Frequently Asked Questions (FAQ)

Does the County of Los Angeles require a minumum setback distance from the trace of an active fault for development of a single family residence in an unincorporated area? Or does the County rely on the setback established by the State?

The County of Los Angeles has not established a minimum setback from the trace of an active fault. Section 113.5 of the Los Angeles County Building Code states that "no building or structure shall be constructed over or upon the trace of a known active earthquake fault... ." The same Section states that a geology investigation is required "when the proposed building is within 50 feet of...the assumed location of a known active earthquake fault..." or "when the proposed building is within 50 feet of...the most probable ground location of a trace of a known active earthquake fault... ."

The State also does not establish a minimum setback. The Alquist-Priolo Earthquake Fault Zones Act of 1972 requires fault studies for subdivisions of land and "projects" consisting of four or more single family residences, but does not require a fault study for a single-family wood frame dwelling (Section 2621.6). The Act also states that "the area within 50 feet of such active faults shall be presumed to be underlain by active fault branches of that fault unless proven otherwise..." (Section 3603(a)).

The building setback from an active fault trace is recommended by the consulting geologist investigating the property. Many geologists recommend a 50-foot fault setback; however, shorter setbacks have been recommended by some consultants.

Alquist Priolo Earthquake Fault Zones building setback active fault LDD regional planning subdivision land development tentative map parcel tract lot split clearance bond drainage concept bulk grading private drain transfer sewer area study LID improvement future surface mining