Try as you might, you really won't be able tell where in the U.S. American Beauty is supposed to be taking place; everything about the Burnhams' hometown seems incredibly generic. Is it California? North Carolina? Aside from being fairly certain it's not Manhattan, we don't know much else—and that's by design.
American Beauty is pretty focused on the (middle-class, white) American lifestyle and American values, so it makes sense that Mendes and Ball would want to keep the setting nonspecific in terms of location—that way, you really can apply the movie's insights and lessons to the American suburban experience as a whole.
For example, the bird's-eye view of the family's neighborhood shows us blocks and blocks of identical-looking housing, all of which looks like it would be at home in pretty much any major American suburb.
When we get down to "earth" (you know, street level), things don't get any more specific—the houses, businesses, and signage all look pretty nondescript. You've got your crummy office space (where Lester works), a real estate office, a fast food joint... nothing you couldn't find in "Anytown, U.S.A." So, that's the name we've given the Burnhams' home turf.
That all said, despite the filmmakers' efforts to keep out specifics regarding the film's setting, some have argued it was supposed to be the suburbs of Chicago, based on the presence of the Chicago Sun Times in the Burnhams' house and the fact that some of the characters have Chicago area codes. However, all shooting for the film took place in California.