If you're thinking about adding an accessory dwelling unit to your property in the Seattle or Tacoma area, one of the first decisions you'll face is whether to build an attached ADU (AADU) or a detached accessory dwelling unit (DADU). Both options can add living space, generate rental income, and increase property value, but they differ significantly in cost, design flexibility, and permitting requirements.
What's the Difference?
An attached ADU is built within or connected to your existing home. Common examples include basement conversions, garage conversions, or additions built onto the side or back of the house. Because they share structural elements with the main dwelling, they tend to be less expensive and faster to build.
A detached ADU (DADU), sometimes called a backyard cottage, is a standalone structure built separately from your primary home. DADUs are fully independent units with their own entrance, kitchen, bathroom, and living space. They offer more privacy for both the homeowner and the tenant, but they cost more to build because they require their own foundation, walls, roof, and utility connections.
Cost Comparison
In the Puget Sound region, here's what you can typically expect to spend:
- Attached ADU (AADU): $150,000 to $250,000. Basement conversions tend to fall on the lower end, while bump-out additions with new foundations cost more.
- Detached ADU (DADU): $200,000 to $400,000. The wide range reflects differences in size (from 400 to 1,000 square feet), finish level, and site conditions like slope, access, and utility distance.
The biggest cost drivers for DADUs are the separate foundation, independent utility connections (sewer, water, and electrical), and the fact that every wall is an exterior wall requiring full insulation and weatherproofing. Attached ADUs save on these elements by leveraging the existing structure.
Permitting Differences
Both types of ADU require building permits from your local jurisdiction. In Seattle, the permitting process for AADUs and DADUs is largely similar, but there are some differences worth knowing:
- Setback requirements differ. DADUs must typically be set back at least 5 feet from rear and side property lines. AADUs follow the setback rules of the primary structure.
- Height limits for DADUs in Seattle are generally capped at 22 to 25 feet, depending on the zone and lot configuration.
- Owner occupancy requirements were eliminated statewide by HB 1337, signed into law in 2023. You no longer need to live on-site to rent out either type of ADU.
- Design review may be triggered for DADUs in certain neighborhoods or if the structure exceeds specific size thresholds.
Pros and Cons
Attached ADU Pros
- Lower construction cost
- Faster build timeline (typically 4 to 8 months)
- Easier utility connections
- Good option for aging parents who want close proximity
Attached ADU Cons
- Less privacy for both parties
- Shared walls mean shared noise
- Design constrained by existing structure
- May require significant renovation of existing space
Detached ADU Pros
- Complete privacy and separation
- Full design freedom for layout and style
- Stronger rental appeal and higher potential rent
- No disruption to your main home during construction
Detached ADU Cons
- Higher cost ($200K+ in most cases)
- Longer build time (6 to 14 months)
- Requires adequate yard space
- Separate utility connections add complexity and expense
Which One Suits Your Property?
If your lot is on the smaller side (under 4,000 square feet), an attached ADU may be your best option, especially if you have an underused basement or garage. Properties with larger yards, rear-alley access, or flat topography are strong candidates for a DADU.
Consider your goals too. If you want to house a family member who values proximity, an AADU makes sense. If you're building primarily for rental income, a DADU's privacy and independence will typically command higher rents and attract longer-term tenants.
For Seattle and Tacoma homeowners, both options are now easier to build than ever thanks to recent state legislation that removed owner-occupancy requirements and streamlined permitting. The best choice depends on your property, your budget, and how you plan to use the space.