Building permits, zoning and land rules in Marin County, California

Most construction projects in unincorporated Marin County require a building permit from Building and Safety¹. A one-story detached, non-habitable accessory structure may be exempt when it is no larger than 120 square feet and complies with zoning; a rural tool shed, workshop or horse stall may qualify at up to 300 square feet on a parcel of at least one acre, again subject to zoning¹. If the work is not on the county's exception list, assume a permit is needed and ask Building and Safety before starting¹. Permitted projects require inspections, and separate reviews or fees may come from Planning, Public Works, Fire, or Environmental Health Services¹². Marin County Code Chapter 19.20 also offers voluntary alternative regulations for limited-density owner-built rural dwellings, and the community development agency issues a certificate of occupancy identifying compliance with that chapter once the work is approved¹³. The County Board of Supervisors adopted the updated California Building Standards Code with Marin amendments on December 9, 2025, and the updated state regulations took effect January 1, 2026¹⁴. The 2025 code and Marin-specific local amendments apply in unincorporated Marin, including updated green-building requirements effective January 1, 2026¹⁴. A Marin-specific restriction also prohibits fire-retardant treated wood in exterior construction within designated Wildland Urban Interface and Very High Fire Hazard Severity Zones, so ask Building and Safety whether the parcel falls in one before choosing materials¹⁴.

Marin County Community Development Agency's Building and Safety Division handles permits, plan review and inspections for unincorporated Marin County¹⁵. The office is at 3501 Civic Center Drive, Suite 308, San Rafael, CA 94903, and its general number is 415-473-6550¹⁵. The county uses two application paths: quick online permits for listed maintenance and minor improvements, and plan review for new buildings, additions, remodels, conversions, ADUs, decks, accessory structures, pools and similar larger work¹⁶. All building permits begin through the county's online application, and plans for larger projects are submitted electronically and drawn to scale^16,17. After issuance, schedule each required inspection with Building and Safety; the inspector checks the approved permit documents and the work covered by the permit¹².

Marin County Title 22 contains the zoning and subdivision rules for every unincorporated area⁴. Inside city and town limits the local council sets planning and zoning instead, as in San Rafael¹⁸, Novato¹⁹, San Anselmo²⁰, Corte Madera²¹ and Tiburon²²; the county's zoning rules below do not apply there². Use the Planning Division's parcel lookup to identify the district before applying a dimensional standard⁴. Agricultural districts have parcel-specific minimum lot areas and height limits; primary structures in the listed agricultural districts generally have a 30-foot limit, while accessory structures in A15 through A60 districts have a 16-foot limit, subject to the code's measurement rules and exceptions⁶. Sleepy Hollow parcels zoned R1:BD or A2:BD require at least 1,300 square feet of habitable floor area, excluding porches and garages⁶. An ADU is allowed in unincorporated Marin: a Category 1 detached ADU is limited to 800 square feet, a Category 2 detached ADU generally to 1,200 square feet, rentals must run longer than 30 consecutive days, and the Director must decide a complete ADU permit application within 60 days; building, zoning, water, wastewater and septic requirements still apply⁶. Ask Planning which standards and overlays apply to the exact parcel²³.

Septic permits for any Marin County address, city or not, are the county's to issue. Marin County Environmental Health Services administers onsite wastewater treatment systems and requires a construction permit for a new septic system³. Permits also cover listed replacement, repair, alteration and abandonment work, while additional permission may be needed in the Coastal Zone or Stream Conservation Area³. Environmental Health Services performs construction inspections, and the septic tank and disposal area cannot be covered before the Health Officer's inspection²⁴. Alternative systems require annual operating permits and professional monitoring²⁴. A property within 400 feet of a public sewer main must connect instead of expanding septic when the Health Officer finds connection physically and legally possible²⁴. Only 1.6% of Marin County soils lack severe limits for a conventional drain field, so a site evaluation should precede a land purchase or house design¹¹. Simple graywater systems below 250 gallons per day may avoid a construction permit but still require advance notice, while larger or treated systems require county permits³.

Marin County has not confirmed a general right to live in an RV on a private parcel, and California generally limits RV occupancy to regulated mobilehome or special-occupancy parks unless a local ordinance provides otherwise⁹. In unincorporated Marin County, camping may occur only in campgrounds and hunting and fishing camps and is limited to 30 days per calendar year per person, except camp staff; the applicable agricultural district may prohibit the use or require a Use Permit⁶. The Board of Supervisors declared a shelter crisis on March 11, 2025, for three years; paired with building-code changes, it allows removable cabins or RVs as temporary emergency shelter only in unincorporated areas and only for someone currently designated as homeless⁷. Private owners may add such shelter without a permanent foundation under that rule, but it is not general authorization for residential RV occupancy⁷. Tiny homes need a recognized legal structure classification under California rules⁹. On a single-family residential parcel, an RV may house dedicated caregivers for occupants who need medical care and help with daily activities, but only under a Temporary Use Permit that cannot run longer than 6 months²⁵. Marin's emergency tiny-home pathway is temporary and cannot be used as a short-term rental⁸. Manufactured-home rules for mobile-home parks do not authorize a unit on every parcel; those parks require the applicable health permit, rezoning petition and Master Plan⁶.

1. Confirm that the parcel is unincorporated, then use the Planning Division's zoning lookup and ask about allowed use, setbacks, flood districts and Coastal Zone constraints^2,4. A qualifying agricultural project in the Coastal Zone needs an approved Coastal Permit Exclusion before building, septic or well permits can issue²⁶.

1. Confirm legal access because every structure or use must reach a public street through frontage, an easement or a recorded mutual-access agreement acceptable to the Director⁶. A driveway in a state highway right-of-way also needs a Caltrans encroachment permit²⁷.

1. Resolve water and wastewater before final design; new and replacement septic systems require county review, and only 1.6% of local soils lack severe conventional-drain-field limits^3,11. A private well in unincorporated Marin needs an Environmental Health Services permit before drilling and another permit before its water is used domestically²⁸.

1. Start the correct online building-permit path and submit scaled electronic plans when plan review is required^16,17.

1. Pay applicable fees, obtain the permit, and complete each required inspection before covering work^12,29.

Check the exact zoning district, jurisdiction and development overlays before relying on a listing description^2,4. In Marin's Primary Floodway district, new buildings and structures are prohibited; the Secondary Floodway district allows underlying uses only under standards and agreements protecting ponding and drainage capacity⁶. On qualifying undeveloped parcels in the San Geronimo Valley Stream Conservation Area, an exception analysis limits total parcel coverage to 1,000 square feet and requires the least possible stream-area encroachment⁶. Verify road access, wastewater feasibility and water before closing because only 1.6% of county soils lack severe conventional-drain-field limitations^6,11. Drilling a private well in unincorporated Marin requires a county permit, and domestic use of the installed well requires a permit to operate²⁸. Splitting a parcel is regulated by the county's subdivision rules in Title 22 Article VI, which work alongside the California Subdivision Map Act and add zoning-district and slope-based minimum lot areas⁴. It needs County approval, generally a Tentative Map and then a Parcel Map or Final Map, and subdivisions creating 5 or more parcels generally need Tentative and Final Maps⁶. If rental income matters, residential rentals shorter than 30 days require county licensing, subject to a countywide cap and township-specific limits³⁰. Marin County's effective property-tax rate is about 0.7%, and the median annual property-tax bill is about $10,001¹¹.