ADU Regulations in Snohomish County: What Homeowners Need to Know (2026)

Published February 2026 | By NW General Contractor

Accessory dwelling units (ADUs) have become one of the hottest topics in Snohomish County real estate and construction. Thanks to sweeping changes in Washington state law and updated local regulations, building an ADU on your property is now easier, more affordable, and more practical than ever before.

Whether you're looking to generate rental income, create a space for aging parents, or simply increase your property value, this guide covers everything you need to know about ADU regulations in Snohomish County as of 2026.

What Is an ADU?

An accessory dwelling unit (ADU) is a secondary housing unit on a single-family residential lot. It has its own kitchen, bathroom, and sleeping area, making it a fully independent living space. ADUs come in three main types:

Detached ADU (DADU)

A standalone structure separate from the primary home, often built in the backyard. This is the most common type of new ADU construction in Snohomish County. DADUs can be purpose-built as a small cottage or built above a detached garage. They offer the most privacy for both the homeowner and the tenant.

Attached ADU (AADU)

An addition or conversion that is physically connected to the primary house. This could be a basement apartment, an above-garage unit, or a bump-out addition with a separate entrance. Attached ADUs are typically less expensive than detached units because they share walls, foundation, or roof structure with the main home.

Garage Conversion

Converting an existing attached or detached garage into a living space. Garage conversions are popular because the basic structure already exists, reducing construction costs. However, they require significant work to meet residential building codes including insulation, plumbing, egress windows, and ventilation.

Washington HB 1337: The Law That Changed Everything

In 2023, Washington state passed House Bill 1337, one of the most progressive ADU laws in the country. This law went into effect in phases through 2023 and 2024, and it fundamentally changed what local jurisdictions can and cannot regulate regarding ADUs.

Key Changes Under HB 1337

• Two ADUs per lot: Homeowners can now build up to two ADUs on a single-family lot — one attached and one detached. Previously, most jurisdictions only allowed one.
• No owner-occupancy requirement: The state eliminated the requirement that the property owner must live on-site. You can now rent both the primary home and the ADU(s). This was a game-changer for investors and homeowners who want to move but keep the property.
• No additional parking required: Local jurisdictions can no longer require extra off-street parking spaces for ADUs. This removed one of the biggest practical barriers to ADU construction, especially on smaller lots.
• Reduced impact fees: Impact fees for ADUs are capped at 50% of the fees charged for the primary dwelling, making the permitting process less expensive.
• Streamlined permitting: Cities and counties must process ADU permit applications within the same timeframe as other residential permits.
• Design flexibility: Jurisdictions cannot impose design requirements on ADUs that are more restrictive than those applied to the primary dwelling.

Snohomish County Regulations Updated March 2025

In March 2025, Snohomish County updated its local ADU regulations to fully comply with HB 1337 and further streamline the process. These updated regulations apply to unincorporated Snohomish County. Cities within the county (Everett, Marysville, Lake Stevens, etc.) have their own codes, though all must comply with state law.

Key Local Updates

• ADUs are now permitted in virtually all residential zones throughout unincorporated Snohomish County
• Lot coverage and setback requirements have been relaxed for ADU construction
• The county created an expedited review process for ADU applications that use pre-approved state building plans
• Connection fees for water and sewer have been adjusted to be more proportional to the ADU's size
• Height limits for detached ADUs have been increased to allow two-story structures in many zones

Where You Can Build ADUs

Under current state and county law, ADUs are allowed in virtually all residential zones where single-family homes are permitted. This includes:

• Single-family residential zones (R-5, R-7, R-9, R-12, etc.)
• Rural residential zones
• Suburban residential zones
• Urban growth areas

The main limitations relate to lot-specific constraints like critical areas (wetlands, steep slopes, flood zones), septic system capacity for rural properties, and utility availability. If your property is on a septic system rather than municipal sewer, you'll need to verify the system can handle the additional load.

Size Limitations

Size limits vary slightly depending on whether you're in an incorporated city or unincorporated Snohomish County:

• Attached ADU (AADU): Up to 1,000 square feet or 50% of the primary dwelling's floor area, whichever is less
• Detached ADU (DADU): Up to 1,000 square feet in most jurisdictions. Some cities within the county allow up to 1,200 square feet
• Height: Detached ADUs can generally be up to 24 feet tall in urban areas, allowing for a two-story structure or a unit above a garage
• Setbacks: Typically 5 feet from side and rear property lines for detached ADUs, though this varies by jurisdiction

A 1,000-square-foot ADU is roughly equivalent to a generous one-bedroom or a compact two-bedroom apartment — plenty of space for a single person, couple, or small family.

Parking Requirements

Thanks to HB 1337, additional parking is largely eliminated as a requirement for ADUs. Local jurisdictions cannot require additional off-street parking spaces specifically for an ADU. This is significant because on many Snohomish County lots, finding space for an additional parking spot was a major obstacle.

You may still need to maintain the existing parking requirement for your primary dwelling (typically two off-street spaces). If you're converting a garage to an ADU, you should verify with your local jurisdiction whether you need to provide replacement parking for the lost garage spaces.

Permit Process and Timeline

Here's what the ADU permitting process looks like in Snohomish County:

1. Pre-application conference (optional but recommended): Meet with the planning department to discuss your project, site constraints, and requirements. This can save significant time and money by identifying issues early. 1-2 weeks.
2. Design and plans: Work with a designer or architect to create construction plans. Using pre-approved state plans can save time here. 2-6 weeks depending on complexity.
3. Submit permit application: File your application with the relevant jurisdiction along with plans, site plans, and fees. The City of Everett, for example, has an online portal for permit applications.
4. Plan review: The jurisdiction reviews your plans for code compliance. Expect 4-8 weeks for standard review, or 2-4 weeks for pre-approved plans.
5. Revisions (if needed): Address any comments or required changes. 1-3 weeks.
6. Permit issued: Once approved, you receive your building permit and can begin construction.
7. Inspections during construction: Foundation, framing, mechanical, electrical, plumbing, and final inspections are required at various stages.
8. Certificate of occupancy: After final inspection, you receive authorization to occupy the ADU.

Total timeline from application to occupancy: 6-14 months is typical for a detached ADU, depending on the complexity of the project and construction schedule.

Cost Estimates by ADU Type

ADU construction costs in Snohomish County vary significantly based on the type and finishes:

Internal Conversion (Basement or Space Within Home)

$80,000 - $150,000

Converting existing finished or unfinished space within the home. Costs depend on whether plumbing and egress already exist. A basement conversion typically requires waterproofing, egress windows, a kitchenette, bathroom, and separate entrance.

Garage Conversion

$180,000 - $220,000

Converting an existing garage into a living space. Requires insulation, drywall, flooring, plumbing (kitchen and bath), electrical upgrades, HVAC, egress windows, and finishing work. The structure already exists, which saves on foundation and framing costs.

Detached ADU (New Construction)

$250,000 - $450,000

Building a new standalone structure from the ground up. This is the most expensive option but offers the most flexibility in design and the highest potential rental income. Costs include site preparation, foundation, framing, roofing, siding, interior finishes, utilities, and landscaping. Premium finishes, difficult site conditions (slope, trees, access), or larger units push costs toward the higher end.

Additional Costs to Budget For

• Permit fees: $5,000-$15,000 depending on jurisdiction and project scope
• Utility connections: $5,000-$20,000 for water, sewer, and electrical connections
• Site work: $3,000-$15,000 for grading, drainage, and access paths
• Design/architecture: $5,000-$15,000 for custom plans (or significantly less with pre-approved plans)
• Landscaping restoration: $2,000-$8,000 to restore the yard after construction

Pre-Approved State Building Plans

Washington state has developed a library of pre-approved ADU building plans to reduce costs and speed up permitting. These plans have already been reviewed for state building code compliance, which means local jurisdictions can expedite their review process.

Benefits of using pre-approved plans:

• Faster permit approval (often weeks instead of months)
• Lower design costs (no need for custom architecture)
• Proven designs that meet all code requirements
• Multiple floor plans and styles available

Pre-approved plans are a great starting point, though some homeowners prefer custom designs that better suit their property and aesthetic preferences. Your contractor can advise on which approach makes the most sense for your situation.

Rental Income Potential

One of the primary motivations for building an ADU is rental income. In Snohomish County, current rental rates for ADU-sized units are strong:

• Studio/efficiency ADU (400-500 sq ft): $1,200-$1,600/month
• One-bedroom ADU (600-800 sq ft): $1,500-$2,000/month
• Two-bedroom ADU (800-1,000 sq ft): $1,800-$2,400/month

These rates mean a well-built ADU can generate $18,000-$28,000+ in annual rental income. For a $300,000 detached ADU, that represents a 6-9% gross return before expenses — a strong return for a real estate investment that also increases your property value.

Short-term rental (Airbnb, VRBO) rates are higher but come with more management overhead and may be subject to local short-term rental regulations. Check with your jurisdiction before planning to use an ADU as a short-term rental.

Impact on Property Value

ADUs generally increase property value in the Snohomish County market. While the exact amount varies, a well-built ADU typically adds 60-80% of its construction cost to the property's appraised value. A $300,000 ADU might add $180,000-$240,000 in property value.

The combination of increased property value and rental income makes ADUs one of the strongest real estate investments available to homeowners in the current market. With the elimination of owner-occupancy requirements, ADU properties have also become attractive to real estate investors.

Frequently Asked Questions

Ready to Build an ADU?

Building an ADU is a significant investment, but the regulatory landscape has never been more favorable. At NW General Contractor, we specialize in ADU construction throughout Snohomish County. We handle everything from design and permitting through final inspection, and we're familiar with the specific requirements of every jurisdiction in the county.