ADU Cost in Washington State in 2026

ADU costs in Washington state range from $80,000 for a basic garage conversion in Spokane to $700,000+ for a high-end detached DADU in Bellevue. The most common build — a 600 sq ft detached mid-range unit in the Seattle metro — costs $250,000 to $380,000. Spokane and eastern Washington markets run $110,000 to $220,000. The huge cost gap between Seattle metro and the rest of the state is driven by labor, permit fees ($20,000–$40,000 in Seattle vs. $4,000–$6,000 in Spokane), and site complexity.

Washington House Bill 1337 (2023, fully effective July 2025) is one of the strongest state ADU laws in the country. It requires all cities over 25,000 to allow two ADUs per lot, eliminates owner-occupancy requirements, caps ADU impact fees at 50% of the primary home charge, eliminates parking requirements, and allows DADUs to be condoized and sold as independent properties. Cities cannot set ADU size limits below 1,000 sq ft or height limits below 24 feet.

Yes — and this is Washington's most powerful ADU feature. HB 1337 allows DADUs to be subdivided and sold with their own independent title as condominium units. In 2022, 44% of Seattle DADUs were permitted as condominiums, with units in premium neighborhoods selling for $500,000–$750,000 independently. This option turns a rental asset into a saleable real estate product and dramatically changes the investment math for high-value Seattle-area lots.

Washington falls in Seismic Design Category D — the same category as most of California. All ADUs require structural engineering that accounts for lateral seismic loads. For a standard detached DADU, this adds $8,000–$20,000 in engineering and structural framing costs compared to lower seismic-risk states. This cost is non-negotiable and is enforced at building permit review.

New ADU connections in King County are subject to a sewer capacity charge of approximately $14,000 (lump sum at permit issuance) or $77.99 per month for 15 years as a utility surcharge. This is one of the most commonly missed line items in Seattle-area ADU budgets. It applies to detached ADUs requiring a new sewer connection — garage conversions and attached ADUs connecting to existing lines may not trigger it. Confirm with King County Wastewater Treatment.

A mid-range 600 sq ft DADU in Seattle costs $300,000–$380,000 all-in and generates $2,000–$2,800 per month in rental income. Cash-on-cash ROI runs 5–7%, with a payback period of 10–16 years on a cash-funded project. The ROI improves significantly with the condoization option — a DADU built for $350,000 and sold as a condo for $600,000 delivers a 71% return at exit. Spokane offers the best cash flow ROI at 8–11% due to low build costs relative to rents.