Detached cottage, basement conversion, or a junior unit inside the existing footprint — Washington HB 1337 requires every city to allow at least two of these per single-family lot. Setback, size, and parking rules still vary city-by-city.
Free lookup. Resolves zoning, setbacks, and which ADU types your parcel allows in seconds.
Sources: Seattle SDCI — Accessory Dwelling Units and Washington HB 1337 (2023), as verified in Kolmo Construction's permit rule data. Build-time range from Kolmo's 2026 estimating model — typical planning ranges, not a schedule commitment; complex sites (steep slopes, critical areas, long utility runs) run longer.
Three flavors with different rules. Most homeowners say “ADU” when they really mean AADU — but the choice changes setbacks, max size, parking, and packet content.
A standalone backyard cottage. Own foundation, own roof, separate from the primary residence. Most flexibility on layout; highest construction cost.
Maps to permit scope adu-dadu
Inside or attached to the primary residence. Basement conversion, garage conversion, or a bump-out addition with kitchen + bath. Lower cost; tighter design constraints.
Maps to permit scope adu-aadu
A smaller AADU (typically ≤500 sf) inside the existing footprint, often sharing a bathroom with the primary unit. Lowest barrier to permit; useful for converting a single bedroom + kitchenette.
Treated as adu-aadu for the permit engine
Effective 2023-07-23, Washington House Bill 1337 requires every city and county planning under the Growth Management Act to allow at least two accessory dwelling units (any combination of detached, attached, or junior) on every lot zoned for single-family residential use.
Cities can no longer require the owner to live on the property as a condition of permitting an ADU. Some still attach owner-occupancy strings to short-term-rental use (Airbnb / VRBO), but the permit itself cannot be denied on those grounds.
Cities must allow at least 1,000 sf gross floor area for detached units. Many go higher (Bellevue: 1,200 sf; Seattle: 1,000 sf or 12% lot coverage, whichever is greater).
Cities may not require more than one off-street parking space per ADU, and zero spaces when the lot is within a half-mile of major transit. This matters in Seattle and parts of Bellevue / Kirkland.
Source: Washington State Legislature — HB 1337 (2023). City-specific rules cited on each city page below.
Verified ADU rules for the 16 cities Kolmo personally serves. Each city page covers max sizes, setbacks, parking, owner-occupancy posture, and HB 1337 conformance.
Enter your address. We pull lot polygon, zoning, setbacks, easements, critical-area overlays, and any existing structure from county GIS — no manual data entry.
Drop a sized ADU footprint on the canvas. Rotate to fit the lot. Setback / coverage / easement / tree-buffer checks update live as you drag — no guessing whether it fits.
The engine assembles a city-filable packet: A1.1 site plan, A2.1 floor plan, A3.1 elevations, A4.1 foundation, A4.2 framing, A5.1 wall section, A6.0 glazing schedule, filled permit forms, Manual J energy compliance — every sheet cited to its source.