A wildfire-safe home must be an ember-ignition-resistant home, so that even if the flames do not reach your home, it will be able to withstand exposure to embers that may have been blown a mile or more in front of a wildfire. To provide maximum wildfire protection for your home, a combination of near-home vegetation management, appropriate building materials, and related design features must be used. These points are summarized the excellent Univesity of California publication, "Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations."
Preparing and maintaining adequate defensible space will guard against flame contact and radiant exposures from nearby vegetation—but because of the likely ember exposure to your home during a wildfire, you cannot ignore building material and design considerations. Similarly, if you ignore your defensible space (i.e., you do not have it or do not maintain it), the wildfire will produce maximum ember, flame, and radiant exposures to your home. It is very unlikely that even hardened buildings can survive such exposure, as a weak link will likely exist somewhere in the building enclosure.