One of the most common questions we hear from homeowners considering a backyard cottage is: What does it actually cost? The honest answer is that DADU costs in Seattle vary significantly based on size, site conditions, design complexity, and material selections. That said, having a realistic framework for the numbers helps you plan and evaluate contractor proposals with confidence. Here is a transparent breakdown of what goes into Seattle DADU builder costs in 2026.
DADU costs are typically divided into “soft costs” and “hard costs.” Soft costs include everything that happens before construction begins: architectural design, structural engineering, soil reports if required, and permitting fees. In Seattle, permitting fees for a DADU can run from several thousand dollars to over ten thousand dollars, depending on the project’s valuation and the number of separate permits required. Design and engineering fees vary based on the complexity of the project, but budgeting 10 to 15 percent of the total project cost for soft costs is a reasonable starting point.
Hard costs cover the physical construction of the DADU. Site preparation, which includes grading, utility trenching, and foundation work, is often the most variable line item because it depends heavily on your lot’s topography and soil conditions. A flat lot with easy utility access will cost significantly less to prepare than a sloped lot that requires retaining walls or a new service connection from the street. Framing, roofing, windows, and exterior finishes are the next major categories, followed by insulation, drywall, and interior finishes. Plumbing, electrical, and HVAC are typically priced separately and can represent 20 to 30 percent of the total hard cost.
Material selections have a meaningful impact on the final number. A DADU finished with standard builder-grade materials will cost less than one with custom cabinetry, engineered hardwood floors, and high-end fixtures. At AmyWorks, we help you prioritize where to invest for maximum durability and appeal, and where standard materials will serve you just as well. We provide detailed, itemized estimates that reflect your specific project so you can make informed decisions. If you are ready to get a real number for your backyard cottage, contact AmyWorks for a site visit and a transparent proposal.