Go Far®Two ADUs per lot — really
A Seattle residential lot can now hold the main house plus two accessory dwelling units. That can be one attached unit (an AADU — typically a basement or in-house apartment) plus one detached backyard cottage (a DADU), or two attached units. For homeowners, that opens real strategies: a basement apartment for rental income and a backyard cottage for family, or the reverse. Statewide law points the same direction — Washington’s HB 1337 requires cities inside urban growth areas to allow two ADUs per lot, and Seattle got there ahead of the state.
Size, height, and setbacks
The current framework allows ADUs up to 1,000 square feet for units with up to two bedrooms, and up to 1,200 square feet for three-bedroom-plus units. Detached units in most Neighborhood Residential zones can now reach roughly 32 feet in height — enough for a genuine two-story cottage. Typical setbacks are five feet at side and rear yards, dropping to zero at an alley. Rules are zone-specific and evolve, so treat these as the shape of what’s possible and confirm your lot’s exact numbers before you design — we do this as part of every feasibility conversation.
The rules that aren’t there anymore
Three former deal-breakers are gone. Owner-occupancy is no longer required — you can rent out both the main house and the ADU. No off-street parking is required for an ADU. And under state law (HB 1337), ADUs can now be condominiumized — legally separated and sold — which changes the long-term math on a backyard cottage from “rental income” to “second sellable home on land you already own.”
Permits: SDCI, and the pre-approved shortcut
ADUs and DADUs go through a standard building permit with the Seattle Department of Construction and Inspections (SDCI). Timelines depend on the review queue, and fees scale with project valuation. The shortcut worth knowing: Seattle maintains a gallery of pre-approved DADU plans that move through review dramatically faster than custom designs. A custom cottage buys you a design matched to your lot and light; a pre-approved plan buys you months. Both are good trades in the right situation, and we build either.
What changed heading into 2026
Seattle folded its ADU rules into the citywide middle-housing framework adopted under Washington’s HB 1110, made permanent in early 2026. The practical effects for homeowners: the more generous size and height limits now apply across residential zones, and ADUs are counted within the same density and lot-coverage math as other housing types. The direction of travel for years has been “easier, bigger, more flexible” — which is why waiting rarely improves the project.
Is your lot a candidate?
Most Seattle lots can hold more than their owners think, but the honest answer lives in the details: zoning, lot coverage, tree protection, sewer capacity, and access. Go Far Construction builds ADUs and backyard cottages across Seattle and the surrounding cities, and we’ll tell you what your lot can carry — and what it will cost — in a free, no-obligation feasibility conversation with itemized pricing.