Building an Accessory Dwelling Unit (ADU) in Snohomish County is an excellent way to enhance your property’s value, provide additional living space, or generate rental income. At CASM Contracting, we specialize in designing and building high-quality ADUs that meet local regulations while maximizing your property’s potential.
This guide covers everything you need to know about ADUs—including definitions, benefits, local regulations, the permitting process, design considerations, and how to choose the right builder.
Understanding ADUs and DADUs
An Accessory Dwelling Unit (ADU) is a secondary housing unit on a single-family residential lot. ADUs can be either attached to the main dwelling or detached as a separate structure, commonly known as Detached Accessory Dwelling Units (DADUs). These units provide independent living spaces with their own kitchen, bathroom, sleeping, and living areas.
In Snohomish County, both attached and detached ADUs are permitted under specific zoning regulations. For detailed definitions and standards, refer to Snohomish County Code 30.28.010.
Benefits of Adding an ADU to Your Property
Adding an ADU to your property offers numerous advantages:
- Rental Income – ADUs provide a steady revenue stream by renting out the unit.
- Housing Flexibility – Perfect for multigenerational families, aging parents, or adult children.
- Increased Property Value – Expanding living space can boost your home’s market value.
- Efficient Land Use – ADUs contribute to local housing solutions without requiring new land development.
For more insights, check out the Master Builders Association’s ADU Overview.
Snohomish County Regulations for ADUs
Understanding local building regulations is crucial for a successful ADU project. Key regulations include:
- Zoning Requirements – ADUs are allowed subordinate to a single-family dwelling in zones where single-family dwellings are permitted under SCC 30.22.100, 30.22.110, and 30.22.120.
- Size Limitations – The floor area of an ADU shall not exceed 1,200 square feet, exclusive of garages, porches, and unfinished basements.
- Design Standards – Attached ADUs must preserve the architectural character of the primary dwelling, matching exterior materials, roof form, window spacing, and proportions. Detached ADUs should approximate these features unless located within an existing accessory structure.
- Parking Requirements – In urban zones, no additional parking is required for ADUs. In rural and other zones, one off-street parking space per ADU is mandatory.
- Utility Provisions – Applicants must provide documentation of the physical and legal availability of potable water and ensure that the existing or proposed sewage or septic system can handle the additional demand.
For a comprehensive list of requirements and standards, consult the Snohomish County Code 30.28.010 and the Detached Accessory Dwelling Units Assistance Bulletin.
Steps to Obtain Necessary Permits
Securing the appropriate permits is a critical step in the ADU construction process:
- Pre-Application Consultation: Engage with the Snohomish County Planning and Development Services (PDS) to discuss your project and understand specific requirements.
- Prepare Application Materials: Compile necessary documents, including a site plan, construction plans, and verification of water and sewage disposal.
- Submit Application: Applications are submitted online through the PDS Permit Portal.
- Application Review: The county reviews your application for completeness and compliance with regulations.
- Address Corrections: If required, make any corrections or provide additional information as requested by the county.
- Permit Issuance: Once approved, pay any remaining fees to receive your building permit.
For detailed guidance, refer to the Residential Building Permits page on the Snohomish County website.
Design Considerations for Your ADU
Thoughtful design ensures that your ADU is both functional and harmonious with your existing property:
- Space Efficiency – Maximize the use of limited space through smart layout planning.
- Privacy – Design the ADU to provide privacy for both occupants and neighbors, considering window placement and sound insulation.
- Aesthetics – Ensure that the ADU’s design complements the architectural style of the primary dwelling.
- Sustainability – Incorporate energy-efficient appliances, insulation, and sustainable materials to reduce environmental impact and operating costs.
- Accessibility – Consider universal design principles to accommodate occupants of all ages and abilities.
Choosing the Right Builder in Snohomish County
Selecting a reputable builder is crucial to the success of your ADU project. A qualified contractor will ensure compliance with local regulations, deliver high-quality craftsmanship, and keep the project on schedule. Here’s what to look for when choosing an ADU builder:
- Experience with ADUs – Not all contractors specialize in ADUs, so it’s important to find one with a proven track record.
- Knowledge of Local Regulations – The builder should be well-versed in Snohomish County’s zoning laws, permitting process, and code requirements.
- Quality of Workmanship – Look at past projects, read customer reviews, and ask for references to assess the builder’s reliability and work quality.
- Transparent Pricing – A trustworthy builder provides clear estimates and a detailed contract outlining materials, labor, and project timelines.
- Licensing and Insurance – Verify that the contractor is licensed, bonded, and insured in Washington State. You can check a builder’s credentials through the Washington State Department of Labor & Industries.
Get Started on Your ADU Project Today
Building an ADU in Snohomish County is a great investment that adds value to your property, creates additional living space, and provides rental income opportunities. Whether you’re considering an attached ADU, a backyard cottage, or a garage conversion, understanding the process and working with experienced professionals will help ensure a smooth and successful project.
For more information on ADU regulations, permitting, and design in Snohomish County, visit the official Snohomish County Planning and Development Services (PDS) website.
If you’re ready to take the next step, contact CASM Contracting today to discuss your project, receive a consultation, and start turning your vision into reality!
