Seattle ADU + DADU Construction — 2019 Ordinance, DCI Permit, $180K-$400K

Seattle's ADU market runs on a different operating system than Los Angeles, Austin, or Phoenix. The 2019 Accessory Dwelling Unit Ordinance — the rule that killed owner-occupancy and legalized two ADUs per single-family lot — flipped the city from one of the most restrictive ADU markets on the West Coast to one of the most permissive. If your contractor is still quoting you Seattle ADU rules from 2018, walk away.

Baily's Seattle ADU filter screens for three things your neighbor's Nextdoor recommendation won't: an active Washington State Department of Labor & Industries (WA L&I) contractor license with the mandatory $12,000 bond, real Seattle Department of Construction and Inspections (SDCI) permit history on DADUs 700+ sqft, and documented Seattle Energy Code 2024 gas-ban experience. One match. Not twelve.

What Seattle's 2019 ADU Ordinance actually unlocked

Pre-2019, Seattle's ADU rules were a drag chute. One ADU per lot. Owner-occupancy required (the owner had to live on-site in either the main house or the ADU — making it effectively impossible for investors or long-distance heirs to legally build one). Lot-size minimums that disqualified thousands of parcels. Off-street parking requirements that priced small lots out of the market entirely.

Then in July 2019, Seattle City Council passed the ADU Reform Ordinance (Ordinance 125854), the housing-production piece carved out of the broader HALA framework (Housing Affordability and Livability Agenda). Post-HALA, the math changed:

• Up to two ADUs per single-family lot — one Attached ADU (AADU) plus one Detached ADU (DADU).
• Owner-occupancy requirement REMOVED. You can build, rent, and live somewhere else. This single change did more for Seattle ADU permit volume than any other policy lever.
• Most lot-size minimums lifted. If you have a conforming single-family lot, you almost certainly qualify.
• Off-street parking requirement eliminated for ADUs within 0.25 miles of frequent transit (which covers most of the urban core).
• DADU size cap raised to 1,000 sqft of gross floor area.
• Height cap at 30 feet / 3 stories (2-story DADU became the market default).

The practical effect: Seattle SDCI permit data shows DADU permits grew more than 130% from 2019 through 2024. The ordinance converted a legal workaround into a mainstream housing product. (seattle.gov/sdci)

AADU vs DADU — what's actually different

Two ADU types, two different cost profiles. Knowing which one fits your lot (and your budget) is step one.

AADU (Attached ADU) — inside or attached to the principal dwelling. Basement conversions, above-garage units, attic conversions, or small attached additions. Smaller footprint (commonly 400–650 sqft in conversion configs), shares mechanical systems with the main house, and sits at the cheap end of Seattle ADU economics: $80K–$150K for a clean garage-to-unit conversion, $120K–$220K for a full basement conversion with egress window, waterproofing, and independent HVAC.

DADU (Detached ADU) — standalone structure in the rear yard, commonly called a "backyard cottage." Max 1,000 sqft gross floor area, max 30 ft height / 3 stories, separate electrical service, separate water meter option. Standalone foundation, standalone mechanical, standalone everything. $180K–$400K new-build depending on grade, tree removal, shoreline overlays, and whether Design Review gets triggered.

Most Seattle homeowners asking Baily about "an ADU" actually want a DADU. The privacy, the resale premium, and the short-term-rental flexibility all favor detached.

WA L&I contractor licensing — what Baily verifies

Washington is one of the states that actually licenses general contractors at the state level. This is a meaningful difference from Texas (no state GC license) or Illinois (city-level only). WA L&I — the Department of Labor & Industries — issues contractor registrations with real teeth behind them.

Baily's WA contractor verification checks:

1. Active WA L&I contractor registration. Searchable at lni.wa.gov. Either "General" or "Specialty" — for a DADU you want General.
2. $12,000+ continuous bond on file. This is the statutory minimum for a General contractor in Washington. Specialty contractors bond at $6,000. If the bond has lapsed, the registration is invalid and your permit can't legally close. (lni.wa.gov)
3. Workers' compensation coverage. WA L&I administers the state's monopoly workers' comp fund. Mandatory for any contractor with one or more employees. Baily flags contractors whose coverage has lapsed — that's your liability if someone gets hurt on your lot.
4. No open L&I complaints or unpaid judgments in the public database.
5. Active Seattle Business License issued by the City of Seattle Department of Finance and Administrative Services. A contractor can be WA L&I-legal and still unlicensed to operate inside Seattle city limits.

That's five checks. A homeowner doing this manually from a Yelp list has to cross-reference five different databases. Baily does it in under a second.

SDCI permit + Design Review path

Seattle ADU permits run through SDCI (Seattle Department of Construction and Inspections) via the Online Permit application system. Here's the realistic timeline:

• Basic residential ADU permit (under 1,000 sqft, no Design Review trigger): 4–8 weeks from complete application to issuance. Faster if your lot is simple (no ECA, no trees, no shoreline, no steep slope).
• Permit with Design Review (most DADUs hitting the 1,000 sqft max, or any project triggering the Design Review threshold): add 4–12 weeks on top of the base permit clock. Seattle Design Review panels meet on a fixed schedule and can send a project back for revisions.
• Permit with ECA overlay (Environmental Critical Area — see below): add 4–8 weeks for ECA review.

Permit fees typically run $2,000–$6,000 for a DADU, scaling with project valuation. SDCI also charges separate plan-review fees, and Seattle City Light and Seattle Public Utilities charge their own connection fees that commonly add another $8K–$18K for a new electric service and water meter.

Seattle Residential Code (SRC) 2021 is the active code, with local amendments. Your contractor needs to be fluent in the Seattle-specific amendments — they're not identical to the Washington State Residential Code. (seattle.gov/sdci/codes)

Seattle Energy Code + the 2024 gas ban

This is where out-of-town contractors get expensive. Seattle Energy Code is more stringent than the Washington state energy code, and the 2024 amendment added a near-total ban on natural gas in new residential construction. If your DADU is new construction, you're on a heat pump. Full stop. Gas appliances are effectively disallowed in new ADUs. (seattle.gov/sdci/codes/changes-to-code)

The energy-compliance premium versus building to bare WA state minimum runs $8K–$25K on a typical DADU. Air-source heat pump HVAC, heat-pump water heater, induction cooking, higher-spec envelope insulation, blower-door testing. A contractor who hasn't built inside Seattle since 2023 will under-quote you by exactly that amount, then issue change orders when the permit reviewer sends them back.

Cost bands + realistic timeline

DADU new-build: $180K–$400K. The low end assumes a flat lot, no tree removal, no ECA overlay, standard 700-sqft two-bed one-bath layout, and a builder who's done 10+ Seattle DADUs. The high end is 1,000 sqft, high-grade finishes, steep-slope engineering, tree-protection fencing, and Design Review compliance.

AADU garage conversion: $80K–$150K. Assumes the garage has a usable slab, the existing electrical service can support the load, and no structural reframe is needed.

AADU basement conversion: $120K–$220K. Egress window, waterproofing, independent HVAC, proper ceiling height (7 ft minimum for habitable space), and independent utilities are where the money goes.

Typical concept-to-move-in timeline: 12–16 months for a DADU, assuming Design Review doesn't add a cycle. Breakdown: design + engineering 2–4 months, permit 2–4 months, construction 6–10 months.

Tree Protection + ECA overlays — the hidden schedule killers

Seattle's Tree Protection Ordinance is serious. A "Significant Tree" — 30+ inches diameter at breast height, or a Heritage Tree of any size — cannot be removed without a tree permit, and some trees cannot be removed at all. If your planned DADU footprint hits a Significant Tree, you're redesigning or you're paying for tree-protection fencing and root-zone engineering. (seattle.gov/trees)

ECA (Environmental Critical Area) overlays cover shoreline (within 200 ft of Puget Sound, Lake Washington, Lake Union, the Ship Canal, or the Duwamish), steep slopes (40%+ grade), flood-prone areas, wetlands, and landslide-prone zones. If your parcel touches any of these, ECA review adds 4–12 weeks and usually requires a geotechnical report. Public Benefit Easement can be triggered in shoreline zones. (seattle.gov/sdci/permits/permits-we-issue/environmentally-critical-areas)

Check your parcel on the Seattle GIS map before you fall in love with a design. Baily does this check automatically when you share your address.

Why Baily matches you with exactly one Seattle ADU specialist

Seattle has hundreds of WA L&I-licensed General contractors. Maybe 40 have closed a DADU permit in the last 24 months. Maybe 15 have closed 5+. Maybe 8 are fluent in Seattle Energy Code 2024 gas-ban construction. Maybe 4 have Design Review panel experience at the 1,000-sqft threshold.

Baily's filter: active WA L&I + $12K+ bond posted, 5+ closed Seattle DADU/AADU permits in the past 24 months, Seattle Energy Code 2024 gas-ban experience documented, Design Review panel experience if your project hits 1,000 sqft or triggers review. Seattle Business License active. No open L&I complaints.

One match. The one you'd have found after six weeks of vetting.

Frequently asked questions

Does my Seattle lot qualify for a DADU after the 2019 ordinance?

Almost certainly yes, if your lot is zoned single-family residential (SF 5000, SF 7200, SF 9600, or RSL). The 2019 ADU Reform Ordinance removed most lot-size minimums, eliminated the owner-occupancy requirement, and cut parking requirements for lots near frequent transit. The remaining disqualifiers are narrow: parcels fully inside an Environmental Critical Area with no buildable envelope, lots where Tree Protection Ordinance removes the feasible footprint, and condo-style lots where the HOA or plat restricts additional structures. SDCI's Online Permit pre-application review will confirm in 48–72 hours whether your specific address qualifies. Expect yes. (seattle.gov/sdci/permits/common-projects/accessory-dwelling-units)

What's the typical cost for a Seattle DADU in 2026?

$180K–$400K for a new-build DADU, with the middle of the market clustering around $260K–$320K for a 700–900 sqft two-bed one-bath on a flat lot with no ECA or tree complications. The low end requires builder efficiency, a simple lot, and no Design Review. The high end adds Seattle Energy Code 2024 compliance premium ($8K–$25K), steep-slope engineering ($10K–$30K), tree protection ($5K–$15K), SDCI permit + plan review ($2K–$6K), and Seattle City Light + SPU utility connection ($8K–$18K combined). Garage-conversion AADUs run $80K–$150K. Basement conversions run $120K–$220K. Get line-item quotes — "turnkey" numbers without utility and permit breakouts are where margin gets hidden.

Can I rent my Seattle ADU as a short-term rental (Airbnb)?

Yes, but with constraints. Seattle's Short-Term Rental ordinance requires a Short-Term Rental Operator License (issued by the Department of Finance and Administrative Services) plus a Regulatory License, and caps most operators at two dwelling units citywide for STR use. One of the two must be your primary residence, with a narrow exception for operators grandfathered under pre-2017 rules. For a DADU owner, the typical play is: live in the main house, operate the DADU as an STR — that's compliant and common. Operating both the main house and the DADU as STRs generally isn't, unless you qualify for the grandfather exception. Seattle also collects occupancy tax on STR revenue. (seattle.gov/business-licenses-and-taxes/business-licenses/short-term-rentals)

How do I verify my Seattle contractor's WA L&I license and bond?

Go directly to lni.wa.gov and use the "Verify a Contractor" tool. Enter the business name or the WA L&I registration number (every legitimate Washington contractor has one — it starts with the business name abbreviation followed by six digits plus two letters). The verification page shows: registration status (active/expired/suspended), bond amount and bond company, workers' comp account status, liability insurance carrier and coverage amount, any complaints filed, and any unpaid L&I judgments. A General contractor must show a $12,000 minimum continuous bond; Specialty contractors show $6,000. If the bond has lapsed even for a day, the registration is invalid and your permit will not close legally. Baily runs this check automatically on every Seattle match. (lni.wa.gov/licensing-permits/contractors/hire-a-contractor-or-tradesperson/verify-a-contractor)

What's the difference between the Seattle gas ban and the statewide WA energy code?

They're two different layers, and both apply inside Seattle city limits. The Washington State Energy Code (WSEC) sets baseline efficiency requirements statewide — adopted and updated on a 3-year cycle by the WA State Building Code Council. Seattle then layers the Seattle Energy Code on top, which is consistently more stringent than the state baseline. The 2024 Seattle Energy Code amendment effectively prohibits natural gas appliances in most new residential construction — meaning new DADUs run on electric heat pumps for space heating and water heating, and induction (or electric) cooking. Retrofits and AADU conversions have more flexibility; new-construction DADUs do not. Budget an extra $8K–$25K versus WA state minimum code. Contractors who don't work inside Seattle regularly get this wrong. (sbcc.wa.gov/state-codes, seattle.gov/sdci/codes/changes-to-code)

Sources cited:

1. Seattle SDCI — Accessory Dwelling Units: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units
2. Seattle SDCI — Seattle Residential Code and Codes: https://www.seattle.gov/sdci/codes
3. Seattle SDCI — Changes to Code (Energy Code 2024): https://www.seattle.gov/sdci/codes/changes-to-code
4. Seattle SDCI — Environmentally Critical Areas: https://www.seattle.gov/sdci/permits/permits-we-issue/environmentally-critical-areas
5. Seattle Office of Sustainability & Environment — Trees: https://www.seattle.gov/trees
6. City of Seattle — Short-Term Rentals Licensing: https://www.seattle.gov/business-licenses-and-taxes/business-licenses/short-term-rentals
7. Washington State L&I — Verify a Contractor: https://lni.wa.gov/licensing-permits/contractors/hire-a-contractor-or-tradesperson/verify-a-contractor
8. Washington State Building Code Council — State Codes: https://sbcc.wa.gov/state-codes
9. Washington State Legislature — ADU and Housing Statutes: https://apps.leg.wa.gov/rcw/