In a previous post, we covered the overall cost to build a Seattle DADU, which can range from around $300,000 for a 450–600 sq ft cottage to $800,000 for a larger, high-end build. While that gives a big-picture number, it doesn’t show where the money is actually spent.
The True Price Tag to Build a DADU in Seattle series breaks the process down phase by phase, starting with pre-construction. Each article provides realistic cost expectations, identifies where it’s worth investing more, and clarifies which expenses are required and where there may be flexibility in the budget.
Seattle’s permitting process for DADUs is detailed and layered. Fees can include land use review (if applicable), building permits, plan review, and inspections.
Typical cost range:
$8,000–$15,000+
Costs vary based on project complexity, review timelines, and whether corrections are required. This cost also varies between choosing a Seattle Pre-Approved design and a custom-built DADU.
The following are the components of permitting and the reasons these steps are necessary.
A land survey is often required to confirm property boundaries, setbacks, easements, and the locations of existing structures before design and permitting can proceed. In Seattle’s dense neighborhoods, surveys help avoid boundary disputes and ensure the DADU is correctly positioned to meet zoning and setback requirements. Surveys may also reveal utility locations, encroachments, or irregular lot conditions that can affect site planning and influence design decisions early in the process.
Typical cost range:
$2,500–$5,000
Sloped lots, dense neighborhoods, or missing historical records can increase this.
Seattle commonly requires a geotechnical report to evaluate soil conditions, slope stability, and foundation requirements. This report helps determine the appropriate foundation design and identifies potential challenges such as poor soils, drainage issues, or slope-related risks. Addressing these conditions early can prevent costly changes during permitting or construction.
Typical cost range:
$3,000–$6,000
Sites with steep slopes, fill, or drainage concerns may require additional analysis.
Some homeowners use pre-approved plans, while others choose custom architectural drawings to address site constraints, design goals, neighborhood context, or simply a desire for a more personalized layout and aesthetic. Custom design can offer greater flexibility in how space is used, how the cottage relates to the main house and yard, and how the home will function long term, but it typically comes with higher upfront design costs.
Typical cost range:
$6,000–$15,000+
Custom designs, revisions, and coordination with consultants can extend this phase.
A structural engineer designs the foundation, framing, and seismic components to meet Seattle code and site conditions. Structural engineering is especially important in Seattle due to earthquake requirements and potentially hazardous ground conditions. A well-coordinated structural design helps prevent costly revisions during permitting and construction, and ensures the DADU performs safely and efficiently over the long term.
Typical cost range:
$4,000–$8,000
More complex designs or challenging soil conditions increase engineering time and cost.
Seattle has strict tree protection regulations, and certain trees can significantly impact where and how a DADU can be built. Exceptional trees—typically defined by species, size, and health—along with mature evergreens and large deciduous trees, often require special review and protection measures. An arborist report identifies which trees are regulated, establishes required protection zones, and determines whether removal is allowed or if design adjustments are needed, all of which can affect site layout, permitting timelines, and overall project cost.
Typical cost range:
$1,500–$4,000
Tree protection measures or removal permits can add additional costs later.
If there’s an existing garage, shed, or structure in the way, it must be removed before construction can begin, along with any remaining foundations, slabs, or footings below grade. Site preparation often includes excavation, removal of unsuitable soil or buried debris, and hauling material off-site to prepare the ground for new foundations. In Seattle, limited access and disposal requirements can add complexity and cost to this phase of the project.
Typical cost range:
$5,000–$15,000+
Access limitations, utility disconnects, and disposal fees can affect pricing.
Before breaking ground, Seattle homeowners often spend $30,000–$60,000 to secure full approval for a DADU project and prepare for construction. These steps are essential, unavoidable, and best planned early to avoid delays or budget shock.
Considering a Seattle backyard cottage? Contact us to learn more about our pre-construction services and how we help homeowners plan with confidence.
