A detached cottage or a unit inside or attached to the house — Washington HB 1337 requires cities to allow two of these on a lot in a single-family zone, except on lots with critical areas. 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
Washington House Bill 1337 (RCW 36.70A.681) requires cities and counties planning under the Growth Management Act to allow at least two accessory dwelling units on a lot in a single-family zone within the urban growth area, attached or detached in any combination. Lots with critical areas or their buffers are excepted.
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.
A city may not cap an ADU below 1,000 sq ft of gross floor area. Many go higher (Bellevue: 1,200 sq ft; Seattle: 1,000 sq ft, or 1,200 with three or more bedrooms).
Cities may not require off-street parking for an ADU within a half-mile walk of a major transit stop, more than one space per unit on a lot under 6,000 sq ft, or more than two on a larger lot.
Source: RCW 36.70A.681 (Washington 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.
