Accessory Dwelling Units: Add Living Space and Income to Your Property
Accessory dwelling units have become one of the most valuable additions a homeowner can make in Snohomish County. Whether you want to generate rental income, house a family member, create a home office, or plan for aging in place, an ADU gives you a self-contained living space on your existing property without the cost and disruption of moving.
NW General Contractor is an ADU specialist serving Everett and communities throughout Snohomish County. We design and build detached ADUs, convert garages into living spaces, and transform basements into fully permitted apartments. We handle every phase of the project, from initial feasibility assessment through permit applications, construction, and final inspection.
Washington State ADU Regulations: What You Need to Know
Washington state has been at the forefront of ADU-friendly legislation. Understanding the current regulatory landscape is essential before starting your project.
HB 1337: The Law That Changed Everything
House Bill 1337, passed by the Washington State Legislature, requires all cities planning under the Growth Management Act to allow at least two ADUs on any residential lot. This landmark legislation effectively eliminated many of the local barriers that previously made ADU construction difficult or impossible. Key provisions include:
- Cities must allow at least two ADUs per lot (one attached and one detached, or two attached, or two detached)
- Owner-occupancy requirements are prohibited for lots with a single ADU
- Off-street parking requirements for ADUs are eliminated in many cases
- Impact fees for ADUs under 800 square feet are prohibited
- Cities cannot require design standards more restrictive than those for the primary dwelling
Snohomish County ADU Rules (Updated March 2025)
Snohomish County updated its ADU regulations in March 2025 to align with state law and further reduce barriers. The updated rules include:
- Removal of owner-occupancy requirements
- Reduced setback requirements for detached ADUs
- Increased maximum ADU size allowances
- Streamlined permit review process for qualifying projects
- Allowance for both attached and detached ADUs on the same lot
The regulatory environment has never been more favorable for ADU construction in Snohomish County. If you explored building an ADU in the past and were discouraged by restrictions, the rules have changed significantly in your favor.
Types of ADUs We Build
Detached ADUs (DADUs)
A detached accessory dwelling unit is a standalone structure built on your property, separate from the main house. This is the most versatile ADU type because it provides complete privacy and separation for both the homeowner and the occupant.
Detached ADUs in Snohomish County typically range from 400 to 1,000 square feet and include a full kitchen, bathroom, living area, and one or two bedrooms. They can be single-story or two-story depending on lot size and zoning. Construction costs for a detached ADU in Everett generally range from $250,000 to $450,000, depending on size, site conditions, utility connections, and finish level.
Site conditions that affect cost include:
- Slope and grading requirements
- Distance from existing utility connections (sewer, water, electrical)
- Soil conditions and foundation requirements
- Access for construction equipment
- Tree removal or protection requirements
Garage Conversions
Converting an existing garage into a living space is often the most cost-effective path to an ADU. The structure is already there, which eliminates foundation and framing costs. The primary work involves insulation, interior framing, plumbing, electrical, HVAC, and finish work.
Garage conversions in Everett typically cost between $180,000 and $220,000. The wide range reflects the condition of the existing garage (some need significant structural upgrades), the extent of plumbing and electrical work required, and the level of finishes you choose.
Key considerations for garage conversions:
- Ceiling height — Most garages have adequate ceiling height, but some may need floor lowering or ceiling modification to meet code minimums.
- Foundation — Garage slabs are often thinner than house foundations and may need reinforcement or waterproofing.
- Insulation — Garages are typically uninsulated. We bring walls, ceiling, and floor up to current energy code standards.
- Parking replacement — Under current regulations, you generally do not need to replace lost parking when converting a garage to an ADU.
Basement Apartments
If your home has an existing basement with adequate ceiling height (typically 7 feet minimum finished), a basement conversion can be an excellent ADU option. Basement ADUs share the foundation and roof of the main house, which reduces construction costs compared to detached units.
Basement conversions typically cost between $100,000 and $180,000 when the space already has sufficient ceiling height and exterior access. The work includes egress windows or doors, waterproofing, insulation, a full kitchen, bathroom, plumbing, electrical, HVAC, and interior finish.
The most important requirement for a basement ADU is a separate entrance. Code requires an independent means of entry and exit for the ADU occupant, which usually means adding an exterior door with a covered entry.
Rental Income Potential
One of the primary motivations for building an ADU is rental income. In Everett and Snohomish County, rental demand is strong, driven by population growth, proximity to major employers (Boeing, Naval Station Everett, Providence Regional Medical Center), and limited housing supply.
Current rental rates for ADUs in Snohomish County typically range from:
- Studio / 1-bedroom ADU (400-600 sq ft): $1,200 – $1,600/month
- 1-bedroom ADU (600-800 sq ft): $1,500 – $2,000/month
- 2-bedroom ADU (800-1,000 sq ft): $1,800 – $2,400/month
At these rates, many ADUs generate enough monthly income to cover a significant portion of the construction loan payment, making the investment cash-flow positive within a few years. The rental income also increases the overall value of your property.
Aging in Place: ADUs for Multi-Generational Living
Beyond rental income, ADUs serve as an excellent solution for multi-generational families. An ADU can provide independent living space for aging parents who want to be close to family while maintaining their own privacy and routines. It can also serve as a space for adult children saving for their own home.
We design ADUs with aging-in-place features including:
- Single-story, no-step entry
- Wide doorways (36-inch minimum) for wheelchair accessibility
- Curbless showers with grab bars
- Lever-style door handles and faucets
- Good lighting throughout
- Low-maintenance finishes and materials
Pre-Approved State ADU Plans
Washington state has developed a catalog of pre-approved ADU plans designed to streamline the permitting process. These plans have been reviewed and approved at the state level for structural and energy code compliance, which can significantly reduce local permit review times.
Pre-approved plans come in a range of sizes and configurations. They are a good option for homeowners who want a straightforward design and a faster permitting timeline. However, they may not work for every lot due to site-specific conditions like slope, setbacks, or utility locations.
We can help you evaluate whether a pre-approved plan is right for your property or whether a custom design makes more sense. In either case, we handle the full permit process from application through final inspection.
Our ADU Construction Process
- Feasibility Assessment — We visit your property, review lot dimensions, setbacks, utility locations, and zoning to determine what type and size of ADU is possible. This initial assessment is free.
- Design & Planning — We work with you to develop the floor plan, select materials, and create construction documents. For custom designs, we coordinate with an architect or designer. For pre-approved plans, we adapt the selected plan to your site.
- Permitting — We prepare and submit all permit applications, including building, plumbing, electrical, and mechanical permits. We respond to plan review comments and schedule all required inspections.
- Site Preparation — Grading, utility trenching, and foundation work. For garage conversions, this phase includes demolition of garage doors and structural modifications.
- Framing & Rough-In — Structural framing, roofing, windows, and doors, followed by rough plumbing, electrical, and HVAC installation.
- Insulation & Drywall — Insulation to current energy code standards, drywall installation, taping, and finishing.
- Finish Work — Cabinets, countertops, flooring, tile, paint, fixtures, and appliances. This is where the ADU comes together as a livable space.
- Final Inspections & Occupancy — We schedule and pass all final inspections, obtain the certificate of occupancy, and walk through the completed ADU with you.
Timeline: How Long Does ADU Construction Take?
The complete timeline from initial design to move-in readiness is typically 6 to 10 months, broken down as follows:
- Design and planning: 2–6 weeks
- Permit review: 4–8 weeks (faster for pre-approved plans)
- Construction (detached ADU): 4–6 months
- Construction (garage conversion): 3–4 months
- Construction (basement conversion): 2–4 months
Factors that can extend the timeline include permit delays, site complications, material lead times, and weather. The Pacific Northwest climate allows year-round construction, but heavy rain periods can slow exterior work.
Why Choose NW General Contractor for Your ADU
We specialize in ADU construction in Snohomish County. We understand the local regulations, the permit process, and the construction challenges specific to this area. Our experience includes detached new-construction ADUs, garage conversions, and basement apartments across Everett, Marysville, Lake Stevens, and surrounding communities.
We are licensed, bonded, and insured in Washington state. We provide detailed written estimates, manage the entire permit process, and communicate proactively throughout every phase of construction. Building an ADU is a significant investment, and we treat it with the care and attention it deserves.