Accessory Dwelling Units have become one of the hottest topics in Seattle real estate. With rising housing costs and new city regulations making it easier than ever to build, thousands of Seattle homeowners are exploring whether an ADU makes sense for their property. If you are one of them, this guide covers everything you need to know about ADU regulations Seattle has in place for 2026
What Is an ADU?
An ADU, or Accessory Dwelling Unit, is a secondary living space on a single-family residential lot. In Seattle, there are two main types:
AADU (Attached ADU):
A unit built within or attached to the existing primary house. This could be a basement conversion, an attic conversion, or an addition to the main structure.
DADU (Detached ADU):
A separate structure built on the same lot, commonly known as a backyard cottage. This is a standalone building with its own entrance, kitchen, bathroom, and living space.
Under current ADU regulations Seattle allows, most single-family lots can have both an AADU and a DADU, meaning up to three dwelling units on one property
Seattle ADU Size Limits
Seattle has specific size limitations for ADUs that depend on the type and your lot size:
| ADU Type | Maximum Size |
| Attached ADU (AADU) | 1,000 sq ft or 40% of primary dwelling (whichever is less) |
| Detached ADU (DADU) on lots under 5,000 sq ft | 800 sq ft |
| Detached ADU (DADU) on lots 5,000+ sq ft | 1,000 sq ft |
| Maximum height for DADU | Generally 12-18 feet depending on roof type and zone |
These limits can change, so always verify current regulations with the Seattle Department of Construction and Inspections before designing your project.
Setbacks and Lot Coverage
Setbacks define how far your ADU must be from property lines. For detached ADUs in Seattle, the typical rear and side setbacks are 5 feet, though this can vary by zone. The total lot coverage, including your primary house and the ADU, cannot exceed the maximum allowed for your zone, which is typically 35 percent in single-family residential zones.
Your lot must also maintain adequate access for emergency vehicles and provide any required parking, although Seattle has eliminated the off-street parking requirement for ADUs in many areas, especially those near frequent transit service.
Permits and the Approval Process
Building an ADU in Seattle requires a building permit. The process typically involves these steps:
- Pre-application research:
Check your zoning, lot size, and existing lot coverage to confirm your property qualifies for an ADU.
- Design and plans:
Work with an architect or designer to create construction drawings that meet Seattle building code requirements.
- Permit application:
Submit your plans to SDCI for review. The review timeline varies but typically takes 4 to 12 weeks depending on the complexity of your project and current backlog.
- Construction:
Once approved, your contractor builds the ADU. A typical DADU takes 4 to 8 months to construct depending on size and complexity.
- Final inspection:
SDCI inspects the completed ADU before it can be occupied.
How Much Does It Cost to Build an ADU in Seattle?
ADU construction costs in Seattle vary widely based on size, design, and site conditions:
| ADU Type | Low Range | High Range |
| Attached ADU (basement/attic conversion) | $80,000 | $200,000 |
| Attached ADU (addition to house) | $120,000 | $280,000 |
| Detached ADU (backyard cottage, 400-600 sq ft) | $180,000 | $300,000 |
| Detached ADU (backyard cottage, 600-1,000 sq ft) | $250,000 | $450,000+ |
| Permits, design, and engineering | $15,000 | $40,000 |
These costs include design, permits, site preparation, construction, and all finishes. The cost per square foot for a new DADU in Seattle typically ranges from $350 to $500 depending on the quality of finishes and complexity of the build.
Is an ADU a Good Investment?
For many Seattle homeowners, the answer is yes. An ADU can generate rental income of $1,500 to $3,000 per month depending on location, size, and finishes. Over time, that rental income can offset or exceed the cost of construction.
ADUs also increase property value. A well-built DADU with a separate entrance, full kitchen, and modern finishes adds significant appraised value to your property. Many homeowners find that the increase in property value alone justifies the construction cost, with the rental income as a bonus.
Build Your ADU with Bad Boyz
Bad Boyz GC has experience building ADUs and backyard cottages across Seattle. Our team handles every phase of the project in-house, from site preparation and foundation work to framing, drywall, painting, and final finishes. We help homeowners navigate the permitting process and deliver quality ADUs on time and on budget.
Call for a free ADU consultation: (206) 434-2893
Email: joe@badboyzseattle.com
Licensed Washington State Contractor #BADBOBG831RA