Design, permit, build — honestly.
The most common regret on the Seattle ADU forums is 'it took twice as long as I was told.' So here's the real sequence, the real timeline, and the one hidden step — the power hookup — that stalls otherwise-finished projects.
Building an ADU is three jobs stacked on top of each other: design it, get it permitted, and build it. Each has its own clock, and the permitting clock is the one homeowners underestimate most. Here’s the whole sequence, with the honest durations.
The steps
- Feasibility & site check. Before any design, we confirm what your lot can take: zoning, setbacks, lot coverage, critical areas (steep slope, wetland overlay), sewer lateral, water and electrical capacity, and protected trees. This is where sewer-pump-station surprises and tree-ordinance conflicts get caught — not mid-build.
- Design + engineering.Drawings to code. For a custom unit that means an architect; for a pre-approved DADU it means selecting a plan from Seattle’s catalog and adapting it to your site. The path you pick here decides your permit timeline.
- Permit package & submission.We assemble the full package and submit it to Seattle SDCI or, in unincorporated areas, King County DPER. This is the “permitting-as-a-service” part — you don’t navigate the city portal or the corrections cycle.
- Permit review & corrections. Reviewers come back with corrections; we resolve them and keep the package moving until the permit issues. A rejected or incomplete application resets the review clock by weeks, which is exactly why the package goes in complete.
- Build. Foundation through finishes — framing, roof, mechanical, electrical, plumbing, interior. Inspections happen along the way.
- Utility connection & power hookup.Sewer and water tie-ins, then the electrical service. In Seattle this is the hidden step: Seattle City Light’s power connection can take up to 6 monthsand can stall a unit that’s otherwise done.
- Final inspection & sign-off. Then you have a legal, connected, rentable unit.
The real timeline
Two numbers, depending on which design path you took:
| Path | Permit | Total, plan to move-in |
|---|---|---|
| Pre-approved DADU | 2–6 weeks | 7–12 months |
| Custom DADU | 4–8 months | 9–18 months |
| Garage / basement conversion | 1–5 months | 5–10 months |
Permitting-as-a-service
The fear of navigating SDCI’s multi-phase review — or King County DPER’s, which is a different jurisdiction with different fees and no pre-approved-plan program — is one of the most consistent reasons homeowners stall. So we handle it end-to-end: package assembly, submission, and the full corrections cycle until the permit issues. You don’t learn the portal, and you don’t lose weeks to a rejected application that resets the clock.
We also offer permitting and feasibility as standalone work — if you’re an owner-builder, or building with another GC, we can run just the documents and the city process. Either way, the permit and plans are yours; we don’t hold them.
Want the full breakdown of each phase, jurisdiction by jurisdiction? Read the Seattle ADU timeline guide → · Or start with a feasibility check →
Plan for 9-18 months from start to move-in for a custom DADU, and 7-12 months if you use a pre-approved plan. Permitting is 2-6 weeks for a pre-approved plan but 4-8 months for a custom one, construction adds several months, and Seattle City Light's power hookup can add up to 6 more. The most common regret is underestimating this.
