Accessory Dwelling Units (ADUs) are becoming increasingly popular in Washington State, offering homeowners flexible living spaces and potential rental income. However, the cost to build an ADU can vary widely based on several factors. Let’s delve into the details to understand the financial commitment involved.
● ADU costs in Washington range from $60K to $400K+
● Detached units are most expensive; basement/garage conversions are cheaper
● Seattle costs are higher, starting around $125K+
● Soft costs (permits, design) can be $15K–$50K
● Cost per sq ft: $150–$500, depending on finish, size, and location
● ADUs add value, generate rental income, and increase property flexibility
The cost of constructing an ADU in Washington typically ranges from $60,000 to $400,000+. This broad range accounts for various factors, including the type of ADU, size, location, and quality of finishes.
Type of ADU | Typical Cost Range |
Detached (new build) | $200,000 – $400,000+ |
Garage conversion | $80,000 – $200,000 |
Basement conversion | $60,000 – $175,000 |
Attached addition | $150,000 – $300,000 |
Seattle Area | Costs can be higher, averaging between $125,000 and $350,000, with some projects reaching up to $600,000 for high-end finishes |
Spokane Area | Estimates for new detached ADUs start as low as $50,000, with per square foot costs around $250-$300 for medium-plus finishes. |
Other Cities | In areas like Tacoma, Bellevue, and Vancouver, costs generally fall within the state range but may lean higher due to proximity to urban centers. |
Several elements can impact the overall cost of building an ADU:
● Garage Conversion: Utilizing existing structures, these conversions can range from $50,000 to $120,000, influenced by market conditions and materials .
● Basement Conversion: Similar to garage conversions, costs can range from $40,000 to $100,000+.
● Attached ADU (Addition): Sharing walls with the main house, these typically cost between $100,000 and $250,000.
● Detached New Construction ADU: Building a standalone structure is the most expensive option, usually ranging from $120,000 to $400,000+.
● ADU Above Garage: Costs can vary widely, often in the higher range due to structural upgrades needed for the existing garage.
● Prefabricated (Prefab) ADU: While the unit itself might be cheaper per square foot, total project costs, including site prep and utility hookups, often range from $150,000 to $285,000.
Larger ADUs naturally cost more due to increased materials and labor. However, fixed costs like permits and utility hookups mean the cost per square foot tends to decrease as the unit gets larger.
High-end finishes, such as premium flooring, custom cabinetry, and luxury appliances, can significantly increase the price. Opting for basic finishes can help keep costs down.
● Excavation and Grading: Sloped lots or those requiring significant earthwork can add $1,500 to $10,500+.
● Utility Connections: Connecting to existing water, sewer, electrical, and gas lines can be a major cost, especially if lines need to be extended. A new septic system can add $30,000-$40,000+.
● Electrical Panel Upgrades: If the main house panel isn’t sufficient, an upgrade can cost several thousand dollars.
● Dedicated Electrical Meter: Some jurisdictions may require a separate meter for the ADU, potentially adding significant costs.
● Seattle Metro: Higher labor rates, permit fees, and demand push average costs higher.
● Permit Requirements: Most counties (King, Pierce, Snohomish) require HVAC permits — usually $100–$500.
● Energy Codes: Washington has some of the strictest efficiency standards, which can affect unit selection.
● Electrical Code Compliance: Local code may require a disconnect switch, weatherproof outlets, or subpanel work.
● Incentives: Many utilities (PSE, SnoPUD, Avista) offer rebates from $200 to $2,000 for qualifying energy-efficient installs.
● Permits & Fees: Essential in Washington State, expect $1,500 to $8,000+ for permits alone. Seattle permits can be $1,500 to $3,500.
● Architectural/Design Fees: Depending on complexity, these can range from $2,000 to $30,000+.
● Engineering Fees: Structural, civil, or geotechnical engineering may add $0 to $5,000+.
Washington State, particularly the Puget Sound region, has higher labor costs than many other parts of the country. This can significantly impact the overall construction budget.
Building an ADU is a major investment — but there are smart ways to reduce costs without compromising quality:
● Interior Painting: Save $2,000–$5,000 by doing this yourself.
● Landscaping: Basic sod, gravel paths, or planters can be DIYed after the build.
● Appliance & Fixture Installation: Buying and installing your own appliances and light fixtures may save 10–20%.
● Finish Selection: Opt for off-the-shelf cabinets, laminate counters, or vinyl plank flooring instead of custom materials.
● Sweat Equity: Demolition, cleanup, or managing permits yourself (if experienced) can cut labor costs.
● Use a simple, boxy layout (fewer corners = lower cost).
● Avoid unnecessary plumbing moves — keep kitchen/bathrooms near each other.
● Minimize windows/sliding doors — these drive up material and energy costs.
● Plan for future expansion, even if you don’t build it now (stub out plumbing, rough electrical).
