ADU Cost Guide: Building an ADU in Snohomish County (2026)

What Does an ADU Cost in Snohomish County?

Building an Accessory Dwelling Unit in Snohomish County is a significant investment that can generate substantial returns through rental income and property value increases. Here's what to expect:

• Garage conversion: $180,000-$220,000
• Basement apartment: $100,000-$180,000
• Detached ADU (new construction): $250,000-$450,000

Costs vary based on size, finishes, site conditions, and whether utility connections need to be extended.

Garage Conversion ADU ($180K-$220K)

Converting an existing garage into a living space is the most cost-effective ADU option because the shell already exists. Typical scope includes:

• Structural reinforcement and foundation work ($15,000-$30,000)
• Insulation, drywall, and interior framing ($20,000-$35,000)
• Plumbing — kitchen and bathroom ($15,000-$25,000)
• Electrical panel upgrade and wiring ($8,000-$15,000)
• Kitchen installation ($15,000-$30,000)
• Bathroom installation ($10,000-$20,000)
• Flooring, paint, and finishes ($10,000-$20,000)
• HVAC system ($5,000-$12,000)
• Windows and exterior door ($5,000-$10,000)
• Permits and design ($5,000-$10,000)

Detached ADU — New Construction ($250K-$450K)

A detached ADU (also called a DADU or backyard cottage) is built from scratch on your property. Costs are higher but you get maximum design flexibility:

• Site preparation and foundation ($25,000-$50,000)
• Framing and structural ($40,000-$70,000)
• Roofing ($10,000-$20,000)
• Plumbing ($15,000-$30,000)
• Electrical ($10,000-$20,000)
• HVAC ($8,000-$15,000)
• Interior finishes ($30,000-$60,000)
• Kitchen and bathroom ($25,000-$50,000)
• Exterior finishes and siding ($15,000-$30,000)
• Utility connections ($10,000-$25,000)
• Permits, design, engineering ($10,000-$25,000)
• Landscaping and site restoration ($5,000-$15,000)

Washington State ADU Regulations (HB 1337)

Washington's HB 1337 has made ADUs significantly easier to build:

• ADUs are now allowed in all residential zones statewide
• No owner-occupancy requirement — you can rent both the main home and the ADU
• Parking requirements largely eliminated near transit
• Cities must allow at least 2 ADUs per lot
• Washington offers 10 pre-approved ADU building plans that can receive permits in 2-6 weeks

Snohomish County further loosened ADU regulations in March 2025, expanding eligibility to duplexes and attached single-family homes.

Rental Income Potential

An ADU in Snohomish County can generate significant rental income:

• Studio/1-bed ADU: $1,200-$1,800/month
• 2-bedroom ADU: $1,800-$2,500/month
• Annual income potential: $14,400-$30,000

At the mid-range, a $200K garage conversion generating $1,500/month pays for itself in approximately 11 years — while simultaneously increasing your property value by 20-30%.

ADU Permit Process in Snohomish County

1. Pre-application meeting — Meet with county planning to confirm your lot qualifies
2. Design and plans — Work with an architect or use WA state pre-approved plans
3. Submit permit application — Include site plan, building plans, and utility plans
4. Plan review — County reviews for code compliance (4-8 weeks typical)
5. Permit issued — Construction can begin
6. Inspections — Required at key stages (foundation, framing, electrical, plumbing, final)
7. Certificate of occupancy — Final approval to occupy the ADU

Using WA state pre-approved plans can reduce the permit timeline to 2-6 weeks.

Is an ADU Worth It in Snohomish County?

For most homeowners, yes. The combination of rental income, property value increase, and flexible use (aging-in-place, home office, housing family members) makes ADUs one of the best investments you can make in your property. With Snohomish County's tight housing market (only 2.4 months of inventory), strong rental demand, and loosened regulations, the conditions for ADU construction have never been better. NW General Contractor specializes in ADU construction throughout Snohomish County. Contact us for a free estimate or call (425) 286-5639.

Frequently Asked Questions