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# ADU vs DADU in Washington State: What's the Difference and Which Should You Build?
Most King County homeowners researching secondary dwelling units quickly hit the same wall: everyone uses "ADU" and "DADU" interchangeably until suddenly they don't, and the distinction starts to matter a lot for permits, costs, and what you can actually build on your lot.
This guide cuts through the confusion. By the end, you'll know exactly what each term means under Washington State code, which type works better for different properties and goals, and what realistic costs look like in the Seattle metro in 2026.
Love Construction is a Passivehaus-certified design-build firm based in SeaTac, WA. We've built ADUs and DADUs across King County, and we field this question on nearly every first call.
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Table of Contents
- • ADU vs DADU: The Basic Definitions
- • Attached ADU vs Detached ADU: What Changes
- • Washington State ADU Laws: What Changed and Why It Matters
- • Cost Comparison: ADU vs DADU in King County
- • Which Type Is Right for Your Property?
- • The Passive House ADU Option
- • FAQ: ADU vs DADU in Washington
ADU vs DADU: The Basic Definitions {#basic-definitions}
ADU stands for Accessory Dwelling Unit. It's the umbrella term for any secondary living space on a single-family residential lot. An ADU has its own kitchen, sleeping area, bathroom, and entrance. It's a complete, independently livable unit.
DADU stands for Detached Accessory Dwelling Unit. It's a specific type of ADU — one that's a completely separate structure from the primary home. Its own foundation, its own roofline, its own entrance. No shared walls with your house.
The difference, then, is physical connection:
| Term | Connection to Primary Home | Common Forms | |------|--------------------------|--------------| | ADU (broad) | Either attached or detached | Basement conversion, garage conversion, addition, backyard cottage | | DADU (specific) | None — fully detached | Backyard cottage, carriage house, laneway house | | AADU (attached ADU) | Shares wall or roof | Basement unit, in-law suite, garage conversion |
In everyday use, "ADU" often means attached unit, and "DADU" always means detached. Washington State code and most King County city documents use the full terms: attached ADU and detached ADU (or DADU). When a builder or city planner says "DADU," they mean a freestanding backyard structure.
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Attached ADU vs Detached ADU: What Changes {#attached-vs-detached}
The ADU vs DADU distinction matters beyond terminology. It affects permitting, construction cost, utility connections, how usable the space feels, and what a future tenant or buyer will pay.
Privacy and Livability
A DADU offers genuine separation from the primary residence. For rental purposes, that separation commands higher rents. A tenant in a backyard cottage has their own address (in many jurisdictions), their own outdoor space, and no shared walls. That privacy typically adds 10–20% to achievable rent compared to an attached basement unit of the same square footage.
For homeowners using the space for aging parents or adult children, the separation often works better for everyone — close but not too close.
Permitting Complexity
In most King County cities — Seattle, Kirkland, Bellevue, SeaTac, Issaquah — attached ADUs and DADUs both require building permits. DADUs typically require:
- • Site plan showing setbacks from property lines
- • Structural plans for a new foundation and framing
- • Separate utility connection review (electrical, water/sewer, sometimes gas)
- • In some zones: design review for exterior appearance
The trade-off: a DADU gives you more square footage potential and better separation, but takes longer to permit and costs more to build.
Construction Timeline
A typical basement or garage conversion to an attached ADU runs 8–12 months from contract to certificate of occupancy. A new DADU runs 12–18 months — longer design phase, longer permit review, and more construction scope.
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Washington State ADU Laws: What Changed and Why It Matters {#wa-state-laws}
Washington State passed significant ADU legislation in 2023 and 2024 that changed what's allowed statewide, regardless of local city rules. Key provisions as of 2026:
Most restrictions on ADUs are now preempted by state law. Cities cannot prohibit ADUs and DADUs outright on single-family lots that can fit them given setbacks and lot coverage rules.
Owner-occupancy requirements have been largely eliminated. Under older rules, many King County cities required the property owner to live in either the primary home or the ADU. That restriction has been removed statewide. You can build a DADU as a pure investment property without living on-site.
Parking requirements cannot be imposed. Cities cannot require additional parking as a condition of ADU approval, with narrow exceptions for transit-poor areas.
Two ADUs per lot is now a baseline right in cities covered by the Growth Management Act. That means, in most of King County, you can have both an attached ADU and a DADU on the same single-family lot — simultaneously.
This is a significant change. A homeowner in Kirkland, Bellevue, or SeaTac with a suitable lot can now legally build both a basement attached ADU and a backyard DADU, creating two rental income streams on land they already own.
Local permit processes still apply — you still need to pull permits and meet setback and lot coverage rules. But the policy environment has shifted strongly in favor of ADU construction across Washington State.
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Cost Comparison: ADU vs DADU in King County (2026) {#cost-comparison}
Costs vary significantly by city, lot conditions, and finish level. These ranges reflect realistic 2026 construction budgets in the King County market.
Attached ADU Costs
| Type | Cost Range | |------|-----------| | Basement conversion (existing space) | $85,000–$175,000 | | Garage conversion (attached garage) | $75,000–$150,000 | | New addition above garage | $150,000–$280,000 |
Attached ADUs cost less because you're adapting existing structure. You're not building a new foundation, a new roof, or running utilities from scratch. The biggest cost variables are the condition of the existing space and how much work it needs to meet code.
DADU Costs
| Size | Cost Range | |------|-----------| | 400–600 sqft | $175,000–$290,000 | | 600–800 sqft | $230,000–$380,000 | | 800–1,000 sqft | $300,000–$460,000 | | 1,000–1,200 sqft | $360,000–$550,000 | | Passive House DADU (any size) | Add 15–25% to above |
These figures include design, permits, construction, and utility connections. They do not include financing.
Why DADUs cost more:
- • New foundation (slab, crawl space, or full basement depending on lot)
- • Full roof system — no shared structure to leverage
- • Separate utility connections: electrical panel or sub-panel, water/sewer tap or sub-meter, possible gas extension
- • Site work: grading, access path, landscaping disruption
Return on Investment
Both types generate rental income, but DADUs typically command more per month because of privacy and independence.
A $250,000 attached ADU generating $1,800/month reaches rental income break-even in approximately 11.5 years. A $320,000 DADU generating $2,400/month reaches break-even in 11.1 years. The larger investment in a DADU often pays back at a similar or faster rate than the less expensive attached option — once you account for the rent premium.
Both types add to resale value, with DADUs generally adding more because they're a discrete, marketable asset separate from the main house.
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Which Type Is Right for Your Property? {#which-type}
The honest answer depends on your specific lot, budget, goals, and timeline. Here's how we think about it during a feasibility assessment:
Choose an attached ADU if:
- • Your budget is under $175,000
- • You have an existing basement or garage with good bones
- • Your lot doesn't have enough rear yard space for a freestanding structure
- • You need the shortest possible timeline to rental income
- • Privacy separation is less important (family member use, office)
- • Your lot has a usable rear yard with adequate setbacks
- • You want maximum rental income potential
- • Privacy for the tenant (or family member) matters
- • You're building for long-term investment value
- • You want to eventually sell the DADU as a separate unit if laws change
- • You're interested in Passive House performance standards (easier to achieve in a standalone structure)
- • Your long-term plan is maximum rental income on a held property
- • You're in a high-demand rental market (Kirkland, Seattle, Bellevue, SeaTac) where two units cash flow well
- • You have the budget and the 18–24 month timeline for both projects
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The Passive House ADU Option {#passive-house-option}
Love Construction is one of the few Passivehaus-certified builders in King County. For both attached ADUs and DADUs, we offer Passive House construction — which means your unit is designed and built to use 60–90% less energy than a code-minimum structure.
For an ADU you're renting out, that matters in two ways:
If utilities are tenant-paid: a Passive House ADU is a genuine competitive advantage. In a market full of drafty basements and poorly insulated backyard sheds, your unit stands out. Tenants pay less to heat and cool it. That commands a premium in a competitive rental market.
If utilities are landlord-paid: the savings go directly to your margin. A code-minimum DADU in Seattle might cost $150–$250/month to heat and cool. A Passive House DADU in the same Seattle climate might cost $20–$50/month. Over 20 years, that difference funds a significant portion of the construction cost premium.
Passive House construction is not right for every project or budget. We'll tell you directly whether it makes financial sense for your goals during our consultation. We build to code minimum too — this isn't an upsell, it's an option worth understanding.
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FAQ: ADU vs DADU in Washington State {#faq}
Q: What's the difference between an ADU and a DADU in simple terms? A: An ADU (Accessory Dwelling Unit) is any secondary living space on a residential lot — attached or detached. A DADU (Detached Accessory Dwelling Unit) is the specific type that's a completely separate structure in your backyard, with no shared walls or roof with the main house.
Q: Can I build both an ADU and a DADU on the same lot in Washington? A: Yes, in most King County cities. Washington State legislation passed in 2023–2024 establishes two ADUs per lot as a baseline right on single-family lots in Growth Management Act jurisdictions. Local setbacks and lot coverage rules still apply. We confirm feasibility for your specific parcel during a site visit.
Q: How much does a DADU cost in King County in 2026? A: A new detached ADU in King County typically runs $175,000 to $550,000 depending on size, lot conditions, and finish level. A 600–800 square foot DADU — the most common size — lands between $230,000 and $380,000 including design, permits, construction, and utility connections.
Q: How long does it take to get an ADU permitted in King County? A: Permit timelines vary by city. In Kirkland and Bellevue, expect 3–6 months. In Seattle, 4–9 months depending on project type and current backlog. In SeaTac, typically 2–5 months. We manage the permit process end-to-end and know the current review timelines for each jurisdiction we work in.
Q: Do I need to live on the property to build or rent a DADU in Washington? A: No. Washington State legislation has eliminated owner-occupancy requirements for ADUs in most jurisdictions. You can build a DADU as a rental investment without living on-site. Confirm your specific parcel's rules during our feasibility consultation.
Q: What's the maximum size for a DADU in King County? A: It varies by city. In Seattle, detached ADUs can be up to 1,000 square feet (or 60% of the primary home's floor area). In Kirkland and Bellevue, the limit is typically 1,000–1,200 square feet depending on zone. SeaTac follows similar limits. We verify the specific rules for your parcel during feasibility.
Q: Which is better for rental income: ADU or DADU? A: DADUs typically generate more rental income because of the privacy and independence they offer tenants. A 700 sqft DADU in Kirkland or SeaTac might rent for $2,200–$2,800/month. A comparable attached ADU in the same neighborhood might fetch $1,600–$2,200/month. The gap varies by location, unit quality, and finish level.
Q: Does Love Construction build both ADUs and DADUs? A: Yes. We build attached ADUs (basement conversions, garage conversions, additions) and fully detached DADUs throughout King County. We're a design-build firm — design, permits, and construction are under one contract with one point of contact. See our location-specific guides for ADU builders in Kirkland, WA and ADU builders in SeaTac, WA.
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Ready to Figure Out What's Right for Your Property?
Whether you're leaning toward an attached ADU or a backyard DADU, the next step is a site visit. We'll review your lot, walk you through what's buildable, give you a realistic cost range, and tell you honestly which approach makes the most financial sense for your goals.
No obligation. No pressure. If your project isn't a fit for us, we'll tell you that too.
Call Aaron at (206) 604-5504 for a free ADU feasibility consultation.
Or schedule your consultation online — we serve SeaTac, Kirkland, Bellevue, Issaquah, Newcastle, and across King County.
Learn more about our ADU and DADU construction services and see completed projects from around the King County area.
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