Lassen County, CA Accessory Structures: ADU Rules (2026)

California's statewide ADU law (Government Code sections 66310 through 66342, recodified in 2025 from former section 65852.2) applies in unincorporated Lassen County and largely preempts local restrictions. A qualifying ADU must be approved ministerially - no public hearing - within 60 days of a complete application. State standards allow an ADU up to 800 sq ft (or larger subject to local development standards), a minimum 4-foot rear and side setback, and a height limit of 16 feet for a detached unit (18 feet near major transit or on a multifamily lot, plus up to 2 feet for matching roof pitch) and up to 25 feet or two stories for an attached unit. A junior ADU is capped at 500 sq ft and must be within the walls of an existing or proposed single-family home. Lassen County's pre-existing Zoning Code provision for a 'second dwelling unit' (LCC 18.108.270), which required a use permit and limited non-R-1 units to 1,400 sq ft or 80 percent of the primary dwelling, is superseded for ADUs that meet state criteria. On rural ranch parcels in agricultural zones, the broad lot sizes generally make meeting setback and size standards straightforward. Confirm current local implementation with Lassen County Planning and Building Services.