California’s Alquist-Priolo fault zone building restrictions bar you from placing any structure for human occupancy across the trace of an active fault, and they presume the ground within 50 feet of that trace is also unsafe to build on. If your parcel sits inside a mapped Alquist-Priolo Earthquake Fault Zone, you can only build an occupied structure after a California-licensed geologist investigates the site and shows it is clear of active faulting. If you’re selling such a parcel, you also have to tell the buyer it’s in the zone before closing.
The Core Rule and the 50-Foot Buffer
The statute itself is short: no structure for human occupancy can be placed across the trace of an active fault.1California Legislative Information. California Public Resources Code 2621.5 State regulations then go a step further. They presume that any ground within 50 feet of an active fault trace is itself underlain by active fault branches, and no occupied structure is allowed inside that 50-foot buffer unless a geologic investigation proves otherwise.2Legal Information Institute. California Code of Regulations Title 14, Section 3603 – Specific Criteria
Two definitions do most of the work here. An “active fault” is one that has shown surface displacement within Holocene time, roughly the last 11,000 years. A “structure for human occupancy” is any building whose expected use exceeds 2,000 person-hours per year, which covers virtually all homes, offices, schools, and commercial buildings.
The zones themselves are drawn by the State Geologist around faults considered active enough to threaten structures with surface rupture or fault creep. The statute specifically names the San Andreas, Calaveras, Hayward, and San Jacinto, and the State Geologist can add any other sufficiently active and well-defined fault.3California Legislative Information. California Public Resources Code 2622 Zones are generally a quarter-mile wide or less, with boundaries typically set about 500 feet from major active faults and 200 to 300 feet from well-defined minor faults.
Checking Whether Your Property Is in a Zone
The California Geological Survey publishes an online map, Earthquake Zones of Required Investigation, that shows fault rupture, liquefaction, and seismic landslide hazard zones at full resolution.4California Department of Conservation. Earthquake Zones of Required Investigation You can search by address or zoom to the parcel. The CGS treats the GIS files in its Information Warehouse as the authoritative source if a boundary is close.
Counties that receive an official map must also post public notice within five days at the offices of the county recorder, county assessor, and county planning commission, identifying where the map can be viewed and when it took effect.3California Legislative Information. California Public Resources Code 2622
Which Projects Trigger the Investigation Requirement
The Act applies to two categories of development inside a fault zone: subdivisions under the Subdivision Map Act that will eventually include occupied structures, and the occupied structures themselves.5California Seismic Safety Commission. California Public Resources Code 2621-2630 – Alquist-Priolo Earthquake Fault Zoning Act
There is an important carve-out for individual homeowners. A single-family wood-frame or steel-frame dwelling of two stories or fewer is excluded from the project definition when it is not part of a development of four or more units. A single-family wood-frame or steel-frame dwelling on a parcel where a geologic report was already approved during subdivision also does not need a second report.6California Legislative Information. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning Mobilehomes wider than eight feet count as single-family wood-frame dwellings for this purpose.
So a homeowner building one modest house on an individual lot often does not need a fault investigation. The moment a developer is subdividing land or building four or more homes, the full requirement applies.
The Geologic Report
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.7Justia Law. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning – Section 2623 The report must be prepared by a California-licensed geologist.8California Department of Conservation. Alquist-Priolo Earthquake Fault Zones It typically involves subsurface trenching to inspect soil layers for evidence of displacement, along with borings to pin down fault locations.
The regulations tell the geologist to focus on locating faults, determining how recently they moved, and characterizing them, all directed at whether surface fault displacement could affect the proposed site.2Legal Information Institute. California Code of Regulations Title 14, Section 3603 – Specific Criteria The fault report can be combined with other geotechnical studies the project needs.
The developer submits the report to the local planning department. A geologist registered in California who works for or is retained by the agency reviews it. There is no fixed statutory review period. The local agency may charge the applicant a reasonable fee for processing and review, and must forward the accepted report to the State Geologist within 30 days, where it becomes part of a permanent open-file record.2Legal Information Institute. California Code of Regulations Title 14, Section 3603 – Specific Criteria
One escape valve exists. If the city or county determines no surface rupture hazard exists at the site, it can waive the report requirement, but only with the State Geologist’s approval.7Justia 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 additional reports are required unless new data warrants it.
Existing Buildings, Additions, and Retrofits
If your building already exists, several exemptions pull you out of the construction-side rules. The disclosure obligation under Section 2621.9 still applies at the time of sale even when these exemptions cover the construction.
- Any building or development that existed before May 4, 1975 is exempt, unless alterations exceed the value threshold below.5California Seismic Safety Commission. California Public Resources Code 2621-2630 – Alquist-Priolo Earthquake Fault Zoning Act
- Renovations and additions are exempt when their 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 create greater life-safety risk than the existing authorized use.6California Legislative Information. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning
- Converting an existing apartment complex to condominiums does not trigger the Act, since no new construction is involved.5California 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 are exempt, provided the retrofit does not increase occupant load and the city or county follows the specific safety conditions in the statute.5California 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 home that is not part of a development of four or more units is excluded from the investigation requirement.6California Legislative Information. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning
Stricter Local Rules Can Apply
The state rules are a floor. The statute explicitly authorizes cities and counties to adopt stricter policies and criteria, impose additional fees beyond what the Act requires, and decline to grant any of the exemptions above.9Justia Law. California Public Resources Code Chapter 7.5 – Earthquake Fault Zoning – Section 2624 A jurisdiction sitting on complex fault systems may require more detailed investigations, wider setbacks, or additional engineering analysis. Before you rely on a state-level exemption, confirm with the local planning department what the city or county adds on top.
Disclosure When You Sell
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 duty applies whether you use an agent or sell directly. It is triggered when the seller or agent has actual knowledge of the zone status, or when the county has received the official map and posted notice at the recorder, assessor, and planning agency offices.
For transactions covered by California’s natural hazard disclosure law, the fault zone question is answered on the Natural Hazard Disclosure Statement.11California Legislative Information. California Civil Code 1103 If the available maps are not detailed enough for a reasonable person to tell whether the property is inside 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 can expose the seller to legal liability and may give the buyer grounds to rescind the purchase. This disclosure duty survives even when the property is otherwise exempt from the Act’s construction restrictions. A pre-1975 building that never needed a geologic report still requires fault zone disclosure at the time of sale.