What Does an ADU Cost in Snohomish County? (2026 Guide)

If you are thinking about adding an accessory dwelling unit to your property, the first question is almost always the same: what is this going to cost? Below is an honest look at what ADUs are running in Snohomish County right now, what actually moves the number, and why two projects on the same street can come in thousands of dollars apart.

What an ADU costs in Snohomish County

Current market data puts detached ADUs in Snohomish County at roughly $336 to $504 per square foot. For a typical 800 square foot unit, that works out to somewhere around $269,000 to $403,000 all-in.

For context, the national average for ADU construction sits closer to $150 to $300 per square foot. The Puget Sound region runs higher, because labor, materials, and permitting all cost more here, though Snohomish County generally comes in below comparable Seattle projects.

Treat those figures as a starting range, not a quote. The spread between the low and high end is wide for a reason, and where your project lands depends almost entirely on the factors below.

What actually drives the number

• Attached vs. detached. A detached unit is its own building with its own foundation, roof, and utility runs. Converting existing space, like a garage or basement, is usually the cheaper path because the shell already exists.
• Site conditions. Slope, drainage, soil, and how far the unit sits from existing water, sewer, and power all change the excavation and utility cost before a single wall goes up.
• Utility connections. This is the line item that surprises people most. Running sewer and water to a detached unit at the back of a lot can add serious cost that has nothing to do with the building itself.
• Kitchen and bath finish level. An ADU is a complete home in miniature. It needs a full kitchen and bath, and finish choices in those two rooms swing the budget more than square footage does.
• Design and permitting. Drawings, engineering, and county review are real costs and real time, and they land before construction starts.

The 2025 rule change worth knowing about

In March 2025 the Snohomish County Council passed Ordinance 25-014, which significantly loosened restrictions on accessory dwelling units in unincorporated parts of the county. For property owners who looked at an ADU a few years ago and were told no, the answer may have changed.

Rules still vary depending on whether your property sits in unincorporated county or inside a city like Everett, Marysville, Lake Stevens, or Snohomish. That is worth confirming for your specific parcel before you get too far into planning. We broke down the full process — what changed, the steps, and where projects stall — in our guide to ADU permits in Snohomish County.

Does an ADU pay for itself?

Local appraisers estimate an ADU adds roughly $200 to $300 per square foot in property value, so an 800 square foot unit might add $160,000 to $240,000. That does not automatically cover the build, but paired with rental income it changes the math considerably compared with most home improvements.

The other return is the one that does not show up on an appraisal: housing a parent, an adult child, or a caregiver on your own property, without buying a second home.

Is an ADU the right call, or is a tiny home?

People often start researching one and end up building the other. The short version: an ADU is permitted, permanent living space tied to your property, and it counts toward your home value. A tiny home is typically a smaller, more flexible structure with a different permitting path and a different resale story. Which one fits depends on your lot, your budget, and what you want the space to do in ten years.

Getting a real number for your lot

Every figure above is a range. The only way to get a number you can plan around is to look at your actual property: the slope, the utilities, the setbacks, and what you want inside the unit.

We are ADU contractors in Snohomish County and we handle design, permitting, and construction as one project, so you are not managing three trades and hoping the drawings match the code. Tell us about your project and we will walk the site and give you a written estimate.