An attached or interior ADU is frequently the cheapest way to add a home, because the shell already exists. The work is in proving the converted space is legally habitable — and in the very common case where someone finished the space years ago without a permit. Here's what the city looks for, and how legalization actually works.
Making a conversion legally habitable
A finished basement is not the same as a permitted dwelling. Several life-safety items have to be demonstrated on the drawings before SDCI will approve it.
- An emergency-escape egress window for each sleeping room
- Minimum ceiling-height verification
- Fire separation from the primary dwelling
- A compliant separate entrance, light, and ventilation
Energy code is where conversions stall
Because you're legalizing conditioned space, the state energy code applies — insulation, air-sealing, and mechanical documentation. Handling this up front is the difference between a clean review and a frustrating round of corrections.
Legalizing work done without a permit
If there's already a unit down there, you're not stuck. Legalizing as-built work — especially before a sale or refinance — is one of the most common things we do. We document existing conditions and bring them through SDCI with the least demolition and disruption the code allows.
- As-built documentation of what exists
- Targeted corrections instead of a full teardown
- Clears title, refinance, and sale obstacles
Common questions
What's the difference between an attached ADU and a DADU?
An attached ADU is inside or attached to your existing house (basement, attic, addition); a DADU is a separate detached structure. Most Seattle lots can now have one of each.