Ask a Seattle homeowner what a pre-approved DADU plan buys them and you will hear one number: two to six weeks. That is the figure SDCI publishes, and it is accurate — for an eligible site. What almost nobody hears is the number on the other side of the comparison, because the City does not publish a single timeline for a custom DADU. So the "versus" question — pre-approved vs custom — gets answered with the fast number alone, and that is how homeowners overpay for the wrong expectation.
Here is the real comparison, from a builder who watched both paths dozens of times. The savings are real. They just are not where most people think they are.
The pre-approved plan saves you design dollars and calendar weeks. It does not discount the SDCI fee, the sewer charge, or the tax bill. Measure the gap you are actually buying.
The timeline gap, measured
SDCI's published figure is that a pre-approved standard plan can be permitted as quickly as 2 to 6 weeks for most sites. The comparison nobody quotes is the custom side, because it varies by corrections, consultants, and site complexity. The City's own permit records close the gap: measured across DADU permits issued since 2024, the median application-to-issue time is 138 days citywide, while the median for pre-approved plans is 62 days. Roughly 20 weeks against 9.
Application to permit issue, in weeks
The published 2–6 week window is the plan; the measured medians are the reality.
Published 2–6 weeks per SDCI. Measured medians from SDCI DADU permit records since 2024: 62 days pre-approved, 138 days citywide. Custom timelines vary by corrections and site complexity.
The extra weeks on the custom side are not wasted — they are design, corrections, and review cycles. But they are calendar weeks you carry while a rental you could be collecting goes unbuilt, and they are the thing the pre-approved path is genuinely selling. For the full custom timeline breakdown, see our line-by-line Seattle DADU cost guide.
The plan-fit report tests every published pre-approved DADU plan against your measured buildable rectangle, in both orientations, and names the eligibility gates that would remove the fast path entirely.
The permit fee is the same schedule; the design fee is not
This is the part most people get backwards. The pre-approved path does not discount the SDCI permit fee. SDCI anchors its fee schedule with a published worked example: on the 2026 schedule, effective 2026-01-01, a 500-square-foot detached ADU costs $3,453 for plan review plus the permit, with 75 percent due at application and the rest before issue. That figure is on the same schedule whether your plan came from the gallery or from a custom architect — and it moves every January, so check the year on any number you are quoted.
What changes is the design money. A pre-approved plan licenses from its designer for up to $1,000, per the ADUniverse gallery. A custom design, drawn to your lot by an architect, commonly runs $8,000–$15,000 before engineering — a builder-reported range that varies by firm and site. That is where the cost savings live: not in the permit, in the drawings.
Design and permit dollars, side by side
The savings are in design; the SDCI fee is the same schedule on both paths.
License fee up to $1,000 per the ADUniverse gallery. Custom design is a builder-reported range and varies by firm. SDCI fee is the 2026 Fee Summary worked example for a 500 sq ft DADU (effective 2026-01-01); a full 1,000 sq ft scales higher, around $4,400–$5,900. All dollars are estimates to confirm at application.
The recurring costs that do not change
Once the cottage exists, the plan you chose is invisible to the billers. Three recurring charges attach to the unit and the lot, and they are identical for pre-approved and custom alike:
- King County sewer capacity charge — $77.99 per residential customer equivalent per month for 15 years, and a DADU is 0.59 RCE. That is $46.01 a month, about $552 a year, roughly $8,283 over the full term. You can prepay the balance at a discount.
- SPU drainage fee — a flat per-parcel charge billed on the property-tax statement: $469.52 a year for a 2,000-to-3,499-square-foot parcel, $976.13 for 6,500 to 9,999 square feet in 2026. Adding a DADU under 10,000 square feet does not change the tier.
- Property tax — the DADU's value lands on the next assessment roll at Seattle's levy code 0010 rate of $9.90845 per $1,000 (about 0.99 percent). A cottage that adds $450,000 of value adds roughly $4,460 a year.
None of these come with a "custom" surcharge or a "pre-approved" discount. They are the cost of the unit, not the cost of the drawing.
Add the three up on a mid-city parcel and a $450,000 cottage and the recurring load lands near $5,480 a year before maintenance: $552 for the capacity charge, $469.52 for a mid-tier drainage parcel, and about $4,460 in added property tax. The number moves with your lot and your assessment, not with which plan you drew.
What the pre-approved plan does not include
Do not read the $1,000 license as "everything but the permit." The pre-approved set is the drawings. What it does not include is the site work that makes those drawings buildable on your lot: the boundary and topographic survey, the foundation design to your soils, the arborist report when a tree sits in the way, the side-sewer and drainage routing, and the electrical service to the cottage. That work is identical whether the plan came out of the gallery or off an architect's desk, and it is where most of the pre-construction dollars actually sit. The plan is the cheap part. The site is the expensive part.
So when someone quotes you the 2-to-6-week number, ask which clock it is on. It is SDCI's review window once a complete application lands. It is not the survey, the WAC, the side-sewer permit, or the correction cycles on the site plan — those run on your calendar and the same people's calendars no matter which plan you picked.
When the pre-approved path does not apply
The comparison only matters if both paths are open to you. The pre-approved route closes the moment any of these is true on your lot: an environmentally critical area on the property, a historic or shoreline overlay, a landmark designation, a non-Neighborhood-Residential zone, or total ground disturbance over 750 square feet. Under SMC 23.42.022 and the current SDCI guidance, those conditions also remove the expedited standard-plan path. On those lots the decision is made for you — it is custom or nothing — and the only question is whether the site can support a cottage at all.
That is the point where the "versus" question stops being about plans and starts being about the lot. The customized feasibility report measures those conditions for your address and tells you which path is even available before you spend on either a license or a custom design.
Those conditions cluster by neighborhood. Ballard's alley grid shortens utility runs and opens the 0-foot rear setback; Queen Anne's slopes force geotech review onto many lots. The neighborhood profiles show the measured patterns behind both, so you know before the first site visit whether your block skews fast path or custom.
How to know if your lot qualifies before you spend
You can resolve the whole pre-approved vs custom decision in about a day of measurement. Confirm the zone and the overlays, check for any mapped ECA, and measure the buildable rectangle and the ground-disturbance footprint. The Plan Fit Report does that work for your address and returns which of the ADUniverse plans fit — and, by extension, whether the fast path is open to you at all. Run that screen before you pay for either path: a $1,000 plan license you cannot use costs money and buys nothing, and a custom design fee on the wrong lot is worse.
Pre-Approved DADU Plan Fit Report
Find out which pre-approved plans fit your lot and whether the fast path is open to you — measured, not guessed. A plan-by-plan fit matrix plus the site flags that force the custom route.
For the program rules and the full catalog, read our guide to Seattle pre-approved standard plans and the companion on which of the plans actually fits your lot. If you would rather screen the basics yourself first, the free homeowner checklist covers the same eligibility questions, and the Pro Guide turns the full review into a scoring template.
Frequently asked questions
How much faster is a pre-approved DADU permit than a custom one?
SDCI says a pre-approved standard plan on an eligible site can be permitted in about 2 to 6 weeks. Measured from the City's own permit records, pre-approved DADU permits have a median of about 62 days from application to issue, against a citywide custom DADU median of about 138 days.
Is a pre-approved DADU plan cheaper than a custom design?
The design side is. A pre-approved plan licenses for up to $1,000, while a custom design commonly runs $8,000–$15,000 before engineering. The SDCI permit fee is on the same schedule either way — SDCI's 2026 Fee Summary worked example is $3,453 for a 500-square-foot DADU, on the schedule effective 2026-01-01.
Do the recurring costs differ between pre-approved and custom?
No. The King County sewer capacity charge (about $46.01 a month for 15 years), the SPU drainage fee, and the added property tax are driven by the lot and the finished unit, not by which plan you choose. The pre-approved savings are in design and time, not in these recurring bills.
How do I know if my lot qualifies for the pre-approved path?
The pre-approved path needs a Neighborhood Residential lot, no environmentally critical areas on the property, and under 750 square feet of ground disturbance. A plan-fit report measures those conditions for your address and shows which pre-approved plans actually fit your buildable rectangle.
This article is educational and preliminary. It is not legal advice, architectural advice, engineering advice, surveying 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 get written bids from licensed contractors and designers before making financial decisions.