Seattle ADU Permit Timeline in 2026: How Long SDCI Review Actually Takes

• The city publishes its own numbers. Since October 1, 2025 SDCI has had permit-issuance goals by project category and reports current performance at the 75th percentile (SDCI, Construction Permit Performance).
• Pre-approved DADU plans: goal 30 days, current 60 days in city control.
• Middle housing (the category a new custom detached unit falls closest to): goal 60 days, current 117 days.
• Single-family addition or alteration — the path a garage conversion or an attached unit usually takes: goal 30 days, current 64 days.
• "Days in city control" is not your calendar. SDCI's own guidance: the total calendar days an applicant experiences is roughly twice the days in city control, because the clock stops while corrections sit with your team.
• Pre-approved eligibility is narrow: NR-1, NR-2 or NR-3 zoning only, no environmentally critical areas on the property, and under 750 sq ft of total ground disturbance including driveways.
• The goals are new, not achieved. The city created the PACT team to close the gap between the 30/60-day goals and the 60/117-day reality; treat the goals as direction, not as a schedule you can bank.

For years the honest answer to "how long does an ADU permit take in Seattle" was a shrug and a range. That changed when SDCI began publishing performance against fixed goals. The table is short and it is the single most useful document a homeowner can read before starting: five project categories, a goal in calendar days in city control, and the current 75th-percentile result beside it.

Read it carefully, because the two columns measure different things than most people assume. Days in city control count only the days the city is holding your application — reviewing, routing, waiting on another department. The moment a correction notice goes out, the clock pauses until you resubmit. That is fair to the reviewers and misleading to homeowners, which is why SDCI adds the caveat itself: the total calendar time you will live through is about double.

So the arithmetic for 2026 is this. A pre-approved DADU: 60 days in city control, roughly four months end to end. A custom detached unit reviewed under the middle-housing category: 117 days in city control, roughly seven to eight months. A garage conversion or attached unit on the addition/alteration path: 64 days, roughly four months. Those are 75th-percentile figures, meaning three projects in four finish inside them — not averages, and not promises.

Seattle's pre-approved standard plans exist to skip the design review argument entirely: the structure has already been approved, so the city is checking your site, not your building. It halves the published review time. It also carries three eligibility gates that disqualify a lot of real parcels.

Your lot must be zoned NR-1, NR-2 or NR-3 — no other zone qualifies. The property must have no environmentally critical areas, which in Seattle quietly removes steep slopes, liquefaction-prone soils, wetland buffers and much of the ravine edge. And total ground disturbance, structure plus driveway plus hard surface, must stay under 750 sq ft. That last one is the sleeper: a 700 sq ft cottage with a new paved path and any parking apron blows through it, and the project falls back to standard review.

The application mechanics matter more than they should. The city's own instructions tell applicants to select Add/Alt as the action type, to answer yes to the standard-plan and DADU questions and no to everything else in that section — including "I'm constructing a new building." Answer that one wrong and the application picks up preliminary steps it never needed, which is a self-inflicted month. If a pre-approved plan fits your lot, our guide to Seattle's pre-approved DADU plans walks the gallery and the submittal set.

Almost nobody loses time to reviewers reading drawings. Time is lost in four predictable places, and three of them are on your side of the table.

Before intake. Survey, geotech if the slope demands it, side sewer capacity, utility availability, the drainage review triggered by hard surface. None of it is in SDCI's numbers because the application does not exist yet. Six to ten weeks is normal, and it is the part a homeowner can compress most by starting it early.

Correction cycles. Each round of comments stops the city clock and starts yours. A clean, complete first submittal that answers the obvious questions — pedestrian path from the right-of-way, ground-disturbance total, sewer capacity charge form — is worth more calendar time than any expediting service.

Other departments. SDCI routes to Seattle Public Utilities, SDOT and sometimes Seattle City Light. A power-service upgrade or a side-sewer conflict is the classic reason a straightforward project sits. Our breakdown of site costs — sewer, power and access covers what those reviews are looking for.

After issuance. The permit is not the finish line; inspections are scheduled against a build sequence. Add the construction window on top: a detached unit typically runs 6 to 9 months of build time after issuance, which is how a project ends up at the 10–16 months that we quote from first call to keys.

Pre-approved DADU, clean NR-2 lot, no critical areas. 4–6 weeks of pre-application work, ~60 days in city control, one correction cycle, permit in hand around month 4. Build 6–8 months. Occupied in roughly 11 months.

Custom detached ADU, standard lot. 8–10 weeks of design and consultants, ~117 days in city control, two correction cycles, permit around month 7 or 8. Build 7–9 months. Occupied in roughly 15 months.

Garage conversion or attached unit. 4 weeks of documentation, ~64 days in city control, permit around month 4. Build 4–6 months. Occupied in roughly 9 months. The comparison in garage conversion vs DADU covers whether the faster path is the better one.

If your parcel is outside Seattle, the numbers above do not transfer — each jurisdiction runs its own queue. Pierce County homeowners should start with Tacoma & Pierce County ADU rules, which covers the envelope rather than the queue.

Four things move the needle, and none of them involve paying for speed.

Test pre-approved eligibility on day one. Zone, critical-area map, ground-disturbance total. It is a thirty-minute check that decides between a four-month and an eight-month permit.

Buy the survey and the geotech before you draw. Every design decision made without them is a decision that may be redrawn during corrections.

Resolve utilities early. Side sewer capacity, water meter size and electrical service are the reviews that hold projects. Ask the questions while the design is still soft.

Respond to corrections in days, not weeks. Your response time is the largest single variable in total calendar days, and it is entirely yours to control.