Bothell, WA Accessory Structures: ADU Rules (2026)
ADU rules in Bothell, WA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- ADUs per lot
- Two
- Size limit
- 1,200 square feet; conversions up to 40 percent of primary unit floor area
- Detached ADU height
- 30 feet, or 33 feet over existing accessory structure
- Critical area lots
- One ADU
- Middle housing lots
- One ADU in any configuration
- Unpermitted ADU
- Apply to city; building inspection if no permit
Summary
In the City of Bothell, Washington, BMC 12.14.135 allows two accessory dwelling units per lot, each up to 1,200 square feet. Detached units are capped at 30 feet in height, or 33 feet over an existing accessory structure, and unpermitted units must apply for city approval and a building inspection.
Accessory dwelling units are permitted subject to the following requirements: A. Two accessory dwelling units are allowed per lot. B. One accessory dwelling unit in any configuration is allowed on a lot developed with middle housing. C. One accessory dwelling unit is allowed on lots designated with critical areas or their buffers. D. The size of an accessory dwelling unit shall be limited as follows: 1. An accessory dwelling unit may be developed up to 1,200 square feet. Conversion of an existing space to an accessory dwelling unit may exceed 1,200 square feet as approved by the community development director, provided this does not exceed 40 percent of the primary dwelling unit’s gross floor area. 2. Detached accessory dwelling units are exempt from the provisions of BMC 12.14.130 that limit the size of accessory buildings to five percent of the lot area. E. Detached accessory dwelling units shall have a height no greater than 30 feet or no greater than 33 feet for units over an existing accessory structure.
Full Breakdown
BMC 12.14.135 is the City of Bothell's accessory dwelling unit section, and it opens by stating that ADUs are permitted subject to its requirements. Subsection A allows two accessory dwelling units per lot, matching the Maximum ADUs per lot row of 2 for R-C, R-L1, R-L2 and R-M1 in the BMC 12.14.030(A) table. Subsection B allows one ADU in any configuration on a lot developed with middle housing, and subsection C allows one ADU on lots designated with critical areas or their buffers.
Size is limited in subsection D. An ADU can be developed up to 1,200 square feet. Converting existing space can exceed 1,200 square feet if the community development director approves it and the unit does not exceed 40 percent of the primary dwelling unit's gross floor area. Detached ADUs are exempt from the BMC 12.14.130 rule that limits accessory buildings to five percent of the lot area. Subsection E caps detached ADU height at 30 feet, or 33 feet for units over an existing accessory structure.
Subsection F covers units built without approval. The owner must apply to the city, and if no building permit was ever issued the city requires a building inspection to decide whether the structure is sound, poses no hazard to people or property, and meets this section and BMC Title 20. The ADU application fee covers that inspection, and permits and inspections can also be required for existing attached and detached ADUs at the building official's direction. Setbacks come from BMC 12.14.070(B): a detached ADU keeps the same rear yard setback as the primary structure, except an ADU on an alley, which follows the alley setbacks in BMC 12.14.060. Building permits for ADUs are also exempt from frontage improvements under BMC 17.09.010(B)(2)(a).
Violations & Fines
An ADU built or used outside BMC 12.14.135 is a zoning violation enforced under Chapter 11.20 BMC by the community development director, with a civil penalty of up to $250.00 per day for each violation from the date set for compliance. Under subsection F, an ADU created without an approved application must be brought to the city for approval and inspection. A notice of violation can be appealed to the hearing examiner within 15 calendar days after service.
Frequently Asked Questions
How many ADUs can I build on my lot in the City of Bothell?
How large can a Bothell ADU be?
How tall can a detached ADU be in Bothell?
What if my Bothell ADU was built without a permit?
Sources & Official References
Other rules in Bothell
How Bothell compares: Easiest Cities to Build an ADU·Compare Bothell to another location·View the Washington accessory structures overview
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ADU Rules in Nearby Cities
How other cities in King County handle adu rules.