California’s PRC 4291, which applies in all unincorporated parts of Nevada County, is the code that mandates defensible space around habitable structures up to the property line. (The definitions of terms used in 4291 are at PRC 4211). The County of Nevada also has a hazardous vegetation ordinance, entitled Nuisance Declared; Duty to Abate Hazardous Vegetation and Combustible Material (Click #7.4) which requires defensible space around habitable structures (100 feet) as well as along roadways 10 feet wide and 15 feet high) beyond the shoulder of roadways (both sides) that serve as primary ingress and egress routes.
County Code for Private Driveway Construction Standards
Follow this path:
Roadside Clearing Regulations: make your roads safe for firefighters to get IN and for you to get OUT. This unofficial one-sheet PDF handout from the Coalition of Firewise Communities tells you the requirements, in the unincorporated County, Grass Valley and Nevada City, for clearing back from the sides of private roads. Diagrams help make it clear. There are three times as many private roads in the County as there are public roads, and the property owners are responsible for meeting these requirements.
Talking to your neighbors about defensible space - helpful PDF from Fire Safe Council with good tips on getting neighbors to buy into the idea of fire safety
Link to all Nevada County Ordinances: http://qcode.us/codes/nevadacounty/
Link to all City of Grass Valley Ordinances: https://library.municode.com/ca/grass_valley/codes/code_of_ordinances
Link to all Nevada City Ordinances: https://library.municode.com/ca/nevada_city/codes/code_of_ordinances