Overview
An Attached Accessory Dwelling Unit (A-ADU) is a self-contained living space built within or physically connected to your existing home. This includes basement conversions, additions to the side or rear of your home, and second-story additions above a garage.
Attached ADUs are often the most cost-effective option because they share structural elements, utility connections, and sometimes HVAC systems with the primary residence. For homeowners on smaller lots where setbacks limit detached construction, an A-ADU may be the best path forward.
Seattle allows basement AADUs without separate lot coverage impact in most SF5000 zones, making conversions of existing below-grade space especially efficient. Attached ADUs are governed by SMC 23.44.025, and our permit specialist handles all submissions through Seattle DCI or King County DPER.
What's Included
- Basement, addition, or above-garage conversion options
- Shared utility connections reduce infrastructure costs
- Separate entrance with direct street access required
- Full kitchen, bathroom, and living space
- Sound insulation between units
- Fire separation per building code
- Permitting handled by our specialist
Need more space? A detached ADU offers maximum privacy and rental value on larger lots. Compare options in our ADU vs home addition guide.
See the full cost breakdown for King County, explore financing options, or visit our pricing page.
Attached ADUs are ideal for aging parents and multigenerational living. Review our construction timeline to plan your project.
Great neighborhoods for attached ADUs: Fremont, Madrona, Fremont, and Mount Baker.
Frequently Asked Questions
How much does an attached ADU cost in Seattle?
Attached ADUs in Seattle typically range from $300,000 to $500,000+ depending on whether you're converting existing space (cheaper) or building an addition (more expensive). Basement conversions tend to be on the lower end since the shell already exists. Seattle permit and impact fees typically add $20,000–$50,000 to the total project cost.
What basement requirements apply in Seattle?
Seattle requires a minimum 7-foot ceiling height for habitable spaces, egress windows meeting IRC specifications, and a separate entrance with direct street access. Basement conversions are classified as attached ADUs (AADUs) under SMC 23.44.025. A building permit through Seattle DCI is required, covering change of occupancy, structural, plumbing, electrical, and egress work.
Can I still build a detached ADU if I have an attached ADU?
Yes. Under HB 1337 and Seattle's ordinance, you can have up to two ADUs per lot in any configuration — two attached, two detached, or one of each. This means a basement conversion plus a backyard cottage is fully permitted.
What are the sound requirements between units?
Building code requires STC (Sound Transmission Class) ratings of 50 or higher between dwelling units. This means proper insulation, double drywall, and resilient channel installations in shared walls and floors.
