Los Angeles issued 1,211 demolition permits in 2001. This ‘erase-atecture,’ as some architectural historians call it, gives builders room to press forward with their perpetual reinvention of the city, and it often protects the public from unsafe structures. But nobody knows just how much valuable history the wrecking balls obliterate each year, because in most cases, nobody’s keeping track. About 85% of the city’s standing structures have never been surveyed for historic or cultural significance.