Accessory dwelling units, commonly called ADUs, have become a practical option for homeowners looking to add living space, house a family member, or generate rental income from an existing property. In Snohomish County, including Everett, the permitting process and construction sequence for an ADU follow a specific path that differs from a standard home remodel in several ways. Understanding what the build involves helps homeowners plan realistically before the first shovel hits the dirt.
Types of ADUs
ADUs generally fall into two categories: attached and detached. An attached ADU is connected to the primary home, either as an addition, a converted garage, or a basement unit with its own entrance. A detached ADU stands separately on the property, typically in the backyard, as a fully independent structure. Snohomish County allows both types in most residential zones, though rules on size, setbacks, and design standards vary by zone and municipality within the county.
Permitting and Design
ADU construction requires building permits and, in most cases, a separate application from the primary home if it’s an existing structure. The permit package typically includes architectural drawings showing the floor plan, elevations, and how the unit relates to the main home; structural drawings for the foundation and framing; and documentation that the utility connections—water, sewer, and electrical—meet applicable codes. Crafthaus Remodel prepares permit packages and coordinates with the county’s planning and building departments to get designs approved before construction begins.
Site Preparation and Foundation
For a detached ADU, the first physical step after permit approval is site preparation: grading the area, addressing drainage, and excavating for the foundation. Foundation type depends on the soil conditions and local requirements, with slab-on-grade being common for smaller detached units and crawl space or stem wall foundations used in areas with specific drainage or slope conditions. The foundation must cure before framing begins, which adds time to the schedule that can’t be rushed.
Framing, Mechanical Rough-In, and Shell
Once the foundation is ready, framing proceeds much like a small home build: walls, roof framing, sheathing, and a weather barrier before any interior work begins. Mechanical rough-in (plumbing, electrical, and HVAC if applicable) happens after framing, while walls are still open. Inspections are required at rough-in before insulation and drywall close the walls. The time from framing to dried-in shell is typically a few weeks for a smaller ADU, though weather and inspection scheduling affect the pace. Homeowners working through ADU construction projects commonly find that the permitting phase takes longer than the construction itself.
Interior Finish and Utility Connections
Interior finish follows the same sequence as any small home: insulation, drywall, paint, flooring, cabinets, and fixtures. Utility connections, water, sewer, and electrical, are tied into the main home’s service or run as separate connections to the municipal infrastructure depending on jurisdiction requirements and how the unit will be used. Some municipalities require a separate meter for a rental ADU; others allow shared metering with a separate submeter. Confirm these details during the permitting stage before construction begins.
FAQ
How Long Does It Take to Build a Detached ADU in Snohomish County?
From initial design to occupancy, a detached ADU in Snohomish County typically takes eight to fourteen months, with permitting review accounting for a significant portion of the timeline. Once permits are approved, construction generally takes four to six months for a standard small unit, depending on size and finish level. Both permitting and construction timelines can stretch if the county review backlog is long or if design changes require plan revisions.
Can a Garage Be Converted Into an ADU?
Yes, attached garage conversions are a common path to an ADU because the structure already exists. The work typically involves adding insulation, heating and cooling, electrical updates, a bathroom, and kitchen facilities, and upgrading the slab if it was designed for vehicles rather than habitation. Garage conversions require permits and must meet the same habitability standards as any new residential construction.
Are There Size Limits for ADUs in Snohomish County?
Yes. Size limits vary by zone and municipality, and Snohomish County’s regulations have been updated several times in recent years as the state expanded ADU permissions. Homeowners should verify current limits with the county’s planning department or a contractor who regularly handles ADU permits in the area, since rules in one zone may differ from a neighboring jurisdiction.
Do ADUs Require Separate Utility Meters?
Whether a separate meter is required depends on the unit’s intended use and the applicable jurisdiction’s utility rules. Some municipalities require separate metering for ADUs rented to non-family members. Others allow shared metering with a submeter that tracks usage for internal billing purposes. The permit process confirms this based on how the owner intends to use the unit.
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