Off-Grid Living in San Mateo County, California

When I started researching off-grid rules for San Mateo County, I quickly discovered that you can't treat this as a single jurisdiction - it's more like 24 separate mini-jurisdictions, each with its own zoning and building rules. The county itself regulates only water, septic, greywater, and wells through Environmental Health. Everything else (zoning, dwelling types, utility disconnection, and applied building codes) falls to your specific city or town. This fragmentation is both a challenge and an opportunity: some municipalities in the county are more permissive than others, so location within San Mateo County matters significantly.

Zoning: Check Your Specific Municipality

San Mateo County has no unincorporated zoning code. This means that if your land is within an incorporated city or town - and most developed or semi-developed land in the county is - your zoning is governed by that municipality's code, not a county baseline. Cities like Palo Alto, San Mateo, Menlo Park, and Redwood City all have different zoning frameworks, minimum lot sizes, and rural/agricultural land definitions.

What this means for off-grid living: You cannot assume that rural acreage in San Mateo County is zoned to allow a detached dwelling, a secondary structure, or an alternative energy system just because it looks remote or undeveloped. I have found no county-level exemption or variance process for off-grid properties. Instead, you must:

1. Identify the specific city or town that has jurisdiction over your parcel 2. Obtain a copy of that municipality's zoning code (available online or at the planning department) 3. Confirm that your intended land use - whether a residence, a secondary dwelling unit (ADU), or an outbuilding - is permitted in your zone 4. Ask the planning department explicitly whether connecting to or remaining disconnected from utilities affects your permitted use 5. If your land is unincorporated (very rare in San Mateo County), contact the county's Planning and Building Department at 650-364-1800 to confirm

The most practical approach is to contact your city or town's planning office *before* you purchase, not after.

Dwelling: Town-Level Rules Apply

The state of California sets a minimum baseline for dwelling types (tiny homes must meet the California Building Code; RV residency is capped at 180 days per year unless connected to utilities), but San Mateo County itself has published no dwelling restrictions. Each municipality has its own rules about whether you can live in an ADU, a cottage, a converted outbuilding, or any alternative dwelling structure.

Practically speaking: If you're thinking of living off-grid in a tiny home, a shipping-container dwelling, or an alternative structure, your first step is to contact your city or town's planning department. Ask them directly: - Is this dwelling type permitted in my zone? - Does the dwelling require a permanent utility connection? - Are there minimum square-footage requirements? - Can I use this as a primary residence?

Some San Mateo County towns have adopted California's ADU laws and allow unpermitted ADUs on owner-occupied land; others have not. There is no blanket rule.

Building: Title 24 Applies; Residential Rules Vary by Municipality

California's Title 24 Building Standards Code applies statewide, including San Mateo County. The county has *also* adopted amendments that require all-electric buildings and solar installation for *multifamily and commercial* projects. These amendments do not automatically extend to single-family homes.

For off-grid residential building: If you are constructing a new residence or substantially renovating, you must meet California's Title 24 standards, which cover structural safety, fire protection, plumbing, electrical wiring, energy efficiency, and sanitation. Title 24 is updated every three years and is the foundation for all building permits.

However, the specific residential building code that applies to your property - and the specific local amendments - depend on which city or town has jurisdiction. Some San Mateo County municipalities may have adopted additional energy or structural requirements. You need to:

1. Confirm which building code your municipality has adopted 2. Contact your city or town's building department for a pre-application consultation 3. Discuss whether off-grid systems (solar, battery backup, composting toilets if applicable) will be approved as part of your plans 4. Obtain a building permit *before* you start construction

Building without a permit in San Mateo County can result in fines, ordered demolition, and difficulty selling or refinancing the property later. This is not a shortcut to pursue.

Septic: County Permit Required; 3-Foot Soil Depth Minimum

This is the first truly county-level rule: all septic systems require a permit from San Mateo County Environmental Health (650-372-6200, smchealth.org). There is no municipal exemption.

San Mateo County's specific requirements are: - At least 3 feet of suitable soil beneath the dispersal trenches to absorb effluent - All system components must remain within your property lines - The system must be easily accessible for maintenance and repairs - You must submit plans to Environmental Health before installation - An operating permit is required, even for alternative systems or holding tanks

What "suitable soil" means: The county will conduct or require a percolation test to confirm that the soil can absorb wastewater at the required rate. Soil that is too clay-heavy, too sandy, or too shallow may not qualify, in which case you cannot use a conventional septic system on that parcel without significant earthwork (import of suitable soil, engineered sand mounds, etc.) or a switch to an alternative system.

Composting toilets: California state law allows composting toilets *only as a supplement* to an approved septic system, not as a standalone replacement for a conventional or alternative system. I found no San Mateo County exemption for composting toilets as primary sewage treatment. If you want to use a composting toilet as your primary system, contact Environmental Health directly to ask whether an alternative treatment system (such as a constructed wetland or advanced treatment unit) might be approved in place of a conventional septic system. This is uncommon and expensive, but it is worth asking.

The realistic sequence: Get a soil evaluation done (hire a licensed soil scientist or septic contractor); submit the results to Environmental Health; receive a permit or a denial; if denied, explore alternative systems; if approved, have a licensed contractor install the system; pass final inspection. This typically takes 2-4 months.

Greywater: County Plan Review Required

Here is where San Mateo County is stricter than California state law: The county requires Environmental Health plan review for any residential greywater reuse inside the home or for surface drip/spray irrigation.

California's state baseline allows laundry-to-landscape systems (washing machine water diverted directly to outdoor drip irrigation on the same property) to operate without a permit if 13 specific guidelines are followed. San Mateo County exceeds this baseline and requires a permit application and plan review for *any* greywater system.

What this means: If you want to reuse greywater from your shower, sink, or washing machine - whether for indoor use or outdoor irrigation - you must:

1. Submit a greywater reuse plan to San Mateo County Environmental Health 2. Include details on the source (which fixtures), treatment method (if any), storage, distribution, and end use 3. Receive approval before installation 4. Have the system inspected upon completion

This adds 4-8 weeks to a greywater project and requires professional plan preparation (typically $500 - $1,500) rather than a DIY installation. However, it ensures that your system is safe and that you have no legal liability if it fails or causes groundwater contamination.

Rainwater harvesting as an alternative: If you cannot use greywater, rainwater harvesting (captured from your roof and stored in a cistern) is a state-permitted activity for outdoor non-potable use and does not require county approval under 360 gallons. See the rainwater section below.

Rainwater: State Law Permits It; County Rules Unclear

California's Rainwater Capture Act of 2012 explicitly legalized rainwater harvesting. You do not need a water-right permit to capture rooftop runoff and store it in a cistern under 360 gallons for outdoor non-potable use (irrigation, outdoor cleaning, etc.).

For San Mateo County specifically: I found no county-level rules that restrict or require permits for rainwater harvesting under the state baseline. However, San Mateo County Environmental Health does oversee water quality and stormwater runoff. If you plan a rainwater system larger than 360 gallons, a system that feeds potable water indoors, or a system that involves substantial earthwork or surface drainage, contact San Mateo County Environmental Health directly to confirm that your plan does not trigger additional review.

Practical approach: A 100-200 gallon cistern for outdoor irrigation is almost certainly fine under state law and unlikely to need county approval. Anything larger, anything intended for drinking water, or anything integrated with your septic or well system should be discussed with Environmental Health first.

Well: County Permit and Site Evaluation Required

All private wells require a permit from San Mateo County Environmental Health, and the county requires a site evaluation to ensure the proposed system will be safe and reliable.

The process: 1. Hire a licensed well contractor (California Water Environment Contractors Board) 2. Have the contractor perform a site evaluation, including depth to groundwater, geological conditions, and proximity to potential contaminants 3. Submit the evaluation and a well permit application to Environmental Health 4. Receive approval or a denial 5. If approved, have the contractor drill and develop the well; submit water samples for drinking-quality testing 6. Receive a final permit

San Mateo County Environmental Health also administers California's Drinking Water Standards, so your water must test safe for human consumption. If it does not, you may need treatment (a filter system, UV disinfection, etc.) approved by Environmental Health.

One critical step I did not find explicitly stated in county documents: verify water rights. Before you buy land with a well, or before you invest in a well permit, confirm whether the parcel has available groundwater rights. In California, water rights are often tied to the land; you may not have a legal right to pump as much water as you want, even if groundwater exists. Contact the local water board or district and ask whether the parcel has riparian rights or an adjudicated water right. If you're unsure, hire a water law attorney to review the deed and property history ($500 - $1,500 well spent).

Utility Disconnection: Check Your Specific City

California state law now explicitly permits off-grid electricity. However, some jurisdictions can still require a connection to sewer or water lines if municipal lines run within a certain distance of the property (typically 100-200 feet, depending on the municipality).

San Mateo County itself has published no rule on utility disconnection. Your city or town's building department (the "Authority Having Jurisdiction") makes the final call. Contact them and ask: - Can I disconnect from the grid? - If the town has sewer lines within X feet, am I required to connect? - If the town has water lines, can I use a well instead? - Are there any other utility connection requirements?

Some San Mateo County municipalities are permissive; others require connection if it's available. This is worth clarifying in writing before you commit to an off-grid plan.

Summary of the Practical Sequence

1. Identify your specific city or town 2. Contact that city or town's planning department and ask about zoning, dwelling types, and building code amendments 3. Contact San Mateo County Environmental Health (650-372-6200) to discuss septic, well, and greywater plans 4. If you want solar or other energy systems, confirm with your city's building department that they are permitted and will pass inspection 5. Hire professional designers (a septic engineer, a licensed well contractor, a solar installer) and submit plans for review 6. Obtain all permits before construction or installation 7. Pass final inspections

This is not fast (plan 3-6 months), but it is the legal path forward and the one that protects your investment.