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

Yes. Colusa County requires permits for new residential, commercial and industrial buildings, additions, decks, pools, water heaters, patio covers, conversions, expansions, reroofing, electrical and plumbing systems, heating and air-conditioning systems, and structures over 120 square feet; the county says that list is not exhaustive¹. The 2025 California Building Standards Code became effective January 1, 2026 and applies to building-code work the county administers¹⁵. After issuance, county inspectors verify that work matches the permit, applicable codes and approved plans¹. At completion, the inspector signs the job card, and the county can issue a Certificate of Occupancy or Notice of Completion upon request¹. An owner-builder option can exempt a qualifying owner from contractor licensing, but it does not remove the county permit or inspection requirements^14,16.

The Colusa County Building Department receives building applications through the county's online permit portal and reviews project plans for code compliance². The coordinated review sends the plot plan to Planning for allowed-use and setback review, Environmental Health for wells and waste disposal, the applicable fire district for State Fire-Safe review, and Public Works for county-road encroachments, flood hazards and significant grading². The Building Department is at 1213 Market Street in Colusa and can be reached at (530) 458-0481². A complete plan set may need electrical and plumbing plans, truss and energy calculations, a plot plan, and applicable sprinkler and solar plans². Public Works publishes separate fees: a county-right-of-way encroachment permit costs 3% of the estimated work, weekend or holiday inspections are billed separately, and a flood-zone development permit has no fee¹⁷.

Yes. The Colusa County Zoning Code applies to all unincorporated land in Colusa County³. Inside city limits, Colusa administers its own zoning ordinance⁴ and Williams sets zoning under its own Title 17⁵; the county's rules below do not apply there, and we could not confirm who issues building permits in either city, so ask the city. The county publishes a countywide map plus maps for named communities and unincorporated parts of Colusa and Williams¹⁸. Planning reviews whether a proposed use is allowed and whether the plot plan meets setbacks². Minimum parcel size is 80 acres in F-A and U-C, 40 acres in E-A and 10 acres in U-T and A-T¹², and 20 acres in A-T-20¹². In those agricultural districts a home (a non-agricultural structure) needs 25-foot front and rear setbacks, and side setbacks of 25 feet in F-A, E-A and U-C or 20 feet in U-T and A-T¹². Non-agricultural structures may reach 30 feet unless a variance is obtained¹². Manufactured or mobile homes are permitted in listed residential and agricultural zones¹². A second unit must have at least 150 square feet; we could not confirm a minimum size for primary homes³. A second unit may be up to 1,200 square feet, or 2,500 in the agricultural districts F-A, E-A, U-C, U-T and A-T³.

Colusa County Environmental Health administers onsite wastewater treatment systems in unincorporated areas for ordinary systems designed at 10,000 gallons per day or less⁶. A permit is required before constructing, reconstructing or repairing a septic system, and site evaluation is part of approval⁹. Only 3.9% of county soils avoid a severe USDA limitation for a conventional drain field, so test the actual homesite early and budget for a design beyond a standard trench system if the evaluation requires one¹⁹. A property with an approved available sanitary sewer within 200 feet must connect when the right-of-way and required sewer-authority and LAFCO approvals can be obtained⁶. Surface-discharge approaches such as surface irrigation or evaporation ponds are not authorized as county OWTS, and alternative systems carry installation, operating, monitoring and inspection requirements⁶. Environmental Health is at 1213 Market Street in Colusa and can be reached at (530) 458-0888²⁰.

You cannot use an RV as a permanent residence in unincorporated Colusa County; temporary RV occupancy during home construction requires a Temporary Use Permit and must fit the circumstances allowed by county code¹⁰. We could not confirm a general private-land camping allowance outside that construction pathway, so ask Planning before placing or occupying an RV. Colusa County's tiny-home ordinance, adopted December 14, 2021, treats a park-trailer tiny home outside an approved mobile-home or special-occupancy park as an accessory dwelling unit, but only if all of these are met: utilities connect to county-approved systems under an issued county permit; the wheels and tongue are removed; a county-approved foundation with its own building permit supports it; and the exterior looks like a house rather than an RV, with matching skirting, to the Community Development Director's satisfaction¹¹. A tiny home on wheels that cannot meet those conditions is not covered by that pathway, so confirm the classification with Planning before buying one¹¹. Manufactured and mobile homes are allowed in listed residential and agricultural districts, while temporary mobile homes require a Temporary Use Permit^12,13.

1. Ask Planning to verify the parcel's zoning district, allowed residential use, setbacks and any required land-use approval before finalizing plans².
2. Start the Environmental Health site evaluation and septic permit if public sewer is unavailable, because poor drain-field ratings make early soil work especially valuable^9,19.
3. Apply to Environmental Health before drilling, modifying or abandoning a water well⁷.
4. Prepare the building plans and a plot plan showing buildings, utilities, sewage disposal, streets, easements and access².
5. Submit the complete building application through the online permit portal².
6. Coordinate with Public Works if the project affects county right-of-way, grading or a flood hazard, and obtain a Caltrans encroachment permit if access enters a state highway^21,22.
7. Keep approved plans onsite and complete the required inspections through final sign-off¹.

First determine whether the parcel is unincorporated or inside Colusa or Williams, then use the county's published maps and have the responsible planning office confirm the exact zone, allowed dwelling type, setbacks and parcel status^8,18. A land division requires county subdivision review in unincorporated Colusa County²³. Check the FEMA flood map, which the county says covers the unincorporated areas, and ask Public Works about floodplain permits and elevation certificates before choosing a building pad; Public Works also reviews every building-permit plot plan for flood hazards, and the flood-zone development permit carries no fee²⁴. The county's pages do not spell out construction standards for flood zones, so get those from Public Works in writing²⁴. Verify legal access too: Public Works issues and inspects permits for work in county right-of-way, while access onto a state highway needs Caltrans approval^21,22. Wells require a county permit, and severe drought occurs in about 41% of years^7,19. Septic feasibility is a major purchase contingency because only about 4% of soils avoid severe limits for a conventional drain field¹⁹.