Go Far®Why a small building carries a big number
A DADU has everything a 3,000-square-foot house has — foundation, roof, kitchen, bathroom, heating, electrical service, plumbing — just less hallway between them. Kitchens and baths are the most expensive rooms per square foot, and a cottage is mostly kitchen and bath. So while the total is far below a house, the per-square-foot number runs higher than people expect: in the Seattle–Snohomish area, complete DADU projects commonly land in the $250,000–$450,000 range depending on size, site, and finish level, with compact or garage-conversion projects below that band.
The four budget blocks
- The building itself — structure, envelope, interior finishes. The block people picture, and usually the largest.
- Site work — excavation, foundation, drainage, and access. Sloped lots, trees, and tight yards move this line more than any other.
- Utilities — extending water, sewer, and power from the main house or street. Trenching distance matters; some projects also trigger electrical service upgrades.
- Soft costs — design, engineering, permits, and fees. Budget real money here; a DADU is a full permitted dwelling, not a shed.
The rental math
Legal detached units in the Seattle metro rent strongly, and a permitted DADU adds appraisable value the way unpermitted space never does. Owners typically weigh three returns at once: monthly rent, property value, and optionality — housing for parents later, a landing pad for kids after that, an office in between. Run the numbers against our ADU & DADU rules guide; the 2023 state law made almost every residential lot in an urban growth area a candidate.
Where owners over- and under-spend
Overspend: square footage for its own sake — a well-planned 600 square feet lives larger than a padded 900 and permits faster. Underspend: sound insulation and storage, the two things every DADU tenant mentions. Sweet spot: durable mid-grade finishes — cottage rents are driven by existence, location, and light far more than by slab-grade countertops.
Price your backyard
Go Far Construction builds DADUs and attached ADUs across Lynnwood, Seattle, and Snohomish & King County. A free feasibility conversation covers your lot's zoning, utility routing, and a real budget range — before you spend a dollar on drawings.