Inside an Alquist-Priolo earthquake fault zone, California building restrictions boil down to two rules: you cannot place a structure for human occupancy across an active fault trace, and for most projects you must pay a California-licensed geologist to investigate the site and prove it is clear before the city or county will issue a permit.1California Department of Conservation. Alquist-Priolo Earthquake Fault Zones A narrow exception spares owners building a single detached home. Every other kind of development, from a four-lot subdivision to a commercial building, has to clear the investigation first.
What the Act Forbids on the Ground
The central rule is simple. No structure for human occupancy can sit across the trace of an active fault.2California Legislative Information. California Public Resources Code 2621.5 State regulations extend that prohibition outward: any ground within 50 feet of an active fault trace is presumed to be underlain by active fault branches, and no occupied building is allowed inside that buffer unless a geologic investigation proves the presumption wrong.3Legal Information Institute. California Code of Regulations Title 14, Section 3603 – Specific Criteria
An “active fault” for these purposes is one that has shown surface displacement within Holocene time, roughly the last 11,000 years. “Structures for human occupancy” is defined by expected use exceeding 2,000 person-hours per year, which sweeps in essentially all homes, offices, schools, and commercial buildings where people spend real time. Sheds, detached garages, and similar low-occupancy structures generally fall outside the definition.
What Counts as a Project That Triggers the Rules
The Act reaches two kinds of development inside a mapped zone. The first is any land subdivision under the Subdivision Map Act where occupied structures will eventually be built. The second is the occupied structures themselves.4California Seismic Safety Commission. California Public Resources Code 2621-2630 – Alquist-Priolo Earthquake Fault Zoning Act
Two carve-outs sit inside that definition. A single-family wood-frame or steel-frame home of two stories or fewer is excluded when it is not part of a development of four or more dwellings. And a single-family wood-frame or steel-frame dwelling built on a parcel where a geologic report was already approved during subdivision does not need a second report.5California Legislative Information. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning A mobilehome wider than eight feet counts as a single-family wood-frame dwelling for this purpose.
The practical upshot: an individual owner building a modest house on a lone lot typically does not need a fault investigation. The moment you are subdividing land, building four or more dwellings, or putting up any other occupied structure, the full investigation requirement applies. Even in the single-home case, the 50-foot buffer and the flat prohibition on building across a fault trace still govern what can actually go where.
The Geologic Investigation You’ll Need
Before approving a project in a fault zone, the city or county must require a geologic report identifying and mapping any surface fault rupture hazard on the site.6Justia Law. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning – Section 2623 The report has to be prepared by a geologist licensed in California.1California Department of Conservation. Alquist-Priolo Earthquake Fault Zones
In practice, the investigation usually involves trenching to expose soil layers for direct inspection and borings to characterize what lies beneath. The regulations require the report to focus on locating faults, dating their most recent movement, and characterizing them, all aimed at whether surface fault displacement could affect the proposed building site.3Legal Information Institute. California Code of Regulations Title 14, Section 3603 – Specific Criteria It can be bundled with the other geotechnical studies a project already needs.
The developer submits the report to the local planning department, where a California-registered geologist working for or retained by the agency reviews it.3Legal Information Institute. California Code of Regulations Title 14, Section 3603 – Specific Criteria There is no statutory deadline for review; timing depends on the report’s complexity and the agency’s queue. The agency can charge a reasonable fee to cover its costs.
One escape valve exists. If the city or county determines that no surface rupture hazard exists at the site, it can waive the geologic report requirement, but only with the State Geologist’s approval.6Justia Law. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning – Section 2623 Once a report is approved or a waiver granted for a site, no further reports are required unless new data warrants it.
Exemptions for Existing Buildings and Small Work
Several categories of work escape the Act’s investigation and building restrictions:
- Buildings or developments that existed before May 4, 1975, unless altered above the value threshold below.4California Seismic Safety Commission. California Public Resources Code 2621-2630 – Alquist-Priolo Earthquake Fault Zoning Act
- Renovations and additions whose value does not exceed 50 percent of the structure’s appraised value. If the work changes the building’s use, the new use cannot increase occupant load or life-safety risk over the existing authorized use.5California Legislative Information. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning
- Converting an existing apartment complex to condominiums, since no new construction is involved.4California Seismic Safety Commission. California Public Resources Code 2621-2630 – Alquist-Priolo Earthquake Fault Zoning Act
- Seismic retrofits of unreinforced masonry buildings, concrete tilt-up buildings, and certain reinforced concrete frame buildings that existed before May 4, 1975, provided the retrofit does not increase occupant load and the jurisdiction follows the safety conditions in the statute.4California Seismic Safety Commission. California Public Resources Code 2621-2630 – Alquist-Priolo Earthquake Fault Zoning Act
- A one- or two-story wood-frame or steel-frame single-family dwelling that is not part of a development of four or more homes.5California Legislative Information. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning
These are exemptions from the construction and investigation rules only. The Act’s disclosure obligation on sellers still applies to every one of them.
Local Rules Can Go Further
The state’s requirements are a floor, not a ceiling. The statute expressly authorizes cities and counties to adopt stricter policies and criteria, to impose additional fees, and to refuse to grant any of the exemptions listed above.7Justia Law. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning – Section 2624 A jurisdiction sitting on complex fault systems can require more detailed investigations, wider setbacks, or additional engineering analysis. Before you commit to a project, confirm the local planning department’s rules alongside the state minimums; a small home that would be exempt statewide might not be exempt in your city.
Checking Whether a Property Is in a Zone
The California Geological Survey’s Earthquake Zones of Required Investigation map application lets you search by address or zoom to a parcel to see whether it falls inside a mapped fault, liquefaction, or seismic landslide zone.8California Department of Conservation. Earthquake Zones of Required Investigation The GIS files in the CGS Information Warehouse are the authoritative source for zone boundaries.
Counties that have received official maps must also post public notice within five days at the offices of the county recorder, assessor, and planning commission, showing where the map can be viewed and when it took effect.9California Legislative Information. California Public Resources Code 2622 Between the online tool and those county postings, most owners can determine zone status without hiring anyone.
If You’re Selling, You Must Disclose
Anyone selling property in a mapped earthquake fault zone must tell the buyer the property is in the zone before the sale closes.10California Legislative Information. California Public Resources Code 2621.9 The obligation applies whether an agent is involved or not, and it is triggered when the seller or agent has actual knowledge of the zone status or the county has received the official map and posted notice.
For transactions covered by California’s broader natural hazard disclosure law, the fault zone disclosure is delivered through the Natural Hazard Disclosure Statement, which has a yes-or-no field for earthquake fault zones.11California Legislative Information. California Civil Code 1103 If the available maps are not detailed enough for a reasonable person to determine whether the property is in the zone, the agent must mark “Yes” unless a third-party expert report confirms the property is outside it.10California Legislative Information. California Public Resources Code 2621.9
Failing to disclose exposes the seller to liability and can give the buyer grounds to rescind. And it applies even when the building is exempt from the Act’s construction rules. A pre-1975 home that never needed a fault report still requires zone disclosure at the time of sale.