Snohomish ADU Basics for Homeowners: What to Know

Thinking about adding an accessory dwelling unit to your Snohomish property? You are not alone. ADUs can unlock rental income, support multigenerational living, and boost long‑term flexibility. In this guide, you will learn the basics, how local rules work in the City of Snohomish and unincorporated Snohomish County, what it may cost, and the steps to move forward confidently. Let’s dive in.

What is an ADU?

An accessory dwelling unit is a secondary home on the same lot as a primary residence. You might picture a basement apartment, a converted garage, or a small detached cottage in the backyard. ADUs include interior or attached units, detached cottages, and conversion units within an existing structure.

ADUs are meant to be subordinate to the main home. They have their own kitchen, bathroom, and sleeping area. The specific definitions and size limits depend on your local code, so always confirm with the city or county that governs your address.

Common ADU uses

ADUs offer flexibility for how you live and invest. Many owners create long‑term rentals where allowed, or use the space for aging parents or adult children. Some choose an ADU as a future downsizing option by moving into the smaller unit later and renting the main home.

You can also combine living and working. For example, you might set up a home office with separate living space. If you are considering short‑term rental use, know that separate rules typically apply and must be confirmed locally.

City vs. County rules

Your first step is to confirm which jurisdiction applies to your property. If your address is inside city limits, the City of Snohomish rules apply. If your property is outside city limits in an unincorporated area, Snohomish County rules apply instead.

• Check the City of Snohomish Planning & Community Development pages to review local zoning and permit requirements and search for “accessory dwelling.” You can start at the City’s website and navigate to Planning and permits using the site search at the City of Snohomish website.
• For unincorporated properties, review the Snohomish County Planning and Development Services Permit Center. Use the county site search for “ADU” or “accessory dwelling” starting from Snohomish County Planning and Development Services.

Washington State offers high‑level guidance and best practices as well. For a broader overview of ADU policy and options, explore the Department of Commerce ADU guidance.

Key rules to confirm

Local rules vary by zone, lot, and overlay districts. Before you commit to a design, confirm the following with your city or county planner:

• Allowed ADU types on your lot: attached, detached, or interior conversion.
• Maximum size, height, and lot coverage for your zone.
• Setbacks from front, side, and rear property lines, plus any roofline rules.
• Owner‑occupancy requirements and whether they apply to your situation.
• Off‑street parking rules for ADUs and any exceptions.
• Utility requirements: water and sewer connections, or septic capacity.
• Short‑term rental rules and any licensing if you plan to host guests.
• Building code items like separate exits, egress windows, fire separation, and ventilation.

If your lot is in a historic district, along a shoreline, or within a critical area, you may face added design review or environmental standards. Ask planning staff early about any overlays that apply to your parcel.

Costs and timelines in Snohomish

Actual costs depend on design, site work, and current bids, but the following ranges are common:

• Interior conversion: often $20,000 to $120,000 depending on kitchen, bath, and code upgrades.
• Attached or detached new construction: often $100,000 to $350,000 or more depending on size, finishes, and site work.

Plan for design and permit preparation to take about 2 to 6 months in many cases. Permit review often takes 4 to 12 weeks depending on complexity and agency workload. Construction can range from 2 to 9 months based on scope and contractor availability.

The Puget Sound region often sees higher construction costs than national averages. Get multiple local estimates so you can refine your budget with real numbers.

Feasibility checklist

Use this quick checklist to gauge whether your property and goals fit an ADU:

• Jurisdiction: Are you in the City of Snohomish or unincorporated Snohomish County?
• Zoning: Does your zone allow ADUs and are there any overlays or floodplain issues?
• Lot layout: Do you have room to meet setbacks, access, and utility routing?
• Utilities: Is sewer available, or does your septic have capacity and reserve area?
• Access and parking: Does your site meet access needs and any parking rules?
• Financing and budget: Will you use cash, a HELOC, or a construction loan?
• Intended use: Will it be owner‑occupied, long‑term rental, short‑term rental, or family use?

If you check most boxes with a yes, your next step is to talk with planning staff and a designer to confirm details.

Permits and process

While each project is unique, most Snohomish‑area ADUs follow a similar path:

1. Pre‑application check. Contact the City of Snohomish or Snohomish County PDS to confirm zoning, setbacks, and whether your ADU type is allowed. Ask about any design review or historic requirements.

2. Site assessment. Verify sewer and water availability. If you have a septic system, consult health regulations and get a septic feasibility check. For statewide onsite sewage rules and resources, start with the Washington State Department of Health onsite sewage guidance.

3. Concept design. Draft a simple site plan and floor plan showing setbacks, parking, and utility routes. Consider hiring an architect or designer for clarity.

4. Permit applications. Prepare building permit documents, energy calculations, and any required land use or zoning reviews. Apply for utility or septic permits as needed through your jurisdiction.

5. Plan review and corrections. Respond to comments from building, public works, and health reviewers.

6. Permit issuance. Pay fees and fulfill any pre‑construction conditions.

7. Construction and inspections. Schedule required inspections at each stage.

8. Final approval. Obtain your certificate of occupancy or final sign‑off for rental use.

Typical timelines include 1 to 2 weeks for an initial zoning check, 2 to 8 weeks for design and application prep, 4 to 12 weeks for permit review, and 2 to 9 months for construction. Complex sites may take longer.

Common hurdles to expect

A smooth ADU project starts with knowing what can slow you down. Keep an eye on these factors:

• Septic capacity. Many projects on septic require system upgrades or added capacity. This can be a major cost item and sometimes a deal‑breaker.
• Historic or critical areas. Properties in historic districts or near wetlands, steep slopes, or shorelines face extra steps.
• Utility connections. Even when sewer is nearby, capacity checks, new laterals, or fees can add cost.
• Parking and access. Curb cut rules and driveway standards may affect your site plan.
• HOA covenants. Private restrictions can limit or prohibit ADUs even if the city or county allows them.
• Code upgrades. Legal rentals often need fire separation, egress, and sound control upgrades that affect budgets.

Address these early to avoid surprises later.

Value drivers and returns

ADUs can add value through rental income and increased resale appeal. Your rent potential depends on unit size, finishes, and location relative to amenities and employment centers. Snohomish is generally desirable for commuters to the Everett and Seattle corridor, which can support demand.

Keep taxes in view. ADU construction can trigger reassessment that may increase your property taxes. For county assessment questions and fee schedules related to utilities, start at Snohomish County Planning and Development Services and use the site search to find the Assessor and Public Works pages.

To control costs, compare a conversion versus new construction, right‑size the unit, and reuse existing utilities where possible. Track expenses closely if you plan to rent, since income is taxable and some costs may be deductible.

Professionals to involve

An experienced team will save you time and risk. Consider speaking with:

• City or county planning staff for a pre‑application check.
• A licensed architect or residential designer for plans and submittals.
• A general contractor with ADU experience for pricing and buildability.
• A civil or structural engineer if your site or structure is complex.
• A septic designer or inspector if your property relies on onsite sewage.
• A land surveyor for property lines and setbacks.
• A lender who understands ADU financing.
• An accountant for rental income and tax planning.

Washington State publishes homeowner‑friendly best practices, model code language, and context that can help you prepare questions for your local planner. You can review these resources at the Department of Commerce ADU guidance.

Next steps and local resources

If you are just starting out, take these simple steps:

1. Confirm jurisdiction and zoning. Contact the City of Snohomish Planning or Snohomish County PDS and ask whether ADUs are allowed on your parcel.

2. Check utilities early. If on septic, schedule a feasibility review as one of your first actions.

3. Build a realistic budget. Ask your contractor for a line‑item estimate that separates site work, utilities, shell, and finishes.

4. Explore financing. Talk with a lender about HELOCs, construction loans, or renovation mortgages and how rental income could affect qualification.

5. Plan for compliance. If you intend to rent, discuss tax and insurance implications with your accountant and insurer.

• City of Snohomish Planning & Community Development: start at the City of Snohomish website and search for “accessory dwelling.”
• Snohomish County PDS Permit Center: begin at Snohomish County Planning and Development Services and search for “ADU.”
• Washington State Department of Commerce ADU resources: Department of Commerce ADU guidance
• Washington State Department of Health onsite sewage systems: Onsite sewage system information

Thinking about how an ADU fits your purchase or sale strategy in Snohomish? You can get local perspective on neighborhood demand, rental potential, and resale value aligned with your goals. If you want a clear plan tailored to your property, connect with Dani Robinett to talk options and next steps.

FAQs

What counts as an ADU in Snohomish?

• An ADU is a secondary home on the same lot as a primary residence, such as an interior, attached, or detached unit, with details defined by your city or county code.

Can I build a detached ADU on a small lot in Snohomish?

• It depends on your zoning, setbacks, lot coverage, and local size caps; confirm with the City of Snohomish or Snohomish County PDS before designing.

Do I need to upgrade my septic system for an ADU in Snohomish County?

• Possibly; septic capacity and reserve areas are common limits, and you may need upgrades or a new design reviewed under state and local health rules.

Do I have to live in one of the units to have an ADU?

• Owner‑occupancy rules vary by jurisdiction and have changed in recent years, so verify the current requirement with your local planning staff.

Can I use an ADU as a short‑term rental in the Snohomish area?

• Short‑term rentals often have separate rules or licensing; confirm local regulations for your address and intended use before you invest.

How will an ADU affect my property taxes in Snohomish County?

• New construction can trigger reassessment and higher taxes; check with the Snohomish County Assessor through the county site for property‑specific guidance.

How much rent can I expect from an ADU in Snohomish?

• Rent depends on unit size, finishes, and location relative to amenities and jobs; review current listings and speak with a local property manager for comps.