5 county-level rules, plus city-specific rules for 1 city in Marin County, California.
Verified from official government sources
In unincorporated Marin County, solid fences and screening walls cannot exceed 6 feet above grade. Within front yards or side yards abutting a street, the limit drops to 4 feet (or up to 6 feet if the portion above 4 feet is at least 50% open).
Marin County Development Code § 22.20.050 (Fencing and Screening Standards)
A fence or screening wall having a maximum height of four feet or less above grade may be located within a required setback for a front yard or side yard that abuts a street... the entire section or portion of the fence or wall above four feet in height above grade has a surface area that is at least 50% open and unobstructed by structural elements.
Fences up to 6 feet tall in unincorporated Marin County generally do not require a building permit if they comply with zoning. Fences exceeding 6 feet, or in coastal/design-review zones, require a Planning Division permit and may require building permits under Title 19.
Marin County Community Development Agency, Building & Safety: Do I Need a Building Permit?
Fences exceeding 6'-0" in height still require a Planning Division permit. If you're building a fence in a rural area located on property of 1 acre or more, you may not need any permit for your fence if you meet all zoning regulations.
Retaining walls in unincorporated Marin County may reach 6 feet above grade when the exposed face points inward, or 4 feet when facing outward (8 feet if 25+ feet from all property lines), under Development Code § 22.20.052.
Marin County Development Code § 22.20.052 (Retaining Wall Standards)
Retaining walls may reach a maximum height of six feet above grade if the exposed face of the retaining wall faces into the center of the property. Retaining walls may reach a maximum height of four feet above grade if the exposed face of the retaining wall faces outward from the center of the property, unless they are at least 25 feet from all property lines, in which case they can reach a max...
Marin County does not categorically prohibit fence materials, but fences taller than 4 feet in front yards or street-side setbacks must be at least 50% open above the 4-foot mark. State Zone 0 wildfire rules also affect non-combustible material requirements within 5 feet of structures.
Marin County Development Code § 22.20.050 (Fencing and Screening Standards)
the entire section or portion of the fence or wall above four feet in height above grade has a surface area that is at least 50% open and unobstructed by structural elements.
1 cities in Marin County have their own fence regulations rules. Each link goes to that city's dedicated page with code citations.
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