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# ADU & DADU Builders in Seattle & the Puget Sound: The Complete 2026 Guide
If you own a single-family home anywhere in Seattle, Bellevue, Kirkland, SeaTac, Renton, or most of South King County, you can almost certainly build an Accessory Dwelling Unit (ADU) or a Detached Accessory Dwelling Unit (DADU) on your property. Seattle's zoning changes over the past several years have made this one of the most accessible and financially rewarding projects available to Washington state homeowners.
Love Construction is a licensed general contractor based in SeaTac, WA — Passivehaus certified and one of the few builders in the South Puget Sound actively specializing in high-performance ADU and DADU construction. WA License LOVECC*802N4. This guide covers everything you need to know before reaching out to a builder.
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ADU vs. DADU: What's the Difference?
These two terms cause consistent confusion — and they matter because zoning rules, costs, and design approaches differ significantly.
ADU (Accessory Dwelling Unit) is the umbrella term for any secondary dwelling unit on a property that already has a primary home. It is a fully self-contained unit with its own kitchen, bathroom, and living space.
DADU (Detached Accessory Dwelling Unit) is specifically a freestanding structure — separate from the main house. In Seattle, this is commonly called a backyard cottage. It sits on its own foundation, has its own exterior walls, and functions entirely independently from the main house.
Attached ADUs are secondary units that share a wall with the primary home — a finished basement apartment, a garage conversion, or an addition above a garage are common examples.
| Feature | Detached ADU (DADU) | Attached ADU | |---|---|---| | Structure | Freestanding building | Shares wall with main house | | Common forms | Backyard cottage, laneway house | Basement apartment, garage conversion | | Maximum size (Seattle) | Up to 1,000 sq ft | Up to 1,000 sq ft | | Cost range | $180,000–$400,000 | $80,000–$250,000 | | Privacy | Highest — separate structure | Moderate — shared wall | | Timeline | 8–14 months | 5–10 months |
Seattle's current zoning allows one attached ADU AND one detached ADU (DADU) on most single-family lots — meaning you can potentially have three dwelling units on your property.
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Why ADU Construction in Seattle is a Major Financial Opportunity
Seattle's housing market is chronically undersupplied. Rents remain among the highest in the country, and the region's ADU-friendly legislation has created a clear path for homeowners to generate significant rental income — or increase their property value substantially — through ADU construction.
Rental income: A well-built DADU in Seattle, Bellevue, or South King County cities like Renton and Burien can command $1,800–$2,800/month in rent depending on size, finishes, and location.
Property value: Real estate appraisers in the Seattle market increasingly apply income capitalization approaches to properties with ADUs, which can add $150,000–$350,000 to assessed value depending on rental income potential.
Aging in place / multi-generational living: Many of our ADU clients build for family rather than rental — a parent moving out of a retirement community, an adult child saving to buy their own home, a family needing a live-in caregiver suite.
ADU legislation: Washington state and City of Seattle have systematically removed barriers to ADU construction, including eliminating owner-occupancy requirements, reducing setback requirements, and streamlining permitting for standard ADU designs.
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ADU Zoning Rules by City in South King County and Seattle
Zoning rules vary by jurisdiction. Here is a quick reference for the cities Love Construction primarily serves:
Seattle
- • Allowed on most single-family (SF) and residential small lot (RSL) zoned parcels
- • One DADU + one attached ADU allowed per lot (up to 3 total dwelling units)
- • DADU maximum: 1,000 sq ft or 60% of primary home size, whichever is smaller
- • Minimum lot size: 3,200 sq ft (most lots qualify)
- • Setbacks: 5 ft side and rear in most zones
- • Owner-occupancy requirement: Eliminated as of 2019
Renton
- • DADUs allowed in R-1 through R-8 zones
- • Maximum 1,000 sq ft
- • Setbacks vary by zone (typically 5 ft side, 20 ft rear)
- • Separate utility connections typically required
Burien
- • ADUs allowed in most single-family residential zones
- • Attached and detached ADUs both permitted
- • Maximum size: lesser of 1,000 sq ft or 40% of primary home sq ft
Tukwila, Kent, Des Moines, Federal Way, Auburn
- • All allow ADUs in single-family zones
- • Rules and setbacks vary — we research current code for each project before design begins
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How Much Does an ADU or DADU Cost to Build in Seattle?
ADU costs in the greater Seattle area in 2026:
| ADU Type | Low End | Mid Range | High End | |---|---|---|---| | Garage Conversion to ADU | $60,000 | $110,000 | $180,000 | | Basement ADU Conversion | $80,000 | $140,000 | $220,000 | | Attached ADU Addition | $120,000 | $180,000 | $280,000 | | Detached ADU (DADU) — Standard | $180,000 | $250,000 | $350,000 | | Detached ADU (DADU) — Passive House | $220,000 | $295,000 | $400,000 |
What drives cost variation:
- • Utilities: Does your lot require a new sewer lateral? New electrical service panel? New water meter? These can add $15,000–$40,000.
- • Site conditions: Sloped lots, tight access, or poor soil drainage increases foundation costs.
- • Size: Every square foot adds cost.
- • Finishes: Entry-level finishes vs. custom tile, hardwood, and premium appliances.
- • Performance level: Standard code-built vs. Passive House construction.
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The ADU/DADU Permitting Process in Seattle
Seattle's ADU permitting has improved significantly but still requires careful navigation. Here's what the process looks like for a standard DADU project:
1. Pre-application site research — We verify your parcel's zoning, lot coverage, setback requirements, utility connections, and any covenant or HOA restrictions.
2. Design phase — Architectural drawings developed to city submittal standards, including site plan, floor plans, elevations, and sections.
3. Permit application submission — Through SDCI (Seattle Department of Construction & Inspections) online portal for Seattle; through jurisdiction-specific portals for Renton, Burien, Kent, etc.
4. Plan review — Typically 8–16 weeks in Seattle for a full plan review. Some jurisdictions offer expedited review for a fee.
5. Permit issuance — Once approved, building permit is issued and construction can begin.
6. Inspections — Multiple city inspections during construction: foundation, framing, rough mechanical/electrical/plumbing, insulation, final.
7. Certificate of occupancy — Issued upon passing final inspection. Your ADU is now legal to occupy and rent.
Total timeline from design start to move-in: 12–18 months for a typical DADU in Seattle. Faster jurisdictions (Renton, Kent, Burien) can come in at 10–14 months.
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Passive House ADUs: Why They Make More Sense Than Standard Construction
Most ADU builders in Seattle build to minimum energy code. Love Construction builds to Passive House standards wherever the client wants it — and here's why it matters specifically for small units:
Small buildings lose proportionally more heat. A 600 sq ft ADU has a much higher ratio of exterior wall area to interior floor area than a 2,500 sq ft house. Passive House super-insulation and airtightness pays off faster in small buildings because the thermal envelope is doing more work per square foot.
Tenants care about energy costs. A Passive House ADU can be marketed with honest numbers: less than $20/month in heating and cooling in Seattle's climate. This is a legitimate competitive advantage in the rental market.
Near-zero energy is achievable with a small solar array. A modest 3–5 kW rooftop or ground-mount solar system combined with a Passive House envelope achieves genuine Net Zero for a typical ADU.
Once construction starts, upgrade costs are minimal. The cost premium for Passive House over standard code construction for an ADU is typically $15,000–$30,000 — a fraction of the total project cost, and recouped in energy savings over 8–12 years.
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How to Choose an ADU Builder in Seattle
1. Verify their Washington State contractor license. Every builder working on an ADU must hold a valid general contractor license through WA Department of Labor & Industries. Check at verify.lni.wa.gov. Love Construction's license is LOVECC*802N4.
2. Ask about ADU-specific permit experience. ADU permitting has its own requirements and quirks — utility connections, owner-occupancy declarations in some jurisdictions, setback calculations. A builder without specific ADU permitting experience in your city will slow your project down.
3. Ask who is on site. Large-volume production builders use project managers who juggle many jobs simultaneously. At Love Construction, Aaron is personally involved throughout the project — not managing from a distance.
4. Get a fixed-price contract. Time-and-materials contracts expose you to unlimited cost overrun. A legitimate ADU builder can provide a fixed-price contract after a thorough design and site investigation.
5. Check references specifically for ADU projects. Ask reference clients about permit timeline accuracy, how surprises were handled, and whether the final product matched what was promised.
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Love Construction's ADU Work
Love Construction has built ADUs and DADUs across the South Puget Sound — from backyard cottages in SeaTac to Passive House DADUs in Bellevue. Our ADU process:
- • Site and zoning research before design begins
- • In-house design coordinated from the start with construction cost in mind
- • All permitting managed by our team
- • Passive House performance available for every ADU project
- • 5-year warranty on all labor
- • Built Green certified construction practices
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Frequently Asked Questions About ADU Construction in Seattle
Can I rent my ADU on a short-term basis (Airbnb, VRBO)?
Seattle's short-term rental regulations are complex and continue to evolve. In most cases, owner-occupied properties can offer one STR unit. Properties without an owner-occupant on site face additional licensing requirements. We recommend consulting with a Seattle real estate attorney before designing for STR use.
Does building an ADU affect my property taxes?
Yes. Adding a new dwelling unit increases the assessed value of your property, which increases your annual property tax bill. The exact increase depends on your county assessor's valuation of the new unit.
Can I finance ADU construction?
Several financing options exist: home equity loans, HELOCs, cash-out refinancing, and ADU-specific construction loans offered by some Washington state lenders. We can discuss project costs in detail during a consultation so you can pursue the right financing path.
Does Seattle require owner-occupancy for ADU properties?
No. Seattle eliminated the owner-occupancy requirement for ADU properties in 2019. You are not required to live on the property that has an ADU.
What is the smallest lot that can support a DADU in Seattle?
Most Seattle single-family lots qualify. The minimum lot size is 3,200 sq ft and most residential lots in Seattle exceed this. Setback requirements (typically 5 ft from side and rear property lines in SF zones) are the more common limiting factor for small lots.
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Get Started with Your ADU Project
Love Construction is based in SeaTac and serves the full South Puget Sound region. If you are considering an ADU or DADU — whether for rental income, multi-generational living, or long-term property value — we are happy to evaluate your site and discuss what is possible.
Explore our ADU & DADU Construction services or contact us to schedule a free site consultation.
Love Construction LLC — SeaTac, WA | WA License LOVECC*802N4 | (206) 339-2718
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