Go Far®The definitions, quickly
An ADU (accessory dwelling unit) is a complete second home on your lot — its own kitchen, bathroom, and entrance. When it lives inside or attached to the main house — a basement apartment, a converted garage, an addition with its own entry — it is an attached ADU (sometimes AADU). When it stands alone in the yard, it is a DADU: a detached accessory dwelling unit, better known as a backyard cottage.
Cost: the biggest practical difference
Attached ADUs win on cost because they reuse structure you already own. In this market, attached ADU conversions and additions typically run $100,000–$250,000 — basement conversions at the lower end, garage conversions in the middle, and new attached additions at the top. A new-build DADU typically runs $250,000–$450,000, because a cottage is a complete small house: foundation, roof, siding, and its own utility connections, often with site work the main house never needed.
Income and value: the DADU case
Detached units generally rent for more than basement apartments of the same size — tenants pay for private entrances, light on four sides, and no shared walls. Under Washington’s ADU law (HB 1337), ADUs can also be condominiumized — legally separated and sold — which makes a well-built DADU something closer to a second sellable home on land you already own. Our HB 1337 guide covers what the law protects.
Privacy, family, and how you will actually use it
For multigenerational living, the right answer usually follows the family. Aging parents often do best in a single-level DADU with a step-free entry — close, but independent. A returning college graduate or a home office does fine in a basement ADU. If the unit is for a caregiver or for rental income while you stay in the main house, dig into whether you want tenants inside your walls or across the yard — that instinct decides more projects than any spreadsheet. Our mother-in-law suite guide walks every option with real costs.
Process and timeline
Attached conversions are usually faster: 5–9 months door to door for a basement or garage conversion, versus roughly 10–15 months for a new DADU including design, permitting, and construction. Both need full building permits; DADUs add siting questions — setbacks, lot coverage, trees, and utility routing — that make an early feasibility check on your specific lot worth its weight.
Or both: the two-unit strategy
HB 1337 requires cities inside urban growth areas to allow two ADUs per lot, and Seattle got there first — so the real question on many lots is not “which one” but “in which order.” A common path we build: convert the basement first (lower cost, faster income), then add the backyard cottage once the first unit is producing rent. The zoning, sewer, and access homework done for unit one carries straight into unit two.
Talk it through with a builder
Go Far Construction builds attached ADUs, basement and garage conversions, and backyard cottages across Lynnwood, Seattle, and Snohomish & King County. A free feasibility conversation looks at your lot’s zoning, utilities, and access, and gives you itemized numbers for the routes that genuinely fit — so you can decide between an ADU and a DADU with real figures, not averages.