lni.wa.gov before signing any contract. The license is state-level; any active L&I-registered contractor can build in Seattle.