5 county-level rules, plus city-specific rules for 2 cities in Monterey County, California.
Verified from official government sources
Unincorporated Monterey County permits one ADU plus one JADU on lots with a single-family dwelling. Inland (Title 21, section 21.64.030) ADUs may not exceed 1,200 sq ft; JADUs max 500 sq ft. Detached ADUs default to a 16-ft height limit. State ADU law (Gov. Code 65852.2/66310+) preempts local rules where they conflict.
Monterey County Code § 21.64.030(E)(5), (6), (9), (10)
Side and rear lot line setbacks for ADUs shall be a minimum of four (4) feet and shall also be sufficient for fire and safety... ADUs shall not exceed 1,200 square feet. JADUs shall not exceed 500 square feet. An ADU detached from the principal dwelling shall be subject to the height regulations for a habitable accessory structure or allowed to be up to 16-feet in height, whichever is greater.
Small one-story sheds in unincorporated Monterey County (240 sq ft or less, no utilities) generally do not need a building permit, but Planning rules such as zoning setbacks and design control still apply. Larger sheds or any structure with electrical, plumbing or mechanical work require a building permit.
Monterey County HCD Building & Planning Services FAQ (citing CBC adopted by MCC § 18.02)
A construction permit is not required for a one-story detached structure accessory to detached one and two family dwellings used as tool and storage sheds, playhouses and similar uses, provided the floor area does not exceed 240 square feet and the structure does not contain any electrical, plumbing or mechanical work.
Unincorporated Monterey County allows converting an existing garage into an ADU under section 21.64.030. No setback is required for a garage converted (or demolished and rebuilt) as an ADU within the existing footprint, and replacement off-street parking is not required when the garage becomes an ADU.
Monterey County Code § 21.64.030(E)(6)
No setback shall be required for an existing garage that is converted or demolished and reconstructed as an ADU within the existing footprint of the structure. Side and rear lot line setbacks for ADUs shall be a minimum of four (4) feet and shall also be sufficient for fire and safety. A minimum setback of four (4) feet must be maintained from the side and rear lot lines for an ADU that is cons...
Monterey County treats carports as accessory structures subject to Title 21 zoning setbacks and building permits. Under the county ADU ordinance and state law, when a carport is demolished or converted to build an ADU, the county does not require the lost off-street parking spaces to be replaced.
Monterey County recognizes a 'tiny home' as an ADU only if it qualifies as a manufactured home under Health & Safety Code section 18007. Per section 21.06.372, a qualifying tiny home is treated as a manufactured home and may serve as an ADU (max 1,200 sq ft inland). Movable tiny homes on wheels are not separately permitted as dwellings.
2 cities in Monterey County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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