Sonoma County, CA Accessory Structures: ADU Impact Fees (2026)
Key Facts
- Fee exemption threshold
- ADUs under 750 sq ft
- Fee basis
- Proportional to ADU/primary sq ft
- Multifamily lots
- Based on average unit size
- Citation
- County Code § 26-88-060(f)
- Applies only to
- Unincorporated Sonoma County
Summary
Sonoma County charges ADU development fees proportionally by square footage compared to the primary residence, and waives them entirely for ADUs under 750 square feet, per County Code § 26-88-060(f).
Applicable development fees shall be charged proportionately by the square footage of the ADU in relation to the square footage of the primary residence. On a lot that contains a multifamily dwelling, the proportionality shall be based on the average square footage of the units within the multifamily dwelling structure. No fees shall be charged for ADUs of less than seventy hundred fifty (750) square feet.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 64 Update 2).
Full Breakdown
Unincorporated Sonoma County ties ADU development fees to unit size instead of billing a flat rate. Under County Code § 26-88-060(f), applicable development fees are charged proportionately by the square footage of the ADU relative to the primary residence's square footage; on a lot with a multifamily building, the proportion is based on the average unit size in that structure. Any ADU under 750 square feet is exempt from these fees outright. This applies only to unincorporated county parcels. Santa Rosa, Petaluma, Rohnert Park, Windsor, Healdsburg, Sebastopol, Cotati, Cloverdale and the City of Sonoma set their own ADU fee rules under separate municipal codes.
Violations & Fines
Unpaid development fees are collected as a condition of permit issuance; Permit Sonoma will not issue a building permit or certificate of occupancy until required fees are paid in full.
Frequently Asked Questions
Do I owe impact fees for a small ADU in Sonoma County?
How are ADU fees calculated if my ADU is over 750 square feet?
Sources & Official References
Other rules in Sonoma County
California rules heatmap·Compare Sonoma County to another location·View the California accessory structures overview
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