Most Seattle homeowners think the DADU permit is a form. It is not. It is a document package — a site plan, a survey, a foundation design, a sewer certificate, and the drawings — that the Seattle Department of Construction and Inspections reviews line by line, then sends back for corrections, sometimes twice. I have watched the same flat, clean lot get permitted in nine weeks and its identical neighbor take nine months. The difference is almost never the lot. It is what was submitted, and which path was chosen.
This is the permit process as it actually runs in 2026, with the measured timelines from the City’s own records and the fee stack from the published schedules. No cheerleading, no shortcuts I would not hand a paying client.
- You submit through the Seattle Services Portal, and about 75% of the SDCI fee is due at application.
- Measured from SDCI records, the citywide median is 138 days from application to issuance since 2024; pre-approved plans run 62 days.
- SDCI’s 2026 Fee Summary worked example prices a 500 sq ft DADU at $3,453 for plan review plus the permit.
- The pre-approved path needs a standard plan, an NR lot, no critical areas, and under 750 sq ft of ground disturbance.
- Two ADUs per lot, no parking required, no owner-occupancy — but every site still gets reviewed.
The permit process, step by step
The permit is a single building permit issued by SDCI, but it moves through several gates, in this order. You are not "approved" once; you are cleared at each gate, and any one of them can send the file back.
- Pre-application prep. Before you touch the portal you need the boundary and topographic survey, the site plan with the parcel number and a pedestrian path from the right-of-way, the foundation design, and — if triggered — a Water, Drainage and Wastewater Availability Certificate (WAC) from Seattle Public Utilities. SPU says the WAC is required for most development projects and is typically issued in about 15 business days at no charge.
- Screening and intake. You create the record on the Seattle Services Portal, answer the intake questions, and upload the drawings. The City screens the submittal for completeness before it assigns a reviewer. An incomplete submittal bounces here, not weeks later.
- Plan review. The drawings go through the disciplines that apply — land use, building, energy, and often drainage. This is where a pre-approved plan genuinely helps, because the design has already been reviewed for code; the site has not.
- Corrections. The reviewer returns comments, you revise, and the file goes back. Two cycles is typical. Each cycle is calendar time you are not building.
- Issuance. Once the corrections clear, you pay the remaining ~25% of the fee and the permit is issued. Only then can the SPU side-sewer permit be issued — as of October 1, 2025, Seattle Public Utilities handles all sanitary and wastewater side-sewer permitting and cannot issue a side sewer permit for new construction until the related construction permit is issued.
Illustrative. Stage durations are planning estimates; the ≈20-week total matches the measured 138-day citywide median. Pre-approved timing is SDCI’s published claim alongside the 62-day measured median. Source: SDCI permit records since 2024; Seattle Services Portal.
Pre-approved or full custom is decided by your zone, your critical areas and your ground disturbance — not by the plan you like. The customized report reads those for your address and names the path.
How long does a Seattle DADU permit take?
A Seattle DADU permit takes 138 days from application to issuance — about 4.5 months — at the citywide median, or 62 days on a pre-approved standard plan. That is measured from the City’s own records: 1,062 detached accessory dwelling unit permits issued since 2024, read 2026-09-22.
The spread matters as much as the middle. Citywide the 25th–75th percentile range is 80 to 209 days; across the 146 pre-approved permits the median is 62 days with a 75th percentile of 104 days. SDCI publishes one timeline figure of its own, and it is the fast one: a pre-approved standard plan on an eligible site can be permitted “as quickly as 2–6 weeks for most sites.” That is the City’s claim rather than a measurement, and it is real — for a clean lot with a pre-approved plan.
Read that gap carefully, because it is where the money is. A pre-approved plan does not discount the SDCI fee or the sewer charge. What it buys is design time and calendar weeks — the drawings are already reviewed, so the plan-review cycle collapses. On a custom plan, those correction cycles are where the extra weeks live.
How much does a Seattle DADU permit cost?
A Seattle DADU permit costs $3,453 for plan review plus the building permit on SDCI’s 2026 worked example for a 500 sq ft DADU, with about 75% of it due at application. That is the building permit alone; trade permits, SPU side-sewer and drainage charges and the recurring King County sewer capacity charge are separate lines.
The permit fee is the visible number and not even the largest one. Here is the whole stack, in the order it shows up on your bill.
The SDCI building and plan-review fee. SDCI’s 2026 Fee Summary worked example prices a 500 sq ft detached ADU at $3,453 for plan review plus the permit, with about 75% due at application. A larger unit scales up from the same schedule — the 1,500 sq ft house-plus-garage anchor is $6,853, so a full 1,000 sq ft cottage lands between those two, around $4,400–$5,900. That interpolation is mine, not SDCI’s: it is an estimate to confirm at the fee estimator, and SDCI resets the schedule every January — the 2026 construction-permit fees went up 18% year over year.
Trade permits. Electrical, mechanical, and plumbing each carry their own small permit — planning hundreds, not thousands, but they add up, and they are separate from the building permit.
SPU side sewer and drainage. The side-sewer connection and the drainage system development charge are quoted per project by Seattle Public Utilities. Trenching from the cottage to the main runs on top of that, at a planning rate of $45–$110 per foot.
Recurring charges. The King County sewer capacity charge is $77.99 per residential customer equivalent per month for 15 years; a DADU is 0.59 RCE, so about $46 a month on top of the property-tax bill — disclosed separately because it is a carrying cost, not a one-time fee. The SPU drainage (stormwater) fee follows the parcel tier — $469.52 a year for a 2,000–3,499 sq ft lot, $976.13 for 6,500–9,999 sq ft.
The full line-by-line number — hard costs, soft costs, and these fees together — is in the Seattle DADU cost guide, built from the same constants.
The pre-approved shortcut, and the four gates
The pre-approved program is the single biggest lever on your timeline. Ten standard plans are published in the City’s ADUniverse gallery — all of them listed, with size, footprint, and construction system, on the pre-approved DADU floor plans page. To use the fast path, the project has to clear four gates:
- A pre-approved plan — the application includes the standard-plan number.
- A Neighborhood Residential lot — verify against current guidance, since the January 2026 code renamed the old single-family zones.
- No environmentally critical areas — any mapped ECA on the property removes the fast path and reverts to standard review.
- Under 750 sq ft of ground disturbance — a stricter threshold than the general stormwater trigger.
Pre-approved does not mean pre-cleared. The City pre-reviews the building package, not your lot. You still submit the site plan, the foundation, and the sewer paperwork, and the site still gets reviewed. The plan-fit report measures which of the ten plans actually fit your buildable rectangle — that is the check most people skip and then regret.
What actually slows a Seattle DADU permit down
Three things account for nearly every stalled file I have seen.
An incomplete submittal. A missing survey, a site plan without the parcel number, or a drawing that skips the pedestrian path from the right-of-way sends the file back at intake. That is the cheapest delay to avoid, and the most common.
Trees and critical areas nobody looked for. A Tier 2 tree near the footprint or a mapped steep-slope designation changes the review and often triggers an arborist or geotechnical report. The setback and coverage rules are the box the cottage has to fit in, but the tree and slope layers decide whether that box is even buildable.
The side sewer. As of October 1, 2025, SPU holds the side-sewer permit, and it cannot be issued until the building permit is issued. An old, undersized side sewer found late in the job is a budget reset, not a paperwork one — the scanned side-sewer card is public, and how to look up your house's permit and utility records shows where to find it before you design around it.
Nobody fails the DADU permit on the building. They fail it on the survey they skipped, the tree they did not look for, and the sewer they found out about in month four.
— Bob's takeFrequently asked questions
How long does a Seattle DADU permit take?
Measured from SDCI DADU permit records since 2024, the citywide median is 138 days from application to issuance, with a 25th-to-75th percentile range of 80 to 209 days. Pre-approved standard plans run a 62-day median. SDCI’s published “2–6 weeks for most sites” is the City’s claim, not a measurement.
How much does a Seattle DADU permit cost?
SDCI’s 2026 Fee Summary worked example prices a 500 sq ft DADU at $3,453 for plan review plus the permit, with about 75% due at application. Trade permits, SPU side-sewer and drainage charges, and the recurring King County sewer capacity charge (about $46 a month for 15 years) are separate.
What is the pre-approved DADU shortcut?
Seattle pre-approves the design of ten standard plans in its ADUniverse gallery, compressing plan review. The expedited path needs a pre-approved plan, a Neighborhood Residential lot, no environmentally critical areas, and under 750 sq ft of ground disturbance. The site still gets reviewed.
Do I need an architect to pull a Seattle DADU permit?
Not necessarily. A pre-approved plan plus a site plan and site-specific foundation design can be submitted without a custom architect, but you still need a boundary survey and utility paperwork, and a licensed professional for the engineering and design work.
Sources
- SDCI DADU permit records, measured since 2024 — 1,062 permits, median 138 days application to issue; 146 pre-approved permits, median 62 days — seattle.gov (Detached Accessory Dwelling Units layer); read 2026-09-22.
- SDCI 2026 Fee Summary worked examples, effective 2026-01-01 — 500 sq ft DADU $3,453 (2025: $2,908); 1,500 sq ft house + garage $6,853 — seattle.gov (SDCI 2026 Fee Summary, read 2026-09-23).
- Seattle Municipal Code § 23.42.022 (Accessory Dwelling Units) — library.municode.com.
- Seattle Municipal Code § 23.44.090 (setbacks) — library.municode.com.
- City of Seattle ADUniverse pre-approved DADU gallery — aduniverse-seattlecitygis.hub.arcgis.com.
- King County sewer capacity charge, 2026 rate ($77.99/RCE/month) — kingcounty.gov.
- Seattle Public Utilities drainage rates and side-sewer / WAC guidance — seattle.gov/utilities.
- RCW 36.70A.681 (state ADU law) — apps.leg.wa.gov.
Get the measured version of all of this for your address: the customized feasibility report pulls your parcel’s permit history, maps the critical areas and trees, and recommends the fastest permit path for your lot. Or start with the free can I build a DADU checklist before you spend on drawings.
Looking backwards rather than forwards — what was already permitted on your lot, including any scanned 1894–1970 microfilm permits SDCI holds for your address from before its digital record? That is a different pull: see every public record on your Seattle house, from the deeds and the tax roll to the tanks, the side-sewer cards and the historical aerials.
This article is educational and preliminary. It is not legal advice, architectural advice, engineering advice, an official cost estimate, or an official determination by the City of Seattle or King County. Fees, rates, and timelines change. Verify all figures against current SDCI, SPU, and King County schedules at the time of your application, and consult qualified professionals before making design, permitting, or construction decisions.