The ADUniverse gallery the City maintains shows you ten pre-approved DADU designs that SDCI has already reviewed. The catch is what the gallery cannot tell you: which pre-approved DADU plan fits my lot. Fit is not a property of the plan — it is a property of your yard, measured in feet.
I have built these cottages around Seattle for years, and the most expensive mistake I saw was not a bad design. It was licensing a fine design and then discovering, on a survey, that it did not fit the buildable rectangle at the back of the lot — a discovery that costs the time and money the fast path was supposed to save.
Start with our guide to Seattle pre-approved DADU standard plans for the program rules and full catalog. This article goes further: how to tell, from your lot's numbers, which plans actually fit.
The City pre-approves the building package. It never pre-approves your yard. Fit is the measurement the gallery leaves to you.
Which pre-approved DADU plan fits my lot starts with a rectangle
Strip the problem down and every fit question in Seattle becomes one measurement: the buildable rectangle. It is what is left of your rear yard after you take off the setbacks and pull the existing house out of the way. The numbers are fixed by code, and they are the same for every lot:
- 15 feet of front yard, which a DADU cannot sit in (SMC 23.42.022).
- 5 feet from each side property line.
- 5 feet from the rear line — or 0 feet if your rear line abuts a public alley.
- 5 feet of separation from the principal house, so the cottage is not jammed against it.
- 50 percent lot coverage and a 32-foot height cap (SMC 23.44) as the outer walls.
Those numbers are the same on every lot and come from the SDCI ADU page and the Neighborhood Residential code. A compact two-storey cottage needs a rectangle about 20 feet wide by 22 feet deep. Anything narrower than 20 feet is a sliver, not a building site.
A 40-ft-wide interior lot, after setbacks
Schematic plan view — the buildable rectangle is what is left, not the whole rear yard.
Setbacks per SMC 23.42.022 and the Neighborhood Residential standards (Ordinance 127376): 15 ft front yard, 5 ft sides, 5 ft rear or 0 ft at a public alley, 5 ft from the house. MOA Family footprint published at 16 × 26 ft. Schematic, not to scale.
On a 40-foot-wide interior lot, 40 minus two 5-foot side setbacks leaves 30 feet of buildable width — a 16-by-26 plan fits with room to spare. On a 30-foot lot the same math leaves 20 feet, the floor, and the plan fits only in its 26-foot orientation. On a 25-foot lot the width left is 15 feet, and no stock plan fits.
The plan-fit report measures your buildable rectangle from Seattle parcel and building outlines and tests every published pre-approved plan against it, in both orientations, with each plan's clearance margin.
The plans, by footprint: one storey spends it, two storeys bank it
The gallery lists ten plans from ten design firms. Here are the plans that matter most for fit, sorted by what each demands from your yard rather than by how they look.
| Firm / plan | Size | Bed / bath | What it asks of your lot |
|---|---|---|---|
| Mobile Office Architects — MOA Family ADU | 850 sq ft | 2 bed / 1 bath | 16 × 26 ft footprint, two storeys, rotating corner entry. Best fit for a narrow rectangle. |
| Bohlin Cywinski Jackson — Edge House | ~300 sq ft footprint | Studio–1 bed / 1 bath | Smallest footprint in the gallery, steel-bolted frame. Fits where little else will. |
| CAST Architecture — Cedar Cottage | 467 sq ft | 1–2 bed / 1 bath | One level, accessible. Spends its full square footage on the ground. |
| Artisans Group — Seattle DADU | 600 sq ft | 1 bed / 1 bath | One level, aging-in-place. Needs a full 600 sq ft of ground. |
| Haas Architectural — Studio / One / Two Story | 288 / 432 / 576 / 940 sq ft | Studio to 2 bed | A kit of parts; the two-storey variants trade footprint for stairs. |
| Ahouse Studio — The Family | 808–964 sq ft | 2–3 bed / 2 bath | Two storeys; mirrors and rotates, built for 30-ft-wide lots and up. |
| Fivedot Architects — Schooner | 1,000 sq ft | 2 bed / 2 bath | Maximum allowed size — 1,000 sq ft, the largest plan in the catalog. |
Plan names, sizes, and designers from the City's ADUniverse gallery, which now lists twelve plans from nine designers. The catalog changes; verify the live gallery and the designer's spec sheet before any decision.
Buildable width left after the 5-ft side setbacks
The 20-ft minimum is where a compact cottage stops fitting.
5 ft side setbacks per SMC 23.42.022 / 23.44. A 20-ft minimum buildable width is the site-geometry floor for a compact two-storey cottage. Fremont carries the largest share of sub-3,200 sq ft parcels measured from City of Seattle open data — about a third of its lots — which is where this math bites hardest.
The lesson is "match the plan's footprint to the rectangle you actually have." A 600-square-foot one-storey Artisans plan needs 600 square feet of ground, which a tight rectangle cannot give even when the same lot would take the two-storey 850-square-foot MOA. Two storeys are the width trick on small lots.
Rotation and mirroring buy width, but only if the rectangle is wide enough
Most gallery designs rotate or flip. The MOA Family ADU has a corner entry specifically so the plan can be rotated and flipped to meet setbacks and entry requirements. That flexibility works only while the rectangle keeps 20 feet on its short side. Rotate a 16-by-26 plan and you trade width for depth; on a 20-foot-wide rectangle you use the 16-foot orientation, on a 30-foot-wide rectangle the 26-foot orientation. Neither helps once the narrow side drops below the floor.
This is why the neighborhood matters more than the plan. Wallingford's tight, sub-5,000-square-foot lots force the two-storey options; Wedgwood's 5,000-to-7,200-square-foot lots fit a one-storey plan without a fight; Queen Anne's narrow, sloping parcels push the question into custom design. The neighborhood profiles show the measured patterns behind each.
What removes the pre-approved path before any plan fits
Fit is not only footprint. Three site conditions quietly remove the expedited path, and all three are knowable from public records before you license a plan:
- Environmentally critical areas. Under current SDCI guidance, any mapped ECA on the property — steep slope, landslide, wetland, peat, shoreline — takes the fast path off the table and adds critical-areas review to a standard permit.
- Historic or shoreline overlays, or a landmark. A historic or special-review district needs a Certificate of Approval first; a shoreline district adds shoreline review; a landmark parcel needs Landmarks Preservation Board approval.
- Protected trees. A Tier 2 tree (24 inches or larger) is removable only as part of the development permit (SMC 25.11.070.A.1) with replacement or a payment in lieu, and its root-protection area can move or shrink the cottage even on a large lot. Our Seattle tree ordinance guide explains the tiers and the root math.
None of these show up in the gallery photo; all of them show up in the City's own GIS layers.
Ground disturbance, not just footprint, gates the fast permit
The expedited path is not purely geometric. SDCI's standard-plan guidance requires total ground disturbance to stay under 750 square feet. A plan that fits on paper can still trip this line once you add the foundation excavation, the sewer trench, the path, and grading. A one-storey plan with a big footprint plus a long trench is more likely to cross it than a two-storey plan with a small footprint.
That is the practical reason to check ground disturbance alongside footprint. The customized feasibility report measures both for your address before you spend on a plan license or a designer.
How the Plan Fit Report measures your lot
You can run all of this yourself against the City's layers, and if you enjoy GIS it will take you an evening. The Plan Fit Report does the same job for your address and hands you the answer on one page: which ADUniverse plans fit your buildable rectangle, which fit only rotated, and which do not — plus the ECA, overlay, tree, and ground-disturbance flags that would knock out the fast path.
Pre-Approved DADU Plan Fit Report
Which of the City's pre-approved DADU plans actually fit your lot — measured, not guessed. A plan-by-plan fit matrix against your buildable rectangle, plus the site flags that remove the fast path.
If you prefer to screen the basics yourself first, the free homeowner checklist walks the same setbacks and eligibility questions, and the Pro Guide turns the whole review into a scoring template. Either way, the order is the same: measure the rectangle, then license the plan.
Frequently asked questions
Which pre-approved DADU plan fits my lot?
The plan that fits is whichever footprint, after rotation or mirroring, sits inside your measured buildable rectangle — the yard left after a 15-foot front yard, 5-foot side and rear setbacks (0 feet at a public alley), and 5 feet of separation from the house. A two-storey plan like the 16-by-26-foot MOA Family ADU fits a narrower rectangle than a one-storey plan of the same square footage. A lot-specific plan-fit report measures that rectangle for your address.
What setbacks apply to a Seattle DADU?
Under SMC 23.42.022 and the Neighborhood Residential standards, a detached ADU sits behind a 15-foot front yard, with 5-foot side setbacks, a 5-foot rear setback (0 feet if the rear property line abuts a public alley), and about 5 feet of separation from the principal house. Lot coverage is capped at 50 percent and height at 32 feet.
Do I have to use a one-storey plan on a small Seattle lot?
No. A one-storey plan spends its full square footage as ground footprint, while a two-storey plan needs only about half as much ground. On a narrow lot the two-storey pre-approved plans — the MOA Family ADU is published at 16 by 26 feet — are often the ones that still fit after the 5-foot side setbacks come off.
What removes the pre-approved DADU plan path?
Any environmentally critical area on the property, a historic or shoreline overlay, a landmark designation, or total ground disturbance over 750 square feet removes the expedited pre-approved path under current SDCI guidance. A protected Tier 2 tree near the footprint can also force a shifted or custom plan even on an otherwise clean lot.
This article is educational and preliminary. It is not legal advice, architectural advice, engineering advice, surveying advice, or an official determination by the City of Seattle. Plan names, dimensions, and program eligibility change; verify the live ADUniverse gallery and current SDCI guidance before making any design or permitting decision, and confirm final siting with a licensed surveyor and SDCI.