Seattle has become one of the most ADU-friendly cities in Washington state, but the permitting process still involves multiple steps, specific requirements, and real costs. If you're planning to build an ADU or DADU in Seattle, here's a clear breakdown of what to expect.
Zoning: Where Can You Build?
ADUs are permitted in all single-family residential zones in Seattle, which covers the vast majority of the city's residential land. Under current regulations, properties zoned as Single Family (SF 5000, SF 7200, SF 9600) and Residential Small Lot (RSL) are eligible for both attached ADUs and detached ADUs.
Thanks to Washington's HB 1337, passed in 2023, all cities planning under the Growth Management Act must allow at least two ADUs per single-family lot. Seattle was already ahead of this requirement, but the state law provides an additional layer of legal backing for homeowners.
There is no minimum lot size required to build an ADU in Seattle. However, practical considerations like setbacks, lot coverage limits, and available space will determine whether your specific property can accommodate a new unit.
Setback Requirements
Setbacks determine how close your ADU can be to property lines and other structures:
- Detached ADU (DADU): Must be at least 5 feet from the rear property line and 5 feet from side property lines. If the DADU is located in the rear yard, it can be as close as 5 feet to the rear lot line regardless of whether there's an alley.
- Attached ADU: Follows the same setback rules as the primary dwelling, which is typically 5 feet from side lot lines and 20 feet from the front lot line in most SF zones.
- Distance between structures: A DADU does not need to maintain a minimum distance from the primary house, but building codes may require fire-rated walls if structures are very close together.
Height Limits
Seattle's height limits for DADUs depend on the roof type and zone:
- Flat roof: Up to 18 feet in most SF zones
- Pitched roof (6:12 or steeper): Up to 22 feet
- Lots with alley access: May allow up to 25 feet in some configurations
These height limits allow for two-story DADUs in many cases, which significantly increases design flexibility and the livable square footage you can fit on a small footprint.
Size Limits
Seattle allows ADUs up to 1,000 square feet of gross floor area. This applies to both attached and detached units. The 1,000-square-foot cap does not count garages, covered porches under 60 square feet, or unconditioned storage areas.
For attached ADUs, the unit cannot exceed 40 percent of the total floor area of the primary dwelling. So if your home is 2,000 square feet, the attached ADU can be up to 800 square feet (subject to the 1,000-square-foot overall cap).
Owner Occupancy: No Longer Required
This is one of the biggest recent changes. Washington's HB 1337, effective since 2023, prohibits cities from requiring owner occupancy as a condition for ADU permits. Previously, Seattle required that the property owner live in either the main house or the ADU. That requirement is now gone.
This means you can build an ADU and rent out both the main house and the ADU, making ADUs a more viable investment property strategy. It also means you can build an ADU before you move and rent both units until you're ready to occupy one.
The Permit Timeline
The permitting timeline in Seattle typically runs 3 to 8 months from application to approval, depending on the complexity of your project and the city's current workload. Here's the general process:
- Pre-application conference (optional but recommended): Meet with the Seattle Department of Construction and Inspections (SDCI) to discuss your project and identify potential issues early. This can save months of back-and-forth later.
- Submit application: File your building permit application with complete construction documents, site plan, and engineering reports if required.
- Plan review: SDCI reviews your plans for code compliance. Expect 4 to 12 weeks for initial review. You'll likely receive correction letters that require revisions.
- Corrections and resubmittal: Address reviewer comments and resubmit. This cycle can happen two or three times.
- Permit issuance: Once plans are approved, you pay remaining fees and receive your building permit.
- Inspections during construction: Multiple inspections at key stages (foundation, framing, electrical, plumbing, final).
- Certificate of occupancy: After the final inspection passes, you receive your CO and the unit is legally habitable.
Permit Costs
Expect to budget $5,000 to $15,000 for all permit-related fees in Seattle. This includes the building permit fee (calculated as a percentage of construction value), plan review fees, mechanical and plumbing permits, and sewer capacity charges. Seattle has waived certain development fees for ADUs, including transportation and parks impact fees, which helps keep costs down.
Pre-Approved Plans
Seattle offers a pre-approved DADU plan program through SDCI. If you choose a pre-approved design, the plan review process is significantly shorter because the city has already reviewed the structural, mechanical, and plumbing details. You still need a site-specific review for setbacks, utilities, and site conditions, but the overall timeline can be cut by several weeks.
Pre-approved plans are available from several local architects and typically cost between $2,000 and $8,000 to license. This is a smart option for homeowners who want to minimize design costs and speed up the permitting process.
Understanding these requirements upfront will help you plan your timeline, set a realistic budget, and avoid surprises during the process. If you're unsure about your property's specific situation, most ADU builders in the Seattle area offer free consultations and can help you navigate the permitting landscape.