Quick answer: An ADU (Accessory Dwelling Unit) is a secondary living unit attached to or inside your existing house, like a basement or above-garage apartment. A DADU (Detached Accessory Dwelling Unit) is a separate, standalone structure in your yard, often called a backyard cottage. Both are legal across King County; the main differences are placement, cost, and size limits.
TL;DR: Choose an attached ADU when you have unused interior space and a tighter budget; choose a DADU when you want a private, standalone unit and have the lot for it. DADUs generally cost more but offer more independence and rental appeal. Both require permits, and 2026 King County rules have made approval easier than ever.
ADU vs DADU at a glance
| Feature | ADU (attached/interior) | DADU (detached) |
|---|---|---|
| Location | Inside or attached to the main house | Separate structure in the yard |
| Also called | In-law unit, basement apartment, MIL | Backyard cottage, granny flat |
| Typical cost | Lower (uses existing structure) | Higher (new foundation + utilities) |
| Privacy | Shares walls with main home | Fully independent |
| Rental appeal | Good | Strongest (standalone) |
| Build complexity | Lower | Higher |
| Best when | You have convertible interior space | You have yard space + want separation |
What is an ADU?
An attached or interior ADU is a complete, independent living unit, with its own kitchen, bathroom, and entrance, created within or attached to your primary home. Common examples are a converted basement, a unit above an attached garage, or a portion of the house walled off with a separate entry. Because it reuses existing structure, an attached ADU is usually the lower-cost path to a second unit. For a deeper primer, see our complete guide to ADUs.
Planning a project?
Wondering what an ADU on your lot would cost?
Send David the address and a few lines about what you want. The first feasibility check is free and usually takes about a day: what the site and the code are likely to allow, and what to check first.
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What is a DADU?
A DADU is a detached, standalone dwelling on the same lot as your main house, the classic “backyard cottage.” It has its own foundation, walls, roof, and utility connections, which makes it more expensive than an attached ADU but also fully private. DADUs are popular for rental income, multigenerational living, and home offices because they function as a separate residence.
Key differences that affect your decision
Cost:
Attached ADUs leverage existing structure and cost less; DADUs are new construction with their own foundation and utility runs, so they cost more per square foot.
Privacy & rental value:
A DADU’s separation commands stronger rent and suits tenants or extended family who want independence.
Lot requirements:
DADUs need available yard area and must meet setbacks and lot-coverage limits; attached ADUs are constrained instead by your home’s existing layout.
Permitting:
Both require permits, but a DADU’s site work (foundation, utilities, stormwater) typically adds steps and time.
King County rules in 2026
Washington’s recent middle-housing and ADU reforms have made it easier to add ADUs and DADUs across King County. Many jurisdictions have raised size allowances, reduced or eliminated off-street parking mandates near transit, and streamlined approvals. Rules still vary by city, Seattle, Kirkland, Bellevue, and Redmond each set their own size, height, and setback standards, so the specifics for your lot depend on your municipality and zone. See our city guides for Seattle DADU rules and Kirkland ADU rules, and always confirm current standards with your city before designing.
Permit timeline (typical)
A realistic King County ADU/DADU timeline runs several months from design to permit issuance. Schematic design and construction documents take a few weeks to a couple of months; municipal permit review commonly takes additional weeks to months depending on the city and completeness of the submittal. DADUs usually take longer than attached ADUs because of site and utility review. An experienced architect shortens this by submitting a complete, code-compliant package the first time.
Which one is right for you?
- Choose an attached ADU if you have convertible interior space (a basement or large garage), a tighter budget, and don’t need full separation.
- Choose a DADU if you have yard space, want maximum privacy and rental value, and can invest more for a standalone unit.
Still deciding? Our team designs both across the Eastside and Seattle. Explore our ADU design services or talk to an ADU architect at Piper Cole for a quick feasibility read on your lot.
FAQ
What is the difference between an ADU and a DADU?
An ADU is attached to or inside your existing home (like a basement or above-garage unit); a DADU is a separate, detached structure in your yard, often called a backyard cottage. Both are independent living units with their own kitchen, bath, and entrance.
Is a DADU more expensive than an ADU?
Generally yes. A DADU is new construction with its own foundation and utility connections, while an attached ADU reuses existing structure, making the attached option lower-cost in most cases.
Can I build a DADU in King County in 2026?
In most residential zones, yes. Washington’s middle-housing reforms have expanded ADU/DADU allowances and streamlined approvals, though size, height, and setback rules vary by city, confirm your parcel’s standards before designing.
Do ADUs and DADUs need permits in King County?
Yes. Both require building permits, and a DADU typically involves additional site, utility, and stormwater review, which can extend the timeline.
Can I rent out an ADU or DADU?
Yes, both can usually be rented in King County jurisdictions. DADUs tend to attract higher rent because they are fully independent units.
Sources consulted: Washington State middle-housing legislation (HB 1110/HB 1337) summaries; Seattle SDCI and King County ADU/DADU permitting pages; City of Kirkland and City of Bellevue ADU standards; Piper Cole Architects ADU/DADU project experience.
AADU vs. DADU: Side-by-Side Comparison for King County 2026
| Factor | AADU (Attached) | DADU (Detached) |
|---|---|---|
| Typical cost range | $80K, $350K | $260K, $480K |
| Lot size required | Any (uses existing footprint) | Varies by city (5,000 to 6,000 sqft typical) |
| Privacy from main house | Shared walls, lower | Fully separate, higher |
| Rental income potential | $1,400 to $2,200/month | $2,000 to $3,800/month |
| Permitting complexity | Moderate (addition permit) | Moderate (new building permit) |
| Allowed per lot (HB 1337) | Up to 2 AADUs | Up to 1 DADU (+ 1 AADU) |
| Best for | Basement conversion, garage, in-law | Rental income, multi-gen, resale value |
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:
