Washington State's ADU laws have changed significantly in recent years, and 2025 brings further updates that make building an ADU easier — and more financially compelling — than ever for Clark County homeowners.
What Is an ADU?
An Accessory Dwelling Unit is a secondary living space on a single-family property. There are three main types:
Detached ADU (DADU)
A separate backyard structure. Most flexible for privacy and rental potential. Typical size: 400–1,000 sq ft.
Attached ADU
Connected to the primary home — a garage conversion, added suite, or addition. Lower cost since you share walls and roof.
Junior ADU (JADU)
Created within the existing home footprint. Maximum 500 sq ft under state law.
Washington State ADU Law in 2025
Senate Bill 5258 now requires most Washington cities to allow ADUs by-right in all single-family zones. Key rules:
- Up to 2 ADUs are allowed per lot in most jurisdictions
- Owner-occupancy requirements have been largely eliminated statewide
- Maximum size: typically 1,000 sq ft or 50% of primary home floor area
- Setbacks: 4–5 feet from rear/side property lines in most Clark County zones
- ADUs cannot be sold separately from the primary home unless subdivided under RCW 58.17
City-specific note: Vancouver, Camas, Ridgefield, and Battleground each have their own ADU ordinances. Always verify with the local planning department before finalizing your design.
What Does It Cost to Build an ADU in Washington?
- Garage conversion: $45,000–$80,000
- Attached addition ADU: $90,000–$160,000
- Detached ADU (400–800 sq ft): $140,000–$250,000
- Permit fees (Clark County): $3,500–$8,000
- Utility connections: $5,000–$20,000
A 1-bedroom ADU in the Vancouver metro renting at $1,400–$1,800/month generates $16,800–$21,600 per year in gross rental income. A $180,000 detached ADU reaches simple payback in roughly 8–11 years — and adds property value from day one.
Permits, Inspections & Timeline
A typical detached ADU in Clark County requires a building permit, plus separate electrical, mechanical, and plumbing permits. Current processing time: 4–8 weeks. Total timeline from permit submission to move-in: 5–9 months. Garage conversions typically run 2–4 months.
Financing Your ADU
- Cash-out refinance or HELOC: Most common for homeowners with equity.
- Construction loan: Short-term financing that converts to a permanent mortgage at completion.
- Washington State Housing Finance Commission: Piloted below-market ADU financing for qualifying homeowners — check their website for 2025 availability.
- Fannie Mae HomeStyle or FHA 203(k): Renovation loans that allow ADU construction to be rolled into a mortgage.
Choosing the Right Contractor
ADU construction involves every trade: site work, framing, roofing, plumbing, electrical, HVAC, insulation, drywall, and finish carpentry. You need a general contractor who manages the full scope, pulls all permits under their license, and coordinates all inspections.
Serden Group has completed ADU projects across Clark County including detached cottages, garage conversions, and in-law suites. We handle permits, engineering coordination, and all inspections start to finish.
Considering an ADU? Start with a site assessment. Schedule your free consultation.