ADU Permits in Snohomish County: How the Process Works in 2026

If you have been thinking about adding an accessory dwelling unit in Snohomish County, the rules are more favorable right now than they have been in decades. In March 2025, the Snohomish County Council unanimously passed Ordinance 25-014, which loosened long-standing restrictions on ADUs in unincorporated areas of the county. Combined with Washington’s statewide ADU reform law (House Bill 1337), the permitting picture in 2026 looks very different from what most homeowners remember.

Here is what actually changed, what the permit process looks like step by step, and where homeowners typically get stuck.

What Ordinance 25-014 changed

Based on the county’s adopted code changes, the highlights for homeowners in unincorporated Snohomish County are:

• Up to two ADUs on a qualifying lot. In urban zones, a lot with a single-family home, attached single-family dwelling, or duplex can now have up to two ADUs — one attached and one detached, two attached, or two detached.
• No additional parking in urban zones. Urban-zone ADUs no longer require an extra off-street parking space. In rural zones, one off-street space per ADU is still required.
• Restrictive design standards removed. The county dropped architectural matching requirements that previously forced ADUs to mimic the main house, which gives you more freedom on design and can lower construction cost.

The state law behind much of this, HB 1337, also barred owner-occupancy requirements and required jurisdictions to cut impact fees for ADUs to no more than half of what a single-family home would pay. Cities across the county had until 2025 to bring their codes into compliance, so if your property is inside city limits (Marysville, Lake Stevens, Arlington, Everett), your city’s rules will be similar in spirit but differ in detail.

The permit process, step by step

1. Confirm what your lot allows. Zoning, critical areas (wetlands, steep slopes), and utility capacity determine what you can build before design ever starts. This is the single most valuable thing to nail down early — it is much cheaper to learn about a setback problem on paper than mid-project.

2. Prove out water and septic (or sewer). Snohomish County requires documentation that potable water is physically and legally available to the ADU, and that your existing or proposed septic system — or sewer connection — can handle the additional demand. On rural lots, septic capacity is the most common early roadblock, and it is far better to know before you fall in love with a floor plan.

3. Design to code. Your plans need to meet setback, lot coverage, and height requirements for your zone. This is where working with a builder who designs around the lot — rather than dropping a catalog plan onto it — pays off in fewer correction cycles with the county.

4. Submit through the PDS Permit Portal. Unincorporated Snohomish County handles ADU permit applications online through Planning and Development Services. A complete, well-prepared submittal is the biggest factor in how fast you get through review — most delays come from incomplete applications bouncing back with correction requests.

5. Build with inspections along the way. Once the permit is issued, construction proceeds through the county’s normal inspection sequence — foundation, framing, mechanical/electrical/plumbing, and final.

How long does it take, and what do permits cost?

Regional market data for the Puget Sound area puts the typical ADU timeline at roughly four to eight months from permit application to completed unit, with permitting review being the largest variable. Permit-related costs — plan review, building permit, utility connections, and impact fees — vary widely by lot and jurisdiction; regional estimates commonly land in the tens of thousands of dollars all-in, though HB 1337’s impact-fee reduction has meaningfully lowered that line item for ADUs compared to a few years ago.

Because these numbers are driven by your specific parcel — zoning, septic vs. sewer, and which jurisdiction reviews the application — treat any figure you read online as a planning range, not a quote. For what the construction itself costs, see our guide to what an ADU costs in Snohomish County. The honest answer for your property comes from a site visit and a written estimate.

Attached, detached, or garage conversion?

The permit path is similar for all three, but the practical considerations differ. Detached ADUs offer the most privacy and rental flexibility but carry the full cost of a foundation, envelope, and utility runs. Attached ADUs share structure with the main house and can be more economical. Converting an existing garage can work well too — though you will want a builder who also handles garage construction to assess whether the existing slab and framing can support conversion to living space, or whether a purpose-built structure makes more sense.

Where homeowners get stuck

After building ADUs across Snohomish County, the pattern is consistent: the projects that stall are almost never stopped by the county — they stall because of incomplete submittals, septic surprises discovered late, or designs that ignored a setback. The projects that move are the ones where feasibility was checked first and the paperwork went in complete.

That is the main reason to have our ADU building team in Snohomish County handle design, permitting, and construction as one job: nothing falls between the trades, and the county sees one coherent package.

Thinking about an ADU on your property?

Every lot is different, and the 2025 rule changes opened doors that were closed a year ago — your property may qualify for more than you think. Tell us about your project and we will walk the site, check what your lot allows, and give you a written estimate with real numbers for your parcel.

Rule references above are based on Snohomish County Ordinance 25-014 and Washington HB 1337 as adopted; requirements vary by jurisdiction and change over time. Verify current requirements with Snohomish County Planning and Development Services or your city before making decisions.