The honest Seattle ADU timeline, month by month
The most-repeated regret in every Seattle ADU forum is some version of “it took twice as long.” It won't, if you know the real steps going in. Plan for 9–18 months for a custom DADU — and here's the reality, laid out month by month, with the hidden steps named.
The fear behind it is specific: “My life will be on hold for 18 months and I don’t know when it ends.” A homeowner on Reddit summed up the regret: “Anticipate that it will take twice as long and cost more (sometimes much more) than planned.”The reason timelines blow up isn’t that builders are slow — it’s that the hidden steps (permit corrections, Seattle City Light’s power-hookup queue, sewer capacity charges) aren’t in the brochure. If you can use a pre-approved plan, the front end gets much shorter — but one delay stays fixed no matter what you do.
The short version
| Phase | Custom DADU | Pre-approved plan |
|---|---|---|
| Design + drawings | 1–3 months | Days (plan already drawn) |
| Permit review (SDCI) | 4–8 months | 2–6 weeks |
| Construction | 3–9 months | 3–9 months |
| Seattle City Light power hookup | Up to 6 months (often runs alongside construction) | Same |
| Realistic total | 9–18 months | 7–12 months (best case) |
The pre-approved path saves you on the permit— Seattle’s own figures are 2–6 weeks vs. 4–8 months. It does not save you on Seattle City Light or construction. A West Seattle homeowner summarized the whole arc in April 2026:
“We submitted in November and our permit just got final approval this week. So 5 months for just the permit. Then it will probably be at least 6 months for construction because Seattle City Light’s current minimum wait time for power hookups is six months.” — a Seattle homeowner on Reddit (r/WestSeattleWA)
Month by month (custom DADU)
Months 0–1: Feasibility + site evaluation
Before any drawing, the ground gets checked: zoning, lot coverage, setbacks, ECA (Environmentally Critical Area) overlays, slope, trees, and where the sewer and water actually connect. This is the step that prevents the worst surprises later. Skip it and you risk the “I’ll spend $5K on plans and then find out I can’t build”outcome. Start your Seattle City Light service application as early as possible here — the clock on that one is long.
Months 1–3: Design + construction documents
Floor plans, elevations, structural engineering, and energy-code compliance (Washington’s energy code is stringent — it drives both cost and the level of detail in drawings). With a pre-approved plan, you collapse most of this to days because the design is already drawn and pre-vetted for city approval. Custom design buys you a unit optimized for a tricky lot; it costs you months.
Months 3–8: Permit review (SDCI or King County DPER)
This is the longest stretch and the one homeowners most underestimate.
- Custom DADU, Seattle SDCI:4–8 months typical. The West Seattle homeowner above hit 5 months.
- Pre-approved plan, SDCI:2–6 weeks.
- Unincorporated King County, DPER:roughly 8–14 weeks (different jurisdiction, different queue).
The hidden step inside this phase: correction cycles.Every time the city returns comments and the plans are revised, the review clock effectively restarts — application rejections can reset the timeline by weeks each time. A builder with real local permit history submits complete the first time, which is the single biggest lever on this phase.
Months 6–12+: Seattle City Light power hookup (the one delay you can’t out-plan)
Months 8–17: Construction
Typically 3–9 months after permit, depending on size, finishes, weather, and subcontractor scheduling (trades in Seattle book months out). Foundation, framing, roof, the full mechanical/electrical/plumbing rough-in, insulation, drywall, finishes, then inspections.
Hidden cost-and-time step inside construction: sewer capacity charges and utility tie-ins.Seattle reports every ADU permit to King County for sewer treatment capacity charges, and the physical sewer/water connections happen here. Utility surprises in the $15K–$40K range are the most-cited mid-build shock — schedule and budget for them as a known step, not a surprise.
Final weeks: Inspections + certificate of occupancy
Final inspections, power energized (if SCL has caught up), and the certificate of occupancy. Only now can someone move in or a tenant sign a lease.
Why people say “twice as long”
It’s almost always one of these four hidden steps stacking up:
- Permit correction cycles that quietly restart the review clock.
- Seattle City Light’s ~6-month hookup queue, discovered too late to run in parallel.
- Sewer capacity charges and utility tie-ins treated as a surprise instead of a scheduled phase.
- Subcontractor scheduling— trades booked months out.
None of these are mysterious once they’re named. Plan for all four and your timeline holds.
How to shorten it honestly
- Use a pre-approved plan if speed matters more than customization.It cuts permit review from 4–8 months to 2–6 weeks. A growing share of Seattle DADU permits use this path; most don’t, and wait.
- File your Seattle City Light application during feasibility.It’s the longest fixed delay. Started early, it overlaps construction instead of extending it.
- Hire a builder with verifiable local permit history. Complete first submissions avoid the correction cycles that reset the clock.
- Get the site evaluation done before design. Catching slope, ECA, sewer, and tree issues up front prevents the redesigns that add months.
- Budget the utility connections as a known phase, not a surprise — so they don’t stall the schedule.
Plan for 9–18 months for a custom DADU, or roughly 7–12 months at best with a pre-approved plan. The breakdown: 1–3 months design, 4–8 months permit review (2–6 weeks pre-approved), 3–9 months construction, plus Seattle City Light's ~6-month power-hookup wait that ideally runs in parallel.
