Home Additions & ADUs — Snohomish County
Home Additions & ADUsin Bothell.
Bothell's sloped lots are the reason ADUs pencil here. A daylight basement that already has a door to the yard is most of a second home before we start.
( Bothell )
Home Additions & ADUs for Bothell homes
The 1980s–2000s two-story homes that fill Canyon Park, North Creek and the Fitzgerald hillsides were built on grade changes, and a lot of them carry a daylight basement with its own exterior door, full-height ceilings and a slab that is already there. Converting that into a legal ADU is the most cost-effective second unit we build anywhere in our service area: the shell, the foundation and often the plumbing rough-ins exist, so the budget goes into a kitchen, a bathroom, fire separation from the main house and a proper egress window if the bedroom lacks one.
Detached units are the other Bothell project. Larger lots on the Snohomish County side take a backyard cottage comfortably; the tighter King County parcels usually need a feasibility read on setbacks, lot coverage and where the sewer runs before anyone draws a plan. Bothell straddles the county line, so the first thing we confirm is which jurisdiction governs your parcel, because the size limits, the review timeline and the fee schedule all differ.
Permits for Bothell projects run through City of Bothell Community Development, and we handle the filing and inspections as part of the job. For the full picture on how we approach this work, see our home additions & adus page — and for everything we build locally, our Bothell contractor page.
Typical Bothell investment
Broad ranges for orientation — your written estimate is free and itemized.
| Tier | Range | What it covers |
|---|---|---|
| Garage conversion | $40k–$150k | Living space at the low end; full ADU with kitchen, bath and entrance at the high end. |
| Ground-level addition | $250–$450/sq ft | Foundation, framing, envelope, systems and finishes, tied into the existing house. |
| Second story / detached ADU | $350–$550/sq ft | Structural work below, or a standalone unit with its own foundation and utilities. |
Bothell questions
- Can I build an ADU in Bothell?
- In most cases yes. Washington's statewide ADU rules apply in Bothell, which allows accessory units on most single-family lots without the old owner-occupancy requirement. Height, setback and lot-coverage limits are local and vary by zone and by which county the parcel is in, so we confirm your specific lot at the free estimate.
- What does an ADU cost in Bothell?
- A daylight-basement conversion with an existing exterior door typically runs $100,000 to $200,000 in 2026 including the kitchen, bath, fire separation, permits and utility work. A detached new-build unit runs $250,000 to $450,000 depending on size, slope and how far utilities have to travel.
- Is my Bothell parcel in King or Snohomish County?
- The city spans both. Permits are filed with City of Bothell Community Development either way, but the county affects utility districts, some code details and the review timeline. We look up your parcel before scoping, so the estimate is built on the right rules.
- Does a basement ADU in Bothell need an egress window?
- Every bedroom does, and many daylight basements already qualify because the walkout wall has full-size windows. If the bedroom sits on the uphill side, we cut a window well into the foundation, which adds roughly $4,000 to $9,000 and is far cheaper than a failed inspection.
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